Factors to Consider when Choosing a Stretcher or Strainer

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Compiler: Barbara A. Buckley

Stretcher as Artifact

Aesthetic and Historical Considerations

The remarkable success of canvas painting supports from the 16th century onward likely owes much to the development of lightweight stretchers. Since their introduction, the design and construction of stretchers¹ have continued to evolve. Early stretcher designs had fixed joints, often a mortise and tenon or lapped construction secured with wooden pegs or nails. Stretchers with expandable joints appeared by the mid-18 th century with the introduction of the key. Some of the early keyed designs were problematic: single-key constructions produced uneven tensions, and keys were easily lost from open notches. This led to the development of the two-key design with grooves to better secure the keys. The 19th century ushered in the age of commercially produced stretchers. Many new stretcher designs were patented and are therefore datable. While there were significant improvements in stretcher design, such as the introduction of beveled edges, the overall level of individual craftsmanship deteriorated. The significance of these evolutions in stretcher design is that the stretcher itself is a valuable source of information in regard to the painting it supports.

As early as 1971, a philosophy that looked at the stretcher as a valuable source of art historical information was already underway. During a conference titled “American Painting to 1776: A Reappraisal,” Eleanor S. Quandt (1971) made a strong case for the stretcher as an important artifact. She describes two general categories of evidence drawn upon in the examination and documentation of an artifact: that which may be separated from the painting and therefore lost to the scholar, and that which is incorporated into the physical structure of the picture:

The first category comprises original painting supports—fabrics or wood panels—and their auxiliary parts: strainers and stretchers, nails and tacks. These ordinarily need no special equipment to be studied or sampled, and they tell far more about the historic character of a painting than one may realize. Most vulnerable in this category are original strainers. These exist in numbers which are diminishing rapidly, so that documentation of remaining examples is now a matter of considerable urgency. (Quandt 346)

Quandt continues this discussion by stating that the importance of stretchers and strainers has been neglected. They have seldom received careful examination, and they have even less often been preserved as a unique part of the painting. Much of this article is devoted to the types of information one may gather from the careful study of a stretcher. Quandt argues that even the best photographic and written documentary evidence may be lost and that the type of information considered important in one time period may change in the next.

Richard Buck in his landmark “Stretcher Design, A Brief Preliminary Survey” also makes a case for the stretcher as artifact, stating that “Examination of stretcher design is useful because it can reveal some evidence as to the origin, either place or time, of a painting—or at least of its restoration. In either case, it can offer clues about the history of the painting” (Buck 1992, 46).

The construction, design, joinery, and wood type of an original stretcher are all important elements of the painting as a whole. They can be used not only to date a painting, but also to place the painting within an artist's oeuvre, such as illustrated by H. von Sonnenburg (1979) in his studies of Rubens. In addition to the design and construction of the stretcher, auxiliary materials—whether intentional or not—may also provide a valuable source of information. The stretcher may contain various kinds of labels, such as auction, framers', exhibition, museum, and dealers' labels. Labels may also contain information related to the sitter or subject matter, provenance, or instructions for care of a painting. They may contain inscriptions by the artist, original or subsequent owners, a dealer, or framer that relate to the painting's provenance, subject matter, or attribution. In some cases, alternative light sources may be used to make inscriptions more legible. The stretcher may contain custom stamps, wax seals, or accession/inventory numbers. It is important to note that the original placement of labels, stamps, and inscriptions may be important information as well. The means of affixing the canvas to the stretcher (such as tacks, nails, or staples) and their placement may be significant or unique. Less commonly, an original stretcher may incorporate artists' materials, such as those found on rare, surviving original stretchers belonging to works by Albert Pinkham Ryder (Grimm 1998). Excess paint and/or ground on a stretcher may provide information related to the artist's technique or choice of materials. Even modifications to a stretcher may provide valuable information, i.e. relating to the reformatting of a painting.

Some questions to consider in terms of historical considerations are

• Is the stretcher original?

• Has a nonoriginal stretcher been documented, considering that it may provide information about the painting's conservation history?

• Has the original stretcher been fully documented and/or photographed, keeping in mind that the information may be lost and that the type of information considered important may change from what we consider important today?

• Does the design or construction of the stretcher provide information about the origins of the painting, such as period, date, region, school or, in unique cases, possibly the artist or the colorman's firm?

• Does the stretcher incorporate valuable information important for provenance or attribution, such as labels, inscriptions, stamps, or signatures?

• Does the stretcher represent a unique example of type or construction, or does it represent an anomaly or change within an artist's oeuvre?

• Does the stretcher incorporate the original artist's materials?

• Is the means used to attach the canvas to the stretcher—such as tacks, nails, or staples—and their placement unique or significant?

• Can the stretcher and/or means of attachment be reused?

• If original or unique, can the stretcher be structurally modified to properly support the painting?

