BPG Encapsulation

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Polyester Encapsulation is a method of stabilizing paper-based objects by sandwiching the object between slightly larger inert films of polyester and sealing the edges together. In this method, the object is not directly altered in any way, unlike lamination. The encapsulation material, and the methods to seal edges, vary by equipment and desired treatment outcomes. The method is an innovative Housings option for paper-based materials, particularly for oversize and fragile objects, and does not need to be replaced or removed for viewing. However, it is not an universally appropriate treatment, particularly for objects with friable media (see also Media Problems, and Matting and Framing: Insecure Media).

The goal of this page is to share information, comments, and experiences about methods of polyester encapsulation. See also Encapsulating in the AIC Lexicon.

Contributors: Rachel Freeman, Katherine Kelly, Evan Knight, Bill Minter (via interview), Cher Schneider, please add your name here

Polyweld.JPGMapInBookInMylar.JPGUltrasonic Welder.JPGEncapsulated Poster.JPGLarge Encapsulated Poster.JPGPolyester Roll.JPG32512-ATR-ledger.jpgPolyester encapsulation minterf deacidification.jpgSpine-pocket-wrapper-01.jpeg

Copyright 2024. The AIC Wiki is a publication of the American Institute for Conservation (AIC). It is published as a convenience for the members of AIC. Publication does not endorse nor recommend any treatments, methods, or techniques described herein. Information on researching with and citing the wiki can be found on the Reference and Bibliography Protocols page.

Cite this page:

American Institute for Conservation (AIC). "BPG Encapsulation." AIC Wiki. June 13, 2024. https://www.conservation-wiki.com/wiki/BPG_Encapsulation.

Factors to Consider[edit | edit source]

There are a number of Factors to Consider prior to deciding to encapsulate an item. Treatment Context must be balanced with the manufacture and condition of the object itself (Object Characteristics), and the available resources (Materials and Equipment). Specific treatment applications may be altered or combined. Readers are encouraged to perform their own evaluation and research before putting to use any of the treatment recommendations and suggestions included in this chapter.

Treatment Context[edit | edit source]

Value of Object

  • Intellectual
  • Historic
  • Aesthetic

Use and Handling

  • Frequency
  • Exhibition

Object Characteristics[edit | edit source]

Primary Support

  • Material
  • Paper
  • Parchment
  • Other materials which benefit from the added support and protection of encapsulation, such as palm leaf or Papyrus

Non-Friable Media

  • Printed
  • Manuscript
  • Photograph

Weight and Size of Object

Condition of Object

  • Condition of object affects choice to encapsulate or choice of method.
  • Deacidification prior to encapsulation? Or include an alkaline reserve sheet with the encapsulated material? (See also related terms in CAMEO).
  • Difficulty of encapsulating non-flat objects.

Current Research[edit | edit source]

  • "The Role of Polyester Film Encapsulation - With and Without Prior Deacidification - a Study Using Long- term, Low-temperature Accelerated Aging" (Minter and Baty 2013)

Materials and Equipment[edit | edit source]

Material for Encapsulation[edit | edit source]

A roll of Mylar®

Mylar® (Mylar) (1950s) or Melinex® 516 (Melinex) (1950s) (Safety Data Sheet) are uncoated, translucent films made from Polyethylene terephthalate (PET), a thermoplastic polyester resin. They are often used to generically refer to encapsulation films, however, both names are registered trademarks owned by Dupont Tejjin Films. The true generic term for these materials are Polyester Film or Plastic Sheet. They are available in rolls or sheets.

  • Chemically Stable
  • Transparent
  • Adds Tensile Strength
  • Reduces Handling of Original Object
  • Range of Thicknesses. 2mil, 3mil, 4mil, 5mil, and others. Selecting the appropriate thickness of polyester for an object is essential for successful treatment.
  • Uniform Structure
  • Resists Mold/Mildew
  • Creates Environmental Barrier


Long-fibered paper can be welded or otherwise sealed to encapsulate paper-based objects.

