Stretchers and Strainers: Addendum

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Cadmium-Containing Corrosion Products on ICA Stretcher Bars - Identification and Abatement

Adapted from the 2019 Paintings Specialty Group Postprints


Authors: Heidi Sobol and Aaron Shugar

Editor: Wendy Partridge

Introduction and Background

Sacrificial coating of mild steel springs has been performed for over a century. Coating is typically done by electroplating and both zinc and cadmium have been used for this purpose. Although zinc coating, or galvanization is more common today for utilitarian artifacts, cadmium has a long history of use in the military and automobile industries and more recently in aerospace and electronics industries. The benefit of cadmium as a coating agent for movable parts lies in its better dry lubricant properties. For this reason alone, the springs used in the stretchers may have been made from cadmium over zinc. Unfortunately, the health hazards associated with cadmium have made this benefit moot. Adding to this problem is the severe level of cadmium corrosion under exposure to acetic acid.1 Red cedar has a typical pH of ~ 3.45 and is considered a fairy high vapour corrosion hazard. In addition, if a backing board is employed, the microclimate surrounding the springs would increase the localized acid concentration. Early studies performed on both zinc and cadmium coatings on steel show that for normal atmospheric corrosion cadmium can deteriorate over twice as fast as zinc.2

Identification of ICA Spring Stretchers that have cadmium-containing corrosion products associated with the spring mechanism is a necessary step in determining the safe handling of all paintings suspected of possessing such a stretcher. It is possible that the springs may be either zinc or cadmium coated. Visually, they look similar, yet the proper identification of which metal is imperative as it changes the hazard risk from non-existent (zinc) to high (cadmium). Identifying a quick way of establishing the presence of cadmium will assist conservators in determining the need for abatement, or at the very least, isolating the contaminated painting from further handling. Considering that cadmium is ten times more toxic than lead (Rossol 2007, pers.comm.), the need to address safe handling of the stretcher components is paramount.

Figure 1. Detail of ICA Spring Stretcher screw and bolt system. Note that aluminum spline is inserted in a type of key-slot, meant to stabilize the corners.


ICA Stretcher

The Intermuseum Conservation Association (ICA) is the nation’s first non-profit regional art conservation center and is currently located in Cleveland, Ohio. From 1952 to 2003, the center was located adjacent to the Allen Memorial Art Museum in Oberlin Ohio. The ICA Spring Stretcher was conceived by Richard Buck, head conservator at the ICA in the early 1950s. The stretchers at the Royal Ontario Museum were fabricated by Glenn Hobbs, Jr., a self-employed contractor and Oberlin resident who fabricated the product known as “The ICA Spring Stretcher”. From the 1990s to 2003 they continued to be manufactured under the name “Superior Spring-Stretcher”. Promotional material showcasing the stretchers never mention any corrosion-inhibiting coatings. Stretchers with expandable corners have an inception date of the mid- eighteenth century as per the writings of Antoine-Joseph Pernety, author of Dictionnaire portratif de peinture, “wrote that keyed stretcher were a new invention” (Callen as quoted by Buckley in Stoner and Rushfield, 2012, 150). The constant tension stretcher is in the family of stretchers with expandable corners. The concept for the ICA stretcher was based on controlling the tension of the stretcher by adjusting the screws in the four corners of the stretcher or at the junction of the cross-member and stretcher member, if applicable (fig.1), whose premise was biaxial interdependent expansion.3 The corner mechanism consists of a stud with a spring, two washers and an adjusting nut. A corner spline and tubular angles are also employed in the mechanism. When installed and engaged, the mechanism applies a force parallel to the stretcher bars with the springs’ main purpose to create a self-regulating tension that reacts to changes in relative humidity. Concern about unnecessary expansion/contraction of the springs resulted in many conservators removing the springs from the stretchers (Albano 2007, pers. comm.).