Discussion of the aesthetic considerations of a stretcher is a bit more ambiguous. While stretchers have been overlooked as less important components of a painting, they do represent some level of choice (or lack thereof). Some artists specifically selected their stretchers; in some schools of thought, conservators will opt for the use of age- or artist-appropriate materials, such as the choice of specific woods and/or construction in the fabrication of a replacement stretcher. Most contemporary conservators would opt to retain significant or unique stretchers and/or tacks, nails, etc., when possible.

Charlotte Seifen Ameringer

Submitted January 2001

Why Use/Reuse a Stretcher/Strainer

The historical integrity of the painting is an important consideration in the decision to reuse a stretcher or strainer. The original size and shape of the picture is indisputably indicated by an unaltered original secondary support. Even if the original secondary support requires some alteration (for example, the addition of a bead to hold the fabric away from the inner edge or the modification of a strainer to allow for expansion), the artist's choice of secondary support, whether deliberate or unconscious, may convey other valuable information, such as the artist's financial state, the presence or absence of studio assistants, location at the time the painting was created, or conformity/departure from a pattern of working practices in the particular painting at hand. What appears to be a meaningless scribble or random mark on the secondary support may, in the context of the entire oeuvre or the provenance, prove to be an important identifier.

We cannot expect to be omniscient regarding the present and future discoveries related to the secondary support; it is therefore an invaluable resource. When the original stretcher/strainer is removed from the composite structure of the picture, although an attempt may be made to retain the essential information by photography, written record/description, or physical storage of all or a portion of the secondary support, it is much less certain that these documents will remain with the picture than that the actual support structure will.

If the painting is on its original stretcher/strainer, is it structurally sound? Can it be modified to make it sound? Has the original secondary support been the source of structural weakness/deformities in the primary support, and can it be modified to correct the problem? Are the original dimensions irregular, therefore increasing the difficulty of replacing the secondary support satisfactorily? If the painting will not be lined and the treatment planned will retain the original creases of the tacking margin, the picture will have the best “fit” on the original stretcher/strainer.

If the painting is unstretched, did the artist use a stretcher/strainer or a planar surface (e.g., a wall) while painting? What is the artist's preference or usual practice for exhibition purposes? If the picture is to be framed, will it be necessary to have a (replaceable) secondary support element (a stretcher/strainer), which can be secured in the rabbet for traditional framing techniques?

Catherine Metzger

Submitted November 2001

When to Replace a Stretcher/Strainer

Is the original stretcher/strainer missing? Is the physical integrity of the picture so compromised by the stretcher/strainer that no method of modification could adequately ensure its continued function as a tensioning device for the primary support? Some examples of loss of function are fractured wooden members, splintered joints, and severe warping. If the stretcher/strainer is not original, it may be replaced simply to supply a stretcher/strainer with improved mechanical properties or of more desirable dimensions. If the picture was originally on a strainer, has a history of requiring adjustment to maintain a planar surface, and the strainer cannot be satisfactorily converted into a stretcher, a new secondary support may be chosen. The selection of the support should take into consideration the future travel and display criteria, as well as the probability that a professional will adjust the tension when necessary.

If the painting has been lined with heavy or stiff adhesives, had interlayers added between the original and lining fabrics, or has been adhered to a lining fabric that is much stiffer than the original fabric, the original stretcher/strainer may not be strong enough to keep the composite support in plane. A new stretcher/strainer is often needed following lining treatments that significantly alter the weight and/or mechanical properties of the support.

Catherine Metzger

Submitted November 2001

Stretcher Design

Profile of a Stretcher

A beveled stretcher profile that slopes continuously from the outer to the inner edge of the stock or a raised bead on the outer edge of rectangular stock raises the reverse of the fabric support away from the stretcher. This helps to reduce the formation of a microclimate between the stretcher and fabric that is different from the climate behind the rest of the support. This discrepancy causes the different regions of the support to expand or contract at different rates. A bead or bevel can also prevent the support from touching the inner edge of the stretcher due to vibration of the support during transit or contact with the stretcher if the support is slack and concave on the stretcher. Impact of the support against the stretcher and repeated formation and cycling of microclimates behind the stretcher result in breakage of the ground and paint films above the inner edges of the stretcher. Eventually this causes the support to form raised creases along the cracks, a phenomenon known to conservators as “stretcher creases.” Separating the support from the stretcher also prevents adhesion of the support to the stretcher when glue size or acrylic priming is applied and squeezes through the fabric's reverse. Intermittent areas where the support is adhered to the stretcher in this manner may result in a dimpled, puckered deformation of the support over the stretcher members.

A stretcher profile that does not adequately separate the support from the stretcher can be retrofitted with strips of beading attached to the outer edges of the stretcher's inner face with glue and brads (fig. 1). An historic stretcher can thus be reused and improved in its function without removing any wood (see section IV.A.1).

Early European and American strainers and stretchers tended to be rectangular in cross section, with some lightweight strainers on small paintings almost square in section.¹ By the early 19th century, the practice of chamfering or beveling the inside face of the strainer or stretcher arose.

