Volume 14, Number 3, Sept 1992, pp.28-29, illustrations

Mounts for Oversized Flat Textiles: Paper Honeycomb vs. HDPE

In summer 1991, we had something rather rare happen in Colorado: heavy rains. After one such rainstorm, on a Friday evening after closing, the roof of the Rocky Mountain Conservation Center leaked. Although the roof did not leak directly over any artifacts, a puddle on the floor caused the humidity in the lab to spike from 55% to 75%, staying at 75% for 3 days before the situation was discovered.

At the time, we had in the lab two large flags mounted on paper honeycomb panels. For each, an attached plexiglas cover, or vitrine, protected the side on which the flag was mounted. When we checked the panels for damage after the roof leak, we saw that they had distorted due to the humidity fluctuations.

After the flags were removed from the honeycomb panels, we discovered that the fronts of the panels were actually more damaged than the backs, which had been directly exposed to the high-humidity environment. The panels had "self-destructed" from shrinkage and expansion, combined with adhesive failure within their corrugated structure.

There are, in fact, numerous problems with acid-free paper honeycomb panels as support for oversize flat textiles:

• They are sensitive to sudden changes in humidity. The outer boards may expand or shrink, pulling away from the internal honeycomb structure. Worse, shrinkage and expansion may take place in the same panel, causing it to delaminate and stress the artifact.
• Paper becomes acidic with time.
• Additional support is necessary, especially at the edges. Usually, wood inserts are attached inside the edges of the panel to prevent the edges from collapsing when the artifact is mounted. A wooden frame is attached to the back of the panel to help prevent bowing from fabric tension. The wooden frame also serves as an attachment point for the vitrine and for hanging.
• The panels cannot be directly joined together; they must be spliced by adhering a wood strip inside the panels. For further reinforcement of the joined panels, an aluminum strip is attached at both sides of the center seam with zinc screws.
• The panels are not puncture-resistant. One slip with a tool can cause irreparable damage to the panel. Similarly, after attachment of the heavy vitrine cover, leaning the panel and vitrine assembly against a hard object can cause damage to the panel simply from the total weight of the mount.
• It is very difficult to cut honeycomb panels so that the edges are square, and nearly impossible with just a mat knife and straightedge.
• They are expensive. Acid-free honeycomb panels cost about $75 each, and an oversized mount (over 46 inches wide) requires two panels.

Acknowledging these shortcomings, we decided to look for a replacement material.

To begin with, a custom stretcher of the type used by our paintings department would be fully adjustable, differing from the wooden frame used in conjunction with the honeycomb panels. This would enable us to fit the mount more closely into the vitrine. The problem then was to find a suitable barrier material. The solid backing is not for support, but serves as an environmental barrier. We considered Fomecor, but we were concerned about the eventual degradation of the foam filling.

Corrulite, a high-density polyethylene (HDPE), was considered next. However, several construction problems needed to be addressed:

1. There is no satisfactory way to attach backing fabric directly to the HDPE panels.
2. Special equipment is necessary to weld panels together.
3. HDPE panels have a slightly ridged surface, which might cause impressions in an artifact mounted in close contact.

Using the HDPE panel in conjunction with a wooden stretcher, however, solves all these problems.

1. The backing fabric can be attached directly to the back of the stretcher.
2. Since the HDPE panels are attached to the stretcher, not directly to each other, the seams do not have to be welded.
3. Due to the construction of the stretcher, the HDPE panel does not come in close contact with the artifact.

Upper: cross section of 1/8-in. Corrulite panel
Lower: cross section of 1/8-in. Coroplast panel

We contacted the Corrulite Company and determined that the 165# weight panels would be suitable for our purposes. Our only problem was the high per-panel cost due to shipping charges: four panels would cost $50 each. I found a local distributor, however, that supplies a similar product called Coroplast for about $9 per panel. Corrulite is a laminated board, while Coroplast is a profile extrusion. Both types seem to be equally strong.