Hollytex® is a spunbonded polyester sheet structured from continuous filament polyester fibers. Calendered for high tensile and tear strength. See Hollytex in the CAMEO Materials Database

  • Chemically Stable
  • Chemical Resistant
  • Air Permeable
  • Smooth
  • Not Sticking
  • Range of Thicknesses
  • Resists Mold/Mildew

Stabiltex In a 1994 BPGA article, the use of Stabiltex is described for encapsulating papyrus fragments (Stanley 1994).

Sealing Methods and Equipment[edit | edit source]

Ultrasonic Welding[edit | edit source]

Ultrasonic welding uses low-frequency energy pulses through a moving applicator-head to seal malleable materials together along a seam.

Welding head of open-throated machine

Ultrasonic Welding machines are used to effectively seal polyester, polypropylene, and other malleable materials of various thickness for library and archival materials. The energy and speed of the machine can be adjusted to ensure an effective weld. Machines designed by Bill Minter are typically 48 inches wide, with 40 inches of working daylight and an open-throat. However, models have changed over time, and each is unique. There are also about 4 or 5 welders that are 6 feet by 6 feet, with closed throats.

One of the adjustable parts of the machine is the distance the welding head is from the table. Older machines utilize manual replacement of shims that vary in thickness while newer machines use a pointer or indicator that can be directly adjusted.

Frequency, and type of frequency, can be adjusted as well.

Bill Minter is the designer and fabricator of most of the welding machines in use in libraries and archives. He has designed, constructed, and set-up ultrasonic welders since circa 1981. The first prototype machine was demonstrated to Peter Waters at the Library of Congress in 1981, who placed the first order. Over time, the Library of Congress purchased newer machines, and the Boston Public Library has since acquired this model, the first production welder. In total, there are over 200 machines in use in the United States and internationally. Although unverified, a welding machine is said to have been installed in a Buddhist monastery with manuscript materials on the top of a mountain in Tibet, circa 2005.


Heat[edit | edit source]

Most heat tools cannot not achieve temperatures high enough to fuse sheets of polyester film together, however the Polyweld Machine

Polyweld machine via ISU Website

can. It has three distinct disadvantages compared with the ultrasonic encapsulator: high heat has risks to objects; it can weld a seam only at the very edge of the film; and it has a rather limited width (60 cm).

Double-sided Tape[edit | edit source]

Double-sided tape encapsulation with adhesive creep

Pressure-sensitive double sided tape can be used to seal edges, however, there are significant risks to this method as the safety and longevity isn't equal to that of heat or ultrasonic welding. Preservation issues that arise from tape in encapsulation are:

  • Difficult to remove all air from encapsulation
  • Unsightly borders
  • Adhesive creep - Sometimes causing object to adhere to edge of tape and causing further damage
  • Object movement - Object may slide into tape during handling and become stuck at edges or corners, requiring careful intervention to release from adhesive mass
  • Tape encapsulation quickly fails when the adhesive becomes wet
  • Eventual adhesive failure

Sewn Edges[edit | edit source]

Treatment Variations[edit | edit source]

Oversize or Rolled Objects[edit | edit source]

Large objects are not always easy to encapsulate. Only a handful of welding machines have a working opening larger than 40 inches.

Large poster encapsulated and tacked, through encapsulation at margins, to wall.

Partial Welding[edit | edit source]

  • Spot Welding
  • L-sleeves

Applied Encapsulation in Book Conservation[edit | edit source]

Post Bindings of Encapsulated Leaves[edit | edit source]

Encapsulated book pages can be rebound into post bindings (Meier-James 1983), and Remay hinges can be created for folded encapsulations (Ruzicka 1983).