FIGURE 1 Stretcher with applied bead

The earliest chamfering treatments were done by hand after the stretcher was assembled by scraping away at the inner edge of the member with a spokeshave or file. These are easily recognized by the rough contours of the worked edges and by the fact that the chamfering stops short of the corners of the stretcher, resulting in an oval-shaped outline to the chamfering. Therefore, the deepest chamfering is generally at the middle of each stretcher member. By the second quarter of the 19th century, stock was milled to create a continuously beveled profile before the corner joints were cut and the stretcher was assembled. After the mid-19th century, stock was sometimes milled to create a raised bead on its outer edges. By 1882, the Pfleger Company in the United States patented the familiar stretcher profile with mitred mortise and tenon corner joints and a raised bead on both the front and reverse outer edges of the stretcher, thus making a symmetrical and reversible profile. This design was later produced by the Anco Company, Bay State Stretchers, and Fredrix, among other American manufacturers. It remained in several variations as one of the most popular stretcher profiles in 20th century America for small to medium-sized paintings and is still in wide use today.

Despite these developments in beveled profiles in 19th century stretcher manufacture, it is common to see many rectangular profiles with no raised beads or bevels on strainers and stretchers from the 19th and 20th centuries. Homemade stretchers and strainers for large contemporary paintings and some auxiliary supports for folk paintings of both the 19th and 20th centuries may exhibit such profiles. Lightweight rectangular strainers with pinned corners and a rectangular profile are frequently seen on small French paintings of the late 19th and early 20th centuries.²

Mark Bockrath

Submitted October 2006

When to Use a Cross-Member

When a painting is larger than 30 to 36 inches in its longest direction, it may become necessary to use a cross-member in the construction of its stretcher, regardless of the stretcher design and member profile, unless the members themselves are very heavy. This is necessary due to the possibility of the stretcher twisting out of plane from the stress of the stretched fabric on its corner joints. The stretcher members may also warp due to the tension of the fabric pulling inward at the centers of the members if no cross-bracing is utilized. The problem of stretcher twisting is, of course, greatly exacerbated by a flimsy, lightweight stretcher profile, weak corner joints, or a very strongly tensioned, densely woven support fabric. Increasing the tension on the fabric by keying out the corners can also cause the stretcher to torque as its corner joints weaken when they are opened up by keying.

The cross-member should be placed perpendicular to the long member of the stretcher, halfway across the member so as to bisect it. Placing the cross-member in this position greatly strengthens the stretcher, and on larger stretchers, multiple cross-members provide greater strength. If a single cross-member does not adequately strengthen the stretcher, a great increase in stability can be achieved by adding another cross-member that is perpendicular to and bisects the first cross-member. The cross-members are half-lapped together at their juncture; the joint here becomes extremely strong when it is glued and screwed together. On very large stretchers, it may be necessary to add several more cross-members in each direction, with more cross-members running parallel to the shorter side of the stretcher if it is considerably longer in one direction than the other.

In addition to providing resistance to twisting of the stretcher, cross-members provide greater ease in evenly tensioning the support when they are themselves keyable. A common way to provide for keyability of the cross-members is to make each of the joints of cross-members keyable to outer members, with mortise and tenon joints, for example. This allows for very precise keying of several parts of the painting. Keying out a cross-member can more efficiently tension the center of the painting, thereby reducing slackness and bulging of the support here than if corner joints only were expanded. Thus, only a slight opening of the cross-member joint can add more tension to the center of the support than a more exaggerated opening of the corner joints. This is because the center of the painting is farther away from the corners than the more centrally located cross-members. Indeed, in many cases, the corner joints would need to be opened so much that they would weaken or break before adequate tension could be provided for the center of the painting if no cross-members are present. Multiple cross-members can allow such precise keying that localized slackness or bulging in the support can often be eliminated with a great degree of specificity and control, with the ultimate goal of having all regions of the support in approximately even tension.

If the cross-member is not joined to the outer member with a mortise and tenon or other joint, a butt-joined cross-member can provide more strength to the stretcher if it incorporates dowels or screws through the stretcher and into the cross-member or is secured over the butt-joint on the stretcher reverse with mending plates and screws. Strength also increases as the thickness of the cross-members approaches that of the outer members. These methods of adding a butt-joined cross-member do not result in a keyable mechanism and therefore provide strength rather than the ability to adjust fabric tension. They can frequently be added to a stretcher with very little work and may flatten a twisted stretcher or strengthen a flimsy one.

The stock used for a cross-member should be shallower in depth than the stock used for the heavier outer members; when the reverse of the cross-member is flush with the reverse of the outer stretcher members, a gap between the cross-member and the reverse of the support fabric is thereby provided. This gap helps prevent the cross-member from pressing against the reverse of the support, with subsequent formation of stretcher creases and lines of cracked paint over the edges of the cross-member.

