Conservation Case Building

Introduction

Acidic environment inside an artifact case can accelerate the rate at which organic artifacts deteriorate.  From a preservation point of view, it is advantageous to reduce or remove any acid content from a case, and to minimize its air exchange rate.

There are two reasons most artifact cases are built from wood. It is inexpensive, and it easy to work. However wood is made up of cellulose tannic acids and lignin, as well as resins and formaldehydes. Wood inside a case must be sealed, and even then sealing does not stop the harmful acid from eventually getting out. Conservation requires either a non-permeable barrier such as Foil type seal, or multiple layers of sealant, but ultimately, wood should be removed entirely from the case.

Some of the most important problems involved in artifact preservation are light levels, ambient vibration, security, and humidity control, but the focus of this study is primarily on: the pursuit of a non-entropic environment through the removal of all wood products from case interiors, and through achieving an airtight seal.

There are many versions of sealed cases available on the market today. Most are cost prohibitive, as well as problematic in terms of lead-time and shipping.

While working here at the Field Museum, along with the help and knowledge of several dedicated people, we have developed specialized methods for creating sealed cases with simple tooling, and common materials, which can be achieved without a high degree of specialized skill. Jonathan Paul Brown has been assisting me with conducting tests, chronicling data and providing design suggestions, while I have been designing cases, formulating material usage, and developing building techniques.

The following paper discusses these techniques developed, and what we have discovered. It should also be noted that this draft is still a work in progress, and there will be more added, as time allows.

Synopsis

Here is how it’s done.

  • Laminate aluminum sheeting to particle-board.
  • Build the case with the particle-board facing in.
  • Integrate any form of deck to the box with metal channel, or laminated board.
  • Cap the case with either an acrylic bonnet, or glazed panels using gasket.
  • Utilize Camger Sancure product 118-1, first to seal your part edges. 
  • Then utilize Camger product 118-1, as a glue substitute for building your case.
  • Cap or glaze your openings using gasket.

The Concept

Our concept was simple, use metal instead of wood to build the case. Initially we welded a box, which served as desiccant chamber, to a platform or deck with air slots and then covered it using an acrylic bonnet, or glazed openings with glass. This approach worked well, but required welding equipment, as well as personnel skilled enough to use it.

The Laminated Box

It was originally suggested by my Shop Assistant, William Rollins, that we could create the same effect as welding a metal box by laminating sheet metal (preferably aluminum) to a piece of particle board, and then build the box with the table saw like we have always done. Theoretically, the metal laminate would be negligible in terms of cutting on a table saw. Most commercial decorative laminates are about .03 inches. The trick was to build with the laminate facing in, and then make sure the inside edges were sealed. This would, in effect, be the same as making a box of metal. 

Building a box from laminated particle-board, however, presented us with several problems. First, Any edges of the particle board exposed to the inside would need to be sealed, this can be done with iron on foil-seal or Acrylic sealant, and second, any gapping in the laminate would be need to be dealt with. Next, we needed to figure out a way to glue it together without allowing the glue to off gas in to the inside of the case, by way of the seams. Standard yellow wood glue is typically as bad as wood in terms of acid content. Silicone and silicone-acrylic caulk, can be used to seal the inside seams, but while conservation approved,  they are not ideal at providing enough holding power on wood products to guarantee the structural integrity of a case over long periods of time. Also, Silicone does not adhere well to aluminum, so we couldn’t just caulk from the inside without it eventually peeling off.  I originally solved this problem by creating a channel along the inner seams.

The Beveled Edge Box

Our preferred method of building was to bevel case walls so they would meet without external visible seams. 

Once a part was cut, it was an easy step to adjust the blade down and run a small channel perpendicular to the bevel.

Once joined the cuts create a channel along the inner seam. Next the case would be built by judiciously applying glue to the outer part of the edges, while trying not to get it in the channel part of the joint. After building, the inner channel is filled with silicone or silicone-acrylic caulk, creating an effective barrier along the inner joints. 

The channel stops off gassing of the glue into the case, and seals it. It also provides a mechanical advantage to keep the caulk adhered indefinitely. We tested other joints, and found the channel wasn’t required for a seal, as long as you were careful about the placement of the silicone and the glue.

This approach to case building proved to be very successful at accomplishing a seal, however, balancing the wood glue and the silicone-acrylic caulk could be tricky. 

Variations of Joinery

Creating a box by beveling the edges, can be difficult. The walls need to be held in position for joining, and this is sometimes problematic, especially on larger cases. After figuring out we could create the same seal as the beveled joint by lapping, we went through a series of testing different types of joints. Depending upon the final deployment of a case, joinery is not always seen.

For The Grainger Hall of Gems, for instance, the case was mounted inside the wall. I discovered that I could utilize laps and channels as guides wherever possible, to make the joinery easier. Walls that have a slot to drop into, are much easier to place and secure. The mechanical advantage helps the wall deploy, and stay, in its final position. During the Grainger Hall of Gems we left the floor and side walls of the case longer than they needed to be, and cut channels where the back wall fit. We cut a lap along the floor for the side walls. The resulting case fit together like a puzzle, and could be joined much easier.

 

We also tried using the caulk as the building adhesive so we wouldn’t have to deal with the wood glue. This proved problematic in several ways. On larger parts, the caulk would start to dry before you could get the wall into position. It was necessary to use water to slow the dry time. The caulk was messy and the results were cases that required masking, and cleaning after they dried. Finally, we needed to increase the holding power of the fasteners joining the sides. Since caulk doesn’t adhere well to wood, these cases needed screws to retain their integrity, instead of just nailing, or stapling.  In instances where nails were used, some of the case walls released after only a few weeks, and needed repair.