High-density polyethylene panels have many advantages over paper honeycomb panels:

• The panels are not sensitive to humidity changes, as shown by experimentation.
• Polyethylene is inert and will not become acidic or degrade.
• A stretcher frame will provide the necessary additional support.
• The panels are puncture resistant, yet they can be cut very easily using a mat knife and straightedge.
• The panels are inexpensive.
• With proper mounting and storage methods, corrugation imprinting on the artifact can be avoided.
• HDPE panels are lighter weight than the paper honeycomb type.

Since HDPE is a non-hygroscopic material and the panel can be fitted tightly into the vitrine cover, there was concern that under certain conditions, condensation could form on the inside of the vitrine cover. An experiment was performed to simulate these conditions. A 16-inch x 22inch panel was constructed in the same manner as a full-sized one. Linen fabric was stretched over this panel and stapled to the back of the wooden stretcher using copper staples. This panel was fitted snugly inside its vitrine, leaving a total internal volume measuring 748 cubic inches. A humidity indicator strip was placed between the panel and the vitrine, and another was placed outside it. The vitrine and panel assembly was laid flat inside a humidity chamber on two braces to simulate the air flow in a display or storage situation. The ambient humidity at the outset of the experiment was 35%. Humidity was introduced into the chamber using two 1000-ml beakers of hot water, and the chamber was sealed. Within 12 hours, the humidity in the chamber had reached 100%, and while there was condensation on the walls and top of the chamber itself, there was none on or inside the vitrine. The humidity inside the vitrine at this time remained at 35%-40%. It took four days in a humidity of 100% to raise the humidity within the vitrine to 80%. The external humidity was kept at 100% for another four days, but the internal humidity never rose above 80%. Next, the top of the humidity chamber was removed and the external humidity dropped from 100% to 35% in a matter of minutes. It took six days for the vitrine's internal humidity to match the exterior level.

There was no condensation on the inside of the vitrine at any time during the experiment. The experiment showed that HDPE panels provide an effective water vapor barrier. During this test, neither the copper staples nor the zinc screws corroded, and the linen showed no corrugation imprints upon removal from the panel.

We have concluded that high density polyethylene panels offer many advantages over paper honeycomb panels in the construction of textile mounts. The HDPE panels provide a buffer to humidity changes, they are chemically inert, they require about half the time to construct, and they are less expensive. Even using the more expensive adjustable stretcher frame, the total cost for mount supplies using HDPE panels is about 75% of the cost of materials for a paper honeycomb panel mount.

This new method has been adopted by the Rocky Mountain Conservation Center for oversized textiles mounted in vitrines. As with all large mounts with plexiglas vitrines, the mount assemblage should be stored vertically to prevent stress fracture to the plexiglas sheet on the front.

Appendix: HDPE Panel Construction

Construction of recommended support for oversized flat textiles

1. Construct an adjustable wooden painting stretcher.
2. Lay the stretcher face up on a flat surface.
3. Place the vitrine over the stretcher. (The vitrine consists of a front pane and sides made of plexiglas.) Adjust the stretcher to fit snugly in the vitrine, allowing for thickness of the backing fabric. Remove the vitrine.
4. Measure the HDPE panel to fit inside the coved edges of the stretcher.
5. Cut the outside edges of the HDPE panel at a 45-degree angle to accommodate the coving. Cut sides that will abut other panel pieces at 90 degrees.
6. Attach the panel to the stretcher around the outside with copper staples placed about 12 to 14 inches apart. Repeat at inner braces. Stapling should not cause indentations in the panel.
7. Heat-attach 2-inch strips of Beva film to the reverse of the stretcher at the outer edges.
8. Turn the panel face-up.
9. Pin the backing fabric (artifact attached) at edges of panel, aligning the straight of grain. We recommend a fine-weave 7- to 9-oz. linen, washed to remove sizing.
10. Turn the panel face down and heat-attach the backing fabric to the panel back with the pre-attached Beva strips. Remove the pins.
11. Trim off excess backing fabric with a scalpel.
12. Staple the backing fabric to the frame at the back for additional stability. The staples should be approximately 4 inches apart.
13. Turn the panel face-up on a flat surface.
14. Place the vitrine over the panel.
15. Attach the panel to the vitrine with screws through pre- drilled holes in the sides of the vitrine.