More substantial stretchers that are commonly used as replacement or new stretchers by conservators vary somewhat in the design of their cross-members, owing to the requirements of their different stock profiles, corner joinery, and keying mechanisms (personal communications, 2000). Firms that manufacture these stretchers vary in their estimate of how long a stretcher member can get before it requires a cross-member. Depending on the stretcher design and the weight of the stock used, this measurement may vary between 24 and 40 inches, with 30 to 36 inches being typical. Expansion-bolt stretchers employ cross-members that are attached to the outer stretcher members with stainless steel dowels. The same expansion-bolt mechanism used in the corners of the stretcher can be used for the cross-member joints. If multiple cross-members are perpendicular to each other, they are joined with half-lap joints. ICA spring stretcher designs employ the use of a cross-member when the stretcher exceeds 35 inches in length. The cross-members include aluminum tubing and redwood stock in combination, with the tubing running through the redwood stock at a right angle. The redwood stock is used to make the longer cross-member. Both cross-members are attached to the outer members with threaded bolts; the joints are expanded by means of springs that are tensioned by tightening nuts along the threaded bolt, repeating the construction of the corner joint mechanisms. More traditional stretcher designs with simple or mitered mortise and tenon corner joints utilize simple mortise and tenon joints to join the cross-members to the outer members. The cross-members are half-lapped together as necessary, and all joints are expandable with wooden keys.

Mark Bockrath

Submitted October 2006

Choice of Materials Used for Stretchers/Strainers

There are many factors to consider when choosing the material for a stretcher or strainer. The material should be lightweight, durable, and dimensionally stable relative to the painting, as well as affordable, readily available, and easy to work with. These qualities can be found in the many types of materials from which stretchers have been made, such as wood, metal, and plastic.

Materials

Wood

It is not surprising that wood has traditionally been used for stretchers and strainers, since it can be lightweight, durable, relatively stable (depending on wood species and cut), inexpensive, and easy to obtain. Although the history of stretchers and strainers has not been well documented, it is possible that the cheapest wood available or scrap pieces were used to construct the earliest auxiliary supports (ICOM 1960, 150). During the 19th century, it is documented that the cost of making stretchers was important to their manufacture. Patents from the 19th century demonstrate this concern: E. H. Collins's patent states his goal was “to furnish a device for stretching prepared canvas for artists' use, in the most simple and economic manner,” and A. Stempel's patent states, “the cost of manufacture is also lessened” (Katlan 1992, 33). The most common type of wood used for stretchers is from the conifer family. Some of the earliest examples of strainers that have survived are made from simple, unfinished, pinewood frames. Mantegna's Presentation in the Temple (c. 1460) has a simple pinewood frame with a panel inserted at the back. The canvas is nailed to the front (ICOM 1960, 151). In America during the 19th century, many artist suppliers, such as A. H. Abbott and Company, advertised “Pine Strips for Stretchers” in 1900 (Katlan 1992, 311) or stretchers “made in our Factory from well seasoned clear pine,” as proclaimed in an 1891 trade catalog from Consolidated Copying Company, Inc., Chicago, Illinois (Katlan 1992, 317).

When conservators search for a stretcher, wood is the most likely choice of material for reasons beyond the financial. A stretcher made of wood that is lightweight, strong, and does not warp is desired to provide the best support. A survey of various contemporary stretcher manufacturers yielded results about the types of wood used to make replacement stretchers. In general, a fine, even-grained, defect-free wood that was kiln-dried is the standard. The kiln drying allows the atmosphere to be controlled so the drying of the green wood is slow and even, minimizing the possibilities of warping and checking (Hoadley 1980, 72). The specific type of cut used to harvest the timber from the rough tree is not described in the literature. Radially cut wood is ideal since its dimensional change is less than that of tangentially cut wood (Hoadley 1980). Both softwoods and hardwoods are used to make the stretcher members, including pine, cedar, redwood, poplar, and basswood. These types of wood are chosen for their low tendency to warp. Sugar pine, white pine, cedars, and poplar all have a low tendency to warp during seasoning, and Douglas fir, other pines, and basswood have an intermediate tendency to warp during seasoning (Hoadley 1980, 80). Since many of these woods have very similar characteristics and some are easier to obtain due to their natural habitat, there does not appear to be one preferred species of wood for making stretchers. As an example, the Tate Gallery uses stretchers made from a softwood, Douglas fir, but they also recommend sugar pine from North America, and the keys are made from beech, a strong, dense hardwood (Booth 1989, 36). Keys can be made from any dense wood, including cherry and maple.

See the appendix that follows for further information on specific softwoods and hardwoods.

Metal

There are several important factors to consider when choosing whether or not to use a metal stretcher: the dimensional stability of the stretcher, its interaction with the environment, and the method of attachment of canvas to metal. The metal chosen to make the stretcher should be strong enough to support the weight of a stretched canvas of various dimensions so that the metal stretcher does not bend or sag under the weight of a large canvas. Fortunately, metal can be manufactured in sections that are perfectly straight and strong and can be fabricated in any desired length and width. Unlike wood, metal will not warp with changes in relative humidity (RH). What may be a concern with raised levels of RH is the possibility of a metal stretcher rusting; therefore, it is essential that the correct type of metal be chosen or that the metal be coated. How the canvas is attached to the stretcher may also affect how a metal stretcher can be used. Either the stretcher design must incorporate wood at the edges so the canvas can be tacked or stapled, or the stretcher needs a specially designed attachment, such as a clip. Another factor to consider is the aesthetics of a painting stretched on a metal stretcher. Since metal is not a traditional stretcher material, a 17th-century painting on a metal stretcher may not be aesthetically pleasing to conservators, museums, or clients.