Detection of Cadmium

The components of the tension mechanism of the ICA Spring Stretcher are aluminum and mild steel springs with a protective galvanic coating. Protective coatings for steel are common and metals with a lower standard Electrode potential such as zinc or cadmium are commonly used. The difference in Electrode potential offers a preferential corrosion of the coating metal, thus maintaining the structure of the coated metal. The result is ideally a slow corrosion of the coating material which often resulting in a visible corrosion product on the surface. The sacrificially coated springs of the ICA stretcher were likely not cadmium by design of ICA, but rather a common coating used for steel springs in another industry (i.e. automotive) and sourced from a third-party supplier. Hobbs probably requested a sacrificial coating be used, assuming it to be zinc – or expecting it to be zinc but ended up with cadmium instead; this may also suggest that some of the springs may not be cadmium-coated but the preferable zinc-coated. Since both zinc and cadmium create a ‘white’ metal coating, it would be difficult to tell the difference from appearance alone. At the ROM in late 2004, a whitish corrosion was observed on some ICA stretcher springs. Samples of the mechanisms were sent to the Canadian Conservation Institute who confirmed the presence of cadmium salts of fatty acids, with particle size as small as 0.5 ums (Duxin and Moffat 2005, 2). Although the cause of the corrosion formation is unknown, it is suspected that the presence of VOC’s in the stretcher wood in combination with varied humidity would instigate their formation (Scott and Derrick 2007, 62). Evolution of acidic gases from the paint layers or canvas may also contribute to the increased presence of VOC’s in vicinity of the cadmium-containing components.

Aside from visual inspection, subjective confirmation using ultraviolet irradiation can be employed in the detection of cadmium corrosion. As seen in Figure 2, the contrast between cadmium-coated and zinc-coated will quickly determine the presence of either type of spring. Using a standard Nikon D7100 DSLR camera with no additional lens filtration, the cadmium components on the spring fluoresce a bright orange while the zinc-coated spring shows no such fluorescence when illuminated with Wildfire UV lights. The zinc-coated spring is courtesy of an outdoor fence hinge from Home Depot and the presence of zinc was confirmed using XRF.4 Examination using UV-induced visible fluorescence, the cadmium-coated spring appears as an intense yellow-orange (fig. 3).5 Examination using visible induced IR luminescence (VILS) also indicates the presence of cadmium between 715 and 1000 nm which is a relatively wide band on the electromagnetic spectrum (fig. 4).6

Figure 2. Image of zinc and cadmium coated spring (f11, ¼ second exposure).
Figure 3. UVA, UVB and UVC images of varying excitation wavelengths.
Figure 4. X-Nite 715nm, 780nm, 850nm, 1000nm passing filters

A cross-section of the spring was examined using scanning electron microscopy.7 Areas of the metal core primarily consisted of low carbon steel. The coat layer of cadmium is about 5 – 10 um thick. Backscatter imaging shows the coating is corroded with a fractured and uneven surface with isolated enriched corrosion pockets, as seen in Figure 5. The coating thickness is within standard electrolytic deposition thickness for cadmium.

Figure 5. Electron Image #2. (cross-section showing metal core, cadmium-crust and surface/air interface).


Health Issues Concerning Cadmium-Containing Compounds

Heavy metals such as cadmium, lead, mercury and arsenic are environmentally present elements that can cause harm to living organisms. While some metals can be beneficial to humans, such as the micronutrient zinc, heavy metals can cause a variety of health problems and exacerbate pre-existing health problems. Cadmium is not readily found in large quantities in the environment and humans typically introduce cadmium into their systems through cigarette smoke inhalation. Cadmium can primarily enter the human body through three methods: inhalation, absorption through skin and ingestion. Once in the body, the liver tries to complex the cadmium in a variety of ways, but it typically ends up being stored in the liver or sent to the kidneys where it may also be stored. Though excretion of cadmium occurs, storage of cadmium in human organs can cause organ impairment, and in increase in co-morbid conditions such as high-blood pressure. Damage caused to organs such as the kidneys is irreversible. Cadmium can also cause impairment in the respiratory system, reproductive system and skeletal system.