The Scrapbooks page discusses the use of encapsulation in rehousing. A blog post from Emory University describes an example of this application ([[#refSawyer2013|Sawyer 2013). Henry Hebert has described a treatment where a scrapbook was disbound, the pages placed in polyester L-sleeves, and bound into a post binding. The advantages of this treatment are its low-cost, its use of polyester when one does not have access to a polyester welder, and the ability to remove each page reasonably easily from the structure (Hebert 2011).

Encapsulation of Individual Leaves[edit | edit source]

Single pages of a book can be encapsulated and bound into a resewn book. This can be a good choice when that page is more fragile than the rest of the book.

Encapsulation of Covering Materials[edit | edit source]

The Book and Paper Gathering has published detailed instructions for "A Spine-Pocket Wrapper for Books with Detached Spines" made of Mylar. (Matsumaru and Signorello 2018). Spine-pocket-wrapper-01.jpeg Spine-pocket-wrapper-02.jpeg

Historical Techniques and Materials[edit | edit source]

Silking[edit | edit source]

Silking was a widely used method of treating paper materials prior to encapsulation (Krueger 2003).

Cellulose Acetate Lamination[edit | edit source]

Cellulose acetate lamination was a widely used method of treating paper materials prior to encapsulation (Krueger 2003).

Double-Stick Tape Sealed Polyester Encapsulations[edit | edit source]

The result of housing paper materials between Mylar sheets adhered with double-stick tape

Bibliography[edit | edit source]

CAMEO: Conservation of Art Materials Encyclopedia Online. Related terms: encapsulation, Mylar, deacidification, Alkalization, Alkaline reserve, Double Sided Tape.
Canadian Conservation Institute. 1995. "Encapsulation CCI-Notes 11/10." CCI Notes. Accessed March 24, 2020.
Hebert, Henry. 2011. "Sleeves and Posts: A Rehousing Option for Scrapbooks." Archival Products NEWS 16(4): 1-4.
Description of a treatment where a scrapbook was disbound, the pages placed in polyester L-sleeves, and bound into a post binding. The advantages of this treatment are its low-cost, its use of polyester when one does not have access to a polyester welder, and the ability to remove each page reasonably easily from the structure.
Krueger, Holly. 2003. "Magnesium Revisited" Book and Paper Group Annual 22.
In addition to discussing the use of magnesium in deacidification treatments, this article reviews the history of silking at the Library of Congress from 1900-1940, the history of lamination at the Library of Congress from the 1940s to the 1960s, and the reasons for their abandonment.
Matsumaru, Mito and Stefania Signorello. November 8, 2018. "A spine-pocket wrapper for books with detached spines." The Book and Paper Gathering. Accessed March 24, 2020.
Meier-James, Barbara. 1983. "Modifications of a Basic Polyester Post Binding." Book and Paper Group Annual 2. Accessed March 24, 2020.
Minter, William and John Baty. 2013. "The Role of Polyester Film Encapsulation - With and Without Prior Deacidification - a Study Using Long- term, Low-temperature Accelerated Aging." Research and Technical Studies Specialty Group Postprints 4. 186-239.
From a paper presented at the AIC Annual Meeting, 2013, in Indianapolis, IN. A summary of the presentation is available through AIC's blog, written by Laura Wahl.
Ruzicka, Glen. 1983. “Polyester Encapsulation in Signatures.” Book and Paper Group Annual 2. Accessed March 24, 2020.
Describes a method of creating a Remay hinge for folded encapsulations.
Sawyer, Suzanne. December 3, 2013 "Preserve & Protect: The Benefits of Polyester Encapsulation to preserve the John Larkin Smith (1882-1936) Scrapbook." Emory University MABRL blog post. Accessed March 24, 2020.
Stanley, Ted. 1994. "Papyrus Storage at Princeton University." Book and Paper Group Annual 13. Accessed March 24, 2020.
Describes the use of Stabiltex to encapsulate papyrus fragments.

External Links[edit | edit source]


Melinex® 516 Safety Data Sheet


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