Research has brought to light three types of metal stretchers. The first is a historical design from 1941 made of Monel metal or steel; the second and third are both currently available stretchers made of aluminum: a stretcher designed and produced by Franco Rigamonti and the Starofix Stretcher.

Monal metal/steel

F. C. Osborn's design (patent #2,244,473, June 3, 1941) was called a stretching frame. The structure consisted of two stretchers/frames that allowed the canvas to be stretched without permanently fastening it to the stretcher/frame. The outer frame would preferably be made of metal, such as Monel metal (an acid-resisting alloy of nickel, copper, iron, manganese, carbon, and silicon) or steel. The inner frame could be metal or wood. The structure would be light and strong, the same qualities desired in wood stretchers (Katlan 1992, 200–202).

Aluminum

The first aluminum stretcher with automatic tension was designed and produced by Franco Rigamonti in 1966. The stretcher is made of extruded aluminum channels that are mitered at the corners and held together by aluminum joints. The joints have screws and helical springs mounted on each end that can be tightened through a slot on the back of the stretcher. The canvas is attached with specially fabricated metallic canvas holders that fit into a channel on the reverse of the stretcher. The specific type of aluminum was not found in the literature, but it is likely an anodized aluminum to prevent corrosion.

The Starofix Stretcher, produced by Starofix North America and developed in 1984, is a continuous tension stretcher and is made of aluminum (specifically an alloy temper 6063-T5 anodic coated 20 mym). This anodized aluminum is corrosion resistant. To solve the problem of attaching the canvas, strips of wood have been fastened to the outside edge allowing the canvas to be tacked or stapled in place. This aluminum stretcher is lightweight and strong, not needing a crossbar at dimensions up to 72" x 72". This makes the stretcher much lighter than wood by comparison, and it does not distort, bend, or sag (Starofix North America brochure).

See section III.B.4.a for further information on these aluminum stretchers.

Plastic

Research to date has uncovered only one source for a stretcher made of plastic. Craft Cut Products of Santa Fe, New Mexico, introduced the WONDERBAR™ in the late 1990s, although the company has since discontinued production. The stretcher was made from a recycled, extruded, composite material that resembles sawdust or wood pulp in a hardened plastic matrix. This recycled wood material was advertised as having no moisture absorption and no toxic emissions and that the stretcher members would not bend, twist, or warp. The stretcher members were cut at the desired angle and assembled with plastic corner inserts that fit into a cavity (molded into the stretcher member) and that hold the stretcher members square. A second plastic mechanism can be added at the corners to allow for keying out of the canvas, one plane at a time. The WONDERBAR™ was a heavy-duty stretcher bar and was quite heavy in weight, especially if assembled into large stretchers. The manufacturers cited weight as one of the reasons for discontinuing it. Aesthetically, this stretcher was the color of wood, being made from wood pulp, but it did have a plastic appearance. As stated earlier, a modern-looking stretcher may not be visually acceptable on some types of paintings (or to some people).

Weight Considerations

The dimensions of a painting determine the ultimate size of the stretcher, so the larger the stretcher, the more weight is added to the stretched painting. A material that is lightweight is desirable when stretching large canvases so that they can be safely handled and mounted on the wall. Wood has proven to be a versatile stretcher material. Some woods, such as those listed below, are light enough that large stretchers can be made and handled. The introduction of metal stretchers has made larger stretchers even more lightweight and easier to handle and mount. The plastic WONDERBAR™ proved to be quite heavy and was only suitable for smaller dimensions.

Alexis Miller

Submitted November 2006

Choice of Materials Used for Keys and Other Joint-Adjusting Mechanisms

Introduction

The purpose of the stretcher key is to control the expansion of the corner joints of a stretcher (Chase and Hutt 1972, 13) in order to adjust the tension of the fabric mounted on the stretcher. Not surprisingly, information on stretcher keys is scarce: little attention has been paid to keys historically, and research on them has always been hampered by the difficulty of determining whether they are original to a painting and/or to its stretcher (see section III.A.2).