Adhesive

Perhaps the most exciting development is the Camger Gloss Waterbourne Finish, Product 1-175 commonly called “Sancure”.  As discussed before, wood inside a case must be sealed. Our Conservation Department recommends three layers of primer, three layers of paint. the Camger’s 1-175 is one of the products used to seal wood, It has proven effective at arresting the escape rate of acid into the air. Imagine the work though! As it happened, For Grainger Hall of Gems, we bought

1-175 for sealing case furniture, and ended up with three 5 gallon buckets left over. During our most recent build “Abott Hall of Conservation Restoring Earth,” it occurred to me that the 1-175 might work like glue. Here is what I discovered.

  • Camger 1-175  is a product very similar to acrylic medium used in painting.
  • Camger’s 1-175 works like wood glue.
  • Upon curing, it is water resilient and somewhat pliable with an excellent balance of strength and elasticity.
  • It can be thinned with water, so initial layers brushed on will wick into any pores, or gaps, creating an extensive and effective vapor barrier.
  • It adheres extremely well to wood, and reasonably well to acrylic, and fair to aluminum and other metals.
  • Camger 1-175  left out for moderate periods of time actually thickens itself to a gel without diminishing its integrity. This is useful for application while gluing parts.
  •  We ran some tests and it actually performed even better than we expected. We could not destroy or pull apart joints any easier than standard wood glued joints.
  • It could be cleaned off the laminate surfaces with water and moderate scrubbing without wrecking the finish.
  • Camger 1-175 can be used to seal joints  after building. It capillaries into missed areas or gaps.
  • 1-175 was less likely to throw off the fit of parts as dramatically as layers of paint, or primer, therefore less tolerance was needed for fitting parts.

 In short, Sancure is the ultimate adhesive, for building conservation cases.

Tested Joinery

Once it was realized Camger 1-175 could be used as a glue substitute, we tested as many joints as we could. We found not only did basic butt joints achieve a seal, but any form of lap did as well.  Seams that create longer paths, and have more surface area per join seem preferable, while it is most likely difficult for air to negotiate a longer, or less likely there will be potential gapping. Once air finds its way into any unsealed part of the board it can travel the pores and permiate through any part of the board face. By far the best method of joinery is to use a locking miter joint. Suggested by my current Shop Assistant Justen Kanthack, it is the best balance of cosmetic seam and structured, fitted mechanical joint.

Gasket and Glazing

How gasket is applied and the deployment of any doors and glazing will compromise the seal unless done correctly. Unless utilizing an acrylic bonnet, it is in you best interest to engineer how the glass will be situated. We have had moderate success with silicone, and have utilized glazing with aluminum h channel siliconed to the edges for easy removal.

Gasket must overlap at the corners or it will not seal. Using a scarf joint at the corner works well for this. At one point we even created a jig to cut gasket consistently. Gasket should be conservation approved, and should be durable, pliable and yet have resilience.

It is very likely that gasket will deteriorate over time and require replacement. So far we are unsure of its durability. Different brands will most likely have different practical life-spans and should be monitored. Currently we have cases built in 2004 that seem to be sustaining their resilience adequately. If there are rotations being performed with artifacts in any given case, perhaps it should be considered an opportunity to replace gasket.  Be cautious of any gasket damaged upon accessing the case, as well.

Openings and Doors

Any openings or doors need to be covered with similar material as the case walls, or other inert non-permeable material. Aluminum plate works nicely, but the laminated particle board works just as well. It is important all doors and openings have been sealed with conservation approved gasket to achieve a seal. Placement of screws or catches should exert pressure from all sides. So far we have tested the sealing capacity of screws spaced out to 12 inches with success. It is important that a door be secured evenly and not over tightened.

Other Concerns

As we solve the problems of sealing a case and removing harmful materials from the interiors, we have started to scrutinize other concerns that should be taken into consideration. Here is some helpful information on some of these developments, which I intend to expand as progress is made.

Ambient Vibration

Vibrations are everywhere. Most of the time they are too subtle for us to notice. In earthquake zones, it is readily apparent, but any building, in any location will have ambient vibrations that travel the structure constantly. Just because they cannot be felt, does not mean they are not there. There are impact waves from traffic, pedestrians and visitors, air conditioning-heating fans, motors and lift trucks. Even sound itself manifests in vibrations that ultimately travel through the building and dissipate by shaking artifacts! There are two important things to think about with vibration control. The first step in controlling vibrations is being able to identify the types of structures that encourage or magnify the effect. Just like a violin or guitar some configurations encourage vibrations. And secondly, once identified, it is important to understand how to not encourage them. Vibrations do not like changes in material density, it dissipates their energy as they travel from one material to the next. Shifts in density can also help waves cancel each other out. We have taken to putting thin layers of ethafoam between surfaces as we build. This helps dissipate waves that reach the case. Vibration can be mitigated by paying attention to the way a wall is structured. Many of us have played with making waves on a clothesline. Freestanding structures tethered on one end (the clothesline model) should be avoided. Delicate artifact cases should have support structures independent of the walls when possible. We insulate cases by putting layers of thin ethafoam between the case structure and the frame, frame and floor or case and wall.

STILL TO COME:
Ease of Access
Actual Builds and Case Studies
Materials Used and Suppliers

CONCLUSION

This undertaking of making better display cases is a constantly evolving process. The techniques we have developed, are simple inexpensive and can be employed by any Institution. It is important to develop and design exhibitions with the concepts put forth here for a proactive approach to building display cases that can help protect and preserve their artifacts. I sincerely hope making this information available to all interested Museums will help protect and preserve their collections. Furthermore, if there are any advancements or discoveries you would like to point out or discuss, please contact me.