Many of the studies involving cadmium-related illnesses contain cohorts that have occupational hazards: individuals who have very high exposure to cadmium-containing materials like rechargeable battery handling. These are observational studies only and are not causative.8 It is difficult to determine what the exposure rates are in terms of contact with cadmium-coated springs because it is unclear how much cadmium-containing products “come off” from the springs and whether or not it is airborne or merely available through physical contact. Thus, it is difficult to determine what the quantitative risk level is for the ICA stretcher-bar handler. HEPA filtration requires that ~99.97% of all materials large than 0.3um, so given that the cadmium particles were determined to be 0.5 ums in size (but could be smaller or larger), HEPA filtration should be sufficient in screening out the cadmium particles. To determine the presence and amount of cadmium in humans, blood and urine analyses can be undertaken. Accidental exposure through ingestion can be treated with emergency procedures but long-term, small dose exposure to cadmium has minimal if any options for treatment.

Abatement

Figure 6. Image of corner of stretcher: note thin aluminum spline at stretcher mid-point.

While removal of the springs would eliminate the source of the issue, the protocols needed to safely remove them are complex. Handling of the affected stretcher should always be done with nitrile or latex gloves. The initial triage for an ICA Spring Stretcher should be to bag and seal the entire painting (including frame, if applicable) with polyethylene plastic and wide tape. If taking the backing board off is not feasible, determine if you can see the tell-tale redwood stretcher and aluminum spline by looking at the side edge (fig. 6). Only in the short term, should a backing board with a warning label be secured to the verso stretcher – a sealed verso environment may create a microclimate, increasing the rate of corrosion of the cadmium coating. Visual examination and ultraviolet light examination can indicate the presence of cadmium-containing components, and p-XRF analysis can also be employed to verify its presence.

In 2008, the ROM’s Painting Conservation Department set out to remove and replace the stretchers that possessed an ICA Spring Stretcher. During a survey of 99 Paul Kane paintings, 18 ICA stretchers were identified. Using a standard fume hood with fresh air generated from behind the fume hood user, the paintings lab was modified to accommodate the safe removal and replacement. From set-up to completion, the entire abatement process took about three days. The workflow was as follows:

  1. Paintings identified to have the ICA stretcher were isolated and placed on a Metrocart (fig. 7).
  2. A large polyethylene tent was created in the “dirty zone” within the fume hood area. An adjacent area was created to allow for the conservator to change into and out of Tyvek suiting as well as removal/cleaning of PPE such as nitrile gloves and goggles. Contaminated disposable PPE was placed in a HAZMAT marked sterling bag, and “hard” PPE such as goggles as well as tools were washed thoroughly in the sink. Because cadmium disassociates in water, removal of the cadmium materials was achieved in this way.
  3. Once in the spray booth, the canvas was removed from the ICA stretcher and all stretcher components were placed in a HAZMAT marked sterling bag for disposal. The verso canvas was vacuumed using a Tiger-Vac with HEPA filter; the filter was disposed of as HAZMAT and the Tiger-Vac components were washed, as applicable, at the end of every day.
  4. The unstretched painting was passed through the Metro cart “window” to a technician, who then re-stretched the canvas onto a new, custom-made, mortise-and-tenon stretcher. Note that the technician wore nitrile gloves, a NIOSH half-face mask respirator with appropriate filtration and a Tyvek suit.
  5. All HAZMAT-labelled sterling bags were placed in a HAZMAT barrel for disposal with a registered third-part service.


Figure 7. Image of lab tent, showing draping of fume hood and staging of contaminated paintings.