Background Information

Stretcher keys made their appearance with the advent of keyable stretchers, first mentioned in France in the mid-18th century (Quandt 1971, 349-350). (There is some debate as to whether expandable stretchers may have appeared as early as the late 17th century Katlan 1999–2000). These stretchers probably developed out of the 17th-century tradition of lacing canvases for paintings to strainers to apply tension to them by tightening the laces (Katlan 1992, 20). Leslie Carlyle mentions prepared canvas available on “plain or wedged frames” in the c. 1842 Winsor & Newton catalog (Carlyle 2001, 186), which implies expandable “frames” were well established by then. Katlan confirms that the term “stretcher” was probably an Americanization of the British “stretcher frame” or “canvas frame” (Katlan 1992, 24). In America, keys first appear in the late 18th century when keyable wooden stretchers of several kinds were being used concurrently with strainers. Single-keyed, wooden stretchers seem to be most prevalent on “naïve” paintings of the early 19th century, and they disappear by the 1850s in America (Katlan 1999-2000). Whether the single-keyed stretcher is a primitive form of the double-keyed stretcher and concurrent with it or is simply the earliest form of the expandable stretcher needs more research. In the 19th century, American inventiveness produced a plethora of designs to improve the traditional single- and double-keyed wooden stretcher, as a review of the patents dating 1849 through 1947 in Alexander Katlan's 1992 book reveals. These patents include many variations on stretching systems and many expandable corners with or without metal additions and with or without single or double keys as an adjunct to the systems.

A discussion of the materials used for keys is rarely part of a discussion of how wooden stretchers are made. A review of the stretcher-keying patents in Katlan's book reveals that none of the patents mention what woods had been or should be used for wooden stretcher keys (Katlan 1992, 16–260). Katlan suggests this was most probably intentionally left out to give the patent its broadest application (Katlan 1999–2000). A brief review of historical artists' manuals also finds no mention of the type of wood from which keys should be made.

Of interest is that the term “wedge” has been used interchangeably with “key” from early times in America and possibly also in Britain, although “key” is the more common term now in both Britain and America. The earliest American patents listed by Katlan use both terms, for example: “and the wedges, or keys, by which the canvas is to be stretched” from an 1849 patent (Katlan 1992, 68) and “avoiding the use of keys or wedges” from an 1866 patent (Katlan 1992, 72). Later in the 19th century, “wedge” predominates, although in an 1893 patent for an improved stretcher “key” appears again interchangeably with “wedge”: “Figure 1 is a perspective of my improved key…; Fig. 2 a plan view of the same showing the wedge B in section” (Katlan 1992, 152). Even today, both terms can appear: in a website ad for Upper Canada Stretchers® appears “keys, wedges are used…to expand the frame in very small increments…” (www.ucsart.com/ourstretchers.html).

A review of the historic American patents reveals the same complaints about wooden keys at that time as exist for conservators now. For example, the Shattuck patent of 1885 states, “These wedges…were very inconvenient in use, as they were quite likely to split in driving, and also to shrink and fall out in a short time” (Katlan 1992, 110).

Materials

Generally all recent sources agree that wooden keys should be made of some sort of hardwood. This is for the ease of cutting hardwood and for its ability to take hammer taps without splitting. Beechwood is recommended by Booth in his article on stretcher construction at the Tate Gallery (Booth 1989, 36); Wingrill, stretcher makers of Ontario, Canada, makes its Conservator's Stretcher's keys of maple (Wingrill 1999). Edgar Kuschan, a stretcher maker in the Washington D.C. area, likes hardwoods such as cherry and walnut because of their attractive appearance and often uses scraps from around his shop (Kuschan 1999). Simon Liu, maker of painting supports in New York City, generally uses good quality poplar, kiln dried, because the wood is of medium hardness. If the wood is too hard, it can be difficult to cut, and possibly the key would be difficult to insert into the stretcher. If the key needs to be very thin, Liu may choose a harder wood such as maple or cherry for strength (Liu 1999).

Alternatives to wooden keys for traditional stretchers have been available for some years, most noticeable recently in the form of plastic keys. These are usually disapproved of by conservators for their lack of aesthetic appeal, lack of similarity to the original, and tendency to slide out of their grooves. Information from one American manufacturer of artists' materials, Tara-Fredrix, formerly the E. H. Friedrich Co., indicated that they started producing plastic stretcher keys (which they call “pegs”) in the early 1970s when first manufacturing plastic stretcher members. The plastic keys could be provided in the form of a T-F (Tara-Fredrix brand) “peg tree” in which eight keys are attached by a thin peg to each side of a thin post and can be broken off for use. This avoided the use of the small polybags containing eight wooden keys that would be included in the back of a shrink-wrapped, pre-stretched canvas on a wooden stretcher (Tara-Fredrix 1999).

However, plexiglass keys (see section IV.L) have been proposed by a conservator to solve the problem of replacing very thin wooden keys (less than 3/32" thick). 1/16" thick plexiglass was used to create the keys and the edges were roughened with coarse sandpaper for tooth, although the recommendations were also made for dipping the keys in Lascaux™ Acrylic Adhesive 360HV before inserting them or tying them in place with nylon monofilament cord anchored to the corner of the stretcher to hold them in place (Cockerline 1991).

