Removal of Shellac and Glyptal from Fossils

I received the following e-mail asking for help about fossils deteriorating due to old hardeners and adhesives. As I see messages such as these quite frequently, from both professionals and amateurs, I've decided to publish the recommended actions as a service to the community. --ed.

Hi, I am an intern at the Royal Tyrrell Museum in Drumheller, Alberta. I am currently looking for information on Glyptal and Shellac, which have often been used on our fossilized specimens in the past. At the museum, we are seeing that these consolidants/adhesives are deteriorating causing damage to surrounding storage cabinets and to the specimens themselves. I am looking for information on how to remove these products or how to slow down their deterioration process. I am also interested in information from other institutions who have had similar problems with these products in their collections. Any information would be helpful. Please send information to Tricia at the Tyrrell Museum at rtmp@dns.magtech.ab.ca, which is my e-mail address. Thanks.

From: San Diego Natural History Museum <libsdnhm@CLASS.ORG>

Definitely, you should stop the use of both Glyptal and shellac, as soon as possible, if in fact you are still using it. Both of them are highly unstable compounds. Shellac will continue to become more brittle and dark throughout its deterioration and can be very acidic, which is damaging in itself. Glyptal is part of the group of compounds derived from cellulose nitrate, which is notoriously unstable over time. They looked good when they were first put on, so people got into the habit of using them because they heard that someone else did, but the results 20 to 50 years later are disastrous. I have seen fossil destroyed by the physical damage caused when a join failed and allowed gravity to take over, and I have seen fossil surfaces pulled away from the bulk of the fossil by shrinkage of the polymer.

You ask about removing these. First of all, if at all possible (and it often isn't), figure out exactly what you are working with. Shellac cross-links over time. It starts out relatively easily soluble in ethanol, but becomes less and less so as it deteriorates. Glyptal should be soluble in the solvent that was originally used as the carrier, usually ethanol or acetone. Test a tiny area first. Second, do *not* dunk the specimens in a bath of solvent. It's unsafe for you, ineffective for the job, and may cause serious damage to the fossil. This is Q-tip work and can take time to do right. Use a fume hood or wear a respirator. The problem took a long time to develop and can't be fixed in an instant. You can use a hypodermic to do a controlled soaking of old joins of massive bones (but please be sure the barrel is not a plastic that will dissolve in your solvent. Yes, that is the voice of experience).

Third, and most important, document the process, the chemicals used, and the results. Someone in your job 50 years from now will bless you.

In the spring newsletter of the Society for the Preservation of Natural History Collections, there will be a technical leaflet on adhesives and consolidants used in preparing/conserving geological materials that will have a range of recommended materials. This will be in a wall chart format. These leaflets are provided as a SPNHC membership benefit, and I will be glad to send information to anyone interested. We will also be sponsoring 4 classes in 1997 on geological conservation: preventive conservation, advanced techniques, site conservation, and identification of geological materials.

From: gbrown@unlinfo.unl.edu (gregory brown)

Just a few additional comments to add to what Sally has already posted.

Shellac was, for many years, thought to be the best material available for consolidating fossils. It was used extensively for this purpose from the 1850's through the last decade, and is still used by some people today. Virtually all of the fossils in our museum (for example) collected before 1970 are consolidated with shellac.

As Sally has said, shellac is at the top of the "not good" list of materials and should be avoided. However, before you decide how to remove it from all your fossils, you should first decide *if* you should remove it. It would be impractical and dangerous (to fossils and preparators) to try to remove the shellac from your entire collection. Attempting to remove any consolidant or coating from a fossil may do more damage than leaving it in place, even if the consolidant is "really bad stuff".

Some characteristics of shellac:

1. It can cross-link to an insoluble solid over time.
2. It darkens over time.
3. It is usually more of a surface coating than a penetrating consolidant (especially on fossils collected during Prohibition!)
4. It attracts and incorporates dust at elevated room temperatures.

Some reasons to remove shellac (or any prior consolidant):

1. Noticeable deterioration. Peeling, shrinking, flaking, spalling.
2. To expose obscured features for research purposes.
3. To improve the aesthetic appearance of a displayed specimen.
4. To break the cycle of dust accumulation/incorporation on the surfaces of specimens exposed to "less than ideal" conditions (often displayed specimens).

Techniques used in removing shellac depend a great deal on the nature of the specimen. Small, fragile specimens and comminuted bone actually being held together by the shellac require considerable care (Sally's "Q-tip" technique.) Larger, more robust material is somewhat easier to deal with safely. Remember, though, that you will only be able to remove surface coatings; shellac that has penetrated the interstices of the bone can not be removed.

Surface buildup of shellac can be removed from "sound" bone as follows:

Materials--

• Ethanol/Acetone 50/50
• Cheesecloth or gauze
• Light-weight polyethylene sheeting
• Small (< 1/2 inch blade) wood, polyethylene, Teflon, or steel tools

Method--

Apply an ethanol/acetone-moistened pad of cheesecloth to a small area of the fossil. Cover with a piece of polyethylene sheeting. Let this remain in place for about 5 minutes, then remove and examine the surface coating. The shellac should have softened enough to gently scrape ("coax" is a better word) some off. Quite often, the shellac surface will bubble up and slide off very nicely, especially on smooth, non-porous bone. Repeat the procedure as needed to soften and remove all the shellac on the surface. Residue can be wiped off with solvent- moistened cheesecloth. You may also try applying the ethanol/acetone mixture to the bone and covering directly with p/e sheeting (no cheesecloth). The idea is to keep the surface coating of shellac exposed to the solvent, but to avoid soaking the bone (risking collapse.) Obviously, the more porous or textured the bone, the less successful you will be.

Cross-linked shellac is insoluble in ethanol and acetone, and can not be removed this way. It has been reported that pyridine may "disrupt" cross-linked films, but I have no experience using this material. We find that removing cross-linked shellac is nearly impossible without damaging the fossil and is seldom worth the risk.

Glyptal:

Glyptal (proprietary cellulose nitrate compound) usually remains soluble over time and is not prone to cross-linking. We have not dealt with removing Glyptal consolidant, but I'd probably test the same techniques used above using acetone or ethyl acetate. The tendency of cellulose nitrate polymers to shrink severely and embrittle over time results in failure of adhesive joints and pulling-away of fossil material from both sides of a join. We see far more damage from Glyptal used as an adhesive than as a consolidant. It might be prudent to remove and repair critical Glyptal joins *before* damage is apparent.

Having said all this, I'm curious that you mentioned that the deterioration is affecting even the cabinets the specimens are stored in. I haven't seen shellac or glyptal do this before. It may be that you are seeing something other than simple consolidant deterioration. Can you describe the damage in more detail??

*****SAFETY******

Get and read Material Safety Data Sheets for all chemicals you use. Follow all safety guidelines...no short-cuts. If you are not trained in handling these materials, or if you do not have adequate facilities, do not use them! Organic solvents, especially mixtures of solvents, can be very hazardous. Glyptal, for example, contains ethyl acetate, isopropyl alcohol, toluene, ethanol, methyl ethyl ketone, and dibutyl phthalate...a mixture I would not feel comfortable working with very much.