Conclusions

While the number of ICA Spring Stretchers still in use is unknown, it can be assumed that many collections may possess affected stretchers circa the second half of the 20th century. The abatement procedure implemented by the ROM should be reviewed and updated to include a review of the appropriate fume hood filtration and monitoring of air flow so that the user is protected from the cadmium components. Due to the rarity of this type of corrosion product coupled with the lack of metrics in terms of exposure levels, it is difficult to determine safe exposure levels. Thus, we must establish a consensus on best practice for this issue and a call to action to make art collectors aware of the possible hazards of cadmium exposure. The first step should be a survey of all painting collections to determine the presence of the ICA spring stretcher(s). Exposing the verso stretcher components to ultraviolet illumination (while wearing UV-blocking goggles) will quickly identify the presence of a cadmium-coating on the exposed springs as they will fluoresce bright orange-yellow. Future study may include investigating tape that binds the cadmium-corrosion products temporarily or installing a temporary backing board so that abatement initiatives such as remove/replace can be more safely conducted at a future point in time. Also, a cleansing regime that would remove the bulk cadmium particles may be considered. However, at this time it is still considered safest to fully remove and replace the stretchers.


Notes

1 Guides to Practice in Corrosion Control: Corrosion of Metals by Wood
2 Hippensteel, C. L., and C. W. Borgmann. "Outdoor Atmospheric Corrosion of Zinc and Cadmium Electrodeposited Coatings on Iron and Steel." Transactions of the American Electrochemical Society 58.1 (1930): 23-41.
3 Tsang, J-S.pp. 91.
4 P-Xrf data was compiled using Bruker Tracer III-SD.
5 Images taken with 30 second exposures at f/32, ISO/250.
6 All images are captured with PowerSmith LED 5000 Lumen Work light with 6mm BG38 filter as the excitation source.
7 SEM – Vega3 Tescan, SEM magnification of 3.02kX, view field of 91.6uM.
8 Swedish. Åkesson et al. 2005, 2006. Thailand. Satarug et al. 2005.

Acknowledgements

The authors would like to thank the Department of Conservation at the Royal Ontario Museum and the staff of the Garman Art Conservation Department at the State University of New York College at Buffalo. Multimodal imaging provided by Ms. J.J. Chen, Associate Professor at the Garman Art Conservation Department at the State University of New York College at Buffalo. XRF compilation and interpretation at the ROM was completed by Branden Rizzuto and assistance during the 2008 abatement campaign was provided by the ROM Ethnology Department technician, Tracey Forester. Research concerning health effects of cadmium exposure was conducted and interpreted by Dr. Paul Sobol.

References

Albano, A. 2007. Personal Communication.

Buckley, B. 2012. “Stretchers, Tensioning and Attachments”. In Conservation of Easel Paintings, edited by J.H. Stoner and R. Rushfield. 2012. New York: Routledge.

Duxin, N. and E. Moffat. 2005. “Analysis of Cadmium-containing Deposits for Royal Ontario Museum Toronto, Ontario, Canada”. Ottawa, Ontario: Canadian Conservation Institute. Report No. ARL 4319.

Rossol, M. 2007. Personal Communication.

Scott D.A. and M.R. Derrick. 2007. “Deterioration of Cadmium-Coated Instruments in Museum Storage”. In Studies in Conservation 52 (1): 59-68.

Tsang, J-S. and I.M.C. Caldeira, D. Williams, R. Pelasara, R. Patterson. 2013. “Modernized Stretcher for Paintings on Canvas: Assessment and Observation”. In AIC Paintings Specialty Group Postprints 26. Washington, DC: AIC. 91-114.

Further Reading

Health & Safety: Heavy Metals, their Salts, and other Compounds. AIC Wiki. 2010.

Minault, D. “Spring Tension Stretchers”. In: PSG Stretchers and Strainers – III. Materials and Equipment, edited by B. Buckley. 2008: AIC.

Rossol, M. “The Artist’s Complete Health and Safety Guide”. 2001. New York: Allworth.

Sobol, P. 2019. Personal Communication.

Toxicologic Assessment of the Army’s Zinc Cadmium Sulfide Dispersion Tests. 1997. National Research Council (US) Subcommittee on Zinc Cadmium Sulfide. Washington (DC): National Academies Press (US).

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