I could find no evidence of the use of metal to fabricate the traditional triangular keys. Most of the various metal joint-adjusting mechanisms that have been proposed over the last two centuries to deal with the shortcomings of stretchers adjustable only with wooden keys were integral parts of new stretcher designs, and most of them are no longer in use (see the American ones in the patents listed in Katlan's book). Information about those metal joint-adjusting mechanisms still in use can be found in other entries describing stretcher designs. However, some of the historic metal pieces are described as keys by their inventors or in the trade catalogs advertising them (“My invention consists of a metallic plate and wedge combined, forming a stretcher key to be applied to the miter-joints of frames used by artists” as stated by Aaron D. Shattuck in his February 13, 1883 patent). Refer to the patent designs in Katlan's book for further examples of metal corner key replacement systems (Katlan 1992).

One modern metal corner-adjusting mechanism that is presently available as a separate item attachable to any inner stretcher corner to expand it is a “tensor-bolt,” brought to my attention by Jay Krueger, Senior Conservator of Modern Paintings at the National Gallery in Washington, D.C. His example, available from a German firm, is a small, brass 4-piece item with an adjusting bolt. It can be screwed to the inner edges of a corner joint of a stretcher, then expanded by turning the bolt (www.deffner-johann.de/). A discussion on independent stretcher expansion bolts took place on the conservation DistList in October-November 1999 during an exchange on the availability of spring stretchers, and it was clear that such items could be, and probably have been, manufactured by independent machine shop workers for artists and conservators.

Form of the Key

The most thorough recent discussion on making wooden keys is in the 1989 article “Stretcher

Design: Problems and Solutions” by Peter Booth of the Tate Gallery:

The chosen angle for the keys will vary between about 14° for heavier stretchers and about 19° for smaller stretchers. The former angle will result from a pair of keys being cut from a slat of Beech with a length to width ratio of 4:1, and the 19° angled key from a slat of 3:1 ratio. Most Tate stretchers bear keys of about 16° cut from slats with a 3 1/2 :1 proportion. If the key angle is made significantly greater than 20° the act of tapping in a key will become less selective with an increased risk of both the joint members being forced away from the canvas centre. Care is taken to ensure that the key is of the same gauge as the key slot to minimize the risks of stretcher splitting. To reduce damage to the key, the key is cut with the key grain running parallel with the receiving angle of the key slot, which in turn will have been cut to the same angle as the key. (Booth 1989, 36)

Most agree that the average key angle should be around 16°. Edgar Kuschan also mentions the importance of the key's fit in its slot: it needs to be precise. If the key is too wide, the stretcher can split; if too small, the key will fall out (Kuschan 1999). All agree that at least the lower edge of the key should be roughened to prevent the key from sliding too easily out of its slot; some simply leave the key edges rough from the original saw cut. Wingrill makes keys with the long edge serrated to seat it better inside the key mortise (Wingrill 1999). Many conservation stretcher makers also recommend rounding or otherwise reducing the sharpness of the keys' corners to reduce the danger of poking holes in the canvas if the key falls between the canvas and stretcher.

Placement of Key in Stretcher

There is some disagreement as to the angle at which the key should be inserted into the stretcher. If the key has been cut with the grain parallel to the shorter of its two long sides, as is usual, the majority of those asked say the key should be inserted into the opening sitting on its longest edge. This requires that the hammer be held at a slight angle to tap the key into place, also recommended to avoid splitting the key. However, some recommend inserting the key sitting on the shorter of its two long edges. Thus, the short edge to be tapped by the hammer is vertically oriented. This enables the hammer to be slid along the inside stretcher edge to tap the key into place, allowing more control of the hammer and reducing the chance that it will hit the canvas prior to hitting the key. All recommend inserting a thin, light protective sheet behind the key, between the fabric and inside stretcher face prior to tapping with the hammer. This can be a piece of cardboard, ragboard, plexiglass, metal plate, etc. This protective sheet avoids contact by the hammer with the canvas, which can produce bulges in the canvas or characteristic crackle in the paint on the front. Simon Liu recommended using a spreader clamp to insert the key in the slot rather than hammering it in, thereby avoiding the problems of vibration and potential damage to the painting (Liu 1999). Techniques for preventing the key's slipping out are covered in another entry (see section IV.K).

Sarah L. Fisher

Submitted October 2006

References

Booth, Peter. 1989. Stretcher design: Problems and solutions. The Conservator 13:31–40.

Bramwell, M., ed. 1982. The international book of wood. London: Artists House.

Buck, Richard D. 1972. Stretcher design, a brief preliminary survey. In American artists' materials, Vol. II. A guide to stretchers, panels, millboards, and stencil marks. 1992. By A.W. Katlan. Madison, CT: Sound View Press.

Carlyle, Leslie. 2001. The artist's assistant: Oil painting instruction manuals and handbooks in Britain, 1800–1900, with reference to selected eighteenth-century sources. London: Archetype Publications.

Chase, William T. and Jeremy R. Hutt. 1972. Aaron Draper Shattuck's patent stretcher key. Studies in Conservation 17(1):12–29.

Cockerline, Neil. 1991. Plexiglass stretcher keys. Painting specialty group postprints. Albuquerque, New Mexico, June 8:56.

Conversations with painting conservators at the National Gallery of Art 1999–2004.

Edlin, H. L. 1969. What wood is that? A manual of wood identification. New York: Viking Press.

Gettens, R. J., and G. L. Stout. 1966. Painting materials: A short encyclopedia. New York: Dover Publications, Inc.

Grimm, C. 1998. Personal communication with author.

Hoadley, R. B. 1980. Understanding wood: A craftsman's guide to wood technology. Newtown, CT: The Taunton Press, Inc.

International Council of Museums (ICOM). 1960. Report on fabric paint supports. Museum 13(3):135–171.

Katlan, Alexander W. 1992. American artists' materials, Vol. II. A guide to stretchers, panels, millboards, and stencil marks. Madison, CT: Sound View Press.

Kuschan, Edgar. 1999. Telephone conversation with author. 24 March.

Liu, Simon. 1999. Telephone conversation with Barbara Buckley. 5 March.

—, 1999–2000. Personal communication with Sarah Fisher. December-January.

Quandt, Eleanor S. 1971. Technical examination of 18th century paintings. American painting to 1776: A reappraisal. Seventeenth Annual Winterthur Conference. Charlottesville: University Press of Virginia.

Straub, Rolf E. 1965. Spannvorrichtungen für das Leinwandbild. Nachrictenblatt der Denkmalpflege in Baden-Würtem-berg. 8(3):75–77.

Tara-Fredrix. 1999. Telephone conversation between representative and author.

Telephone conversations with manufacturer's representatives from the following firms: Archival Art Services, Washington, D.C.; Conservator's Stretchers by Wingrill, Ltd., Brighton, Ontario; and Simon Liu, Inc., Brooklyn, NY, February, 2000.

Von Sonnenburg, H. 1979. Rubens' Bildaufbau und Technik, I, Bildtrager, Grundierung und Vorskizzierung. Maltechnik Restauro 85:77–100.

Wingrill. Information pamphlets “Conservator's Stretcher,” examinations of sample stretchers. Mailed to author in 1999.

Further Reading

Verougstraete-Marcq, H. and R. Van Schoute. 1986. Painting technique: Supports and frames. PACT: Art History and Laboratory Scientific Examination of Easel Painting 3:13–34.

Appendix

The following appendix is not a comprehensive list of all woods used to make stretchers and keys. The ones listed below are those that conservators use or encounter most frequently.

Softwoods

Cedar is exceptionally light in weight when seasoned, but the timber is also strong for its weight. The straight grain of the wood makes cedar very stable, and it has a good surface finish (Edlin 1969, 110). Cedar is also resistant to termite and fungus attack (Bramwell 1982, 235).

Douglas fir has a fine texture. It is soft, easy to work, and lightweight (Edlin 1969, 116). It is strong and moderately resistant to decay (Bramwell 1982, 264).

Pine has no pores or vessels in its internal structure, so the wood from a pine tree is even-grained. The timber is light, soft, easy to work, yet strong. Since knots usually occur in the tree in groups, with knot-free timber between each group, it is possible to harvest defect-free timber in long lengths (Edlin 1969, 144).

Redwood, also known as Scots pine, is of medium weight and dries well, resulting in timber that is stable in use. It is a strong wood, although it is not resistant to decay (Bramwell 1982, 264). At the time of research, this wood was used for ICA Spring Stretchers; however, at the time of publication, the ICA Spring Stretcher is no longer in production.

Hardwoods

Basswood is also known as lime. The wood from the linden tree is called basswood in America and lime in Europe. In this wood the pores are fine and diffuse, and there is no distinction between the heartwood and the sapwood, making the timber smooth and even-grained. The wood is soft and lightweight. Once seasoned, basswood is very stable and free from both warping and shrinkage (Edlin 1969, 127). Basswood is not a very strong wood, but it has excellent working properties (Bramwell 1982, 259).

Beech was not mentioned in the literature as a wood for stretcher members, but the hard, strong, and heavy wood is used to make keys for stretchers. The even growth and small pores of beech make it a workable wood, and it has a smooth surface that resists wear (Edlin 1969, 99).

Cherry is a fine-textured and generally straight-grained wood. European cherry is slightly heavier in weight than American cherry. Once seasoned, cherry is moderately stable in use. It can be worked easily and gives an excellent finish (Bramwell 1982, 252). This wood is often used for keys.

Maple does not readily warp or shrink and is a very hard wood. The fine grain makes it easily worked. Maple timber (sugar or hard maple) is more commonly used in America (Gettens and Stout 1966, 240). This wood is not often used for stretchers, but it may be used for keys.

Poplar is soft, fine-grained, and fine-textured. The timber is light in weight and has a low tendency to warp due to its even grain (Gettens and Stout 1966, 254). It will decay in adverse conditions, but is a “tough [wood] for its weight and does not easily split or splinter” (Bramwell 1982, 252).