Sometimes it is necessary to stabilize rocks before cutting and polishing them. Stones with fractures, porous stones or brittle stones will benefit from stabilization, allowing you to cut and shape them without risking the loss of the stone. Typically, I hear of three primary methods of stabilizing stones. I will summarize these methods below. Then, I would like to introduce you to a material that is used by museums to stabilize fossils and artifacts. This material, butvar, is a polyvinyl acetate and is used by museums because it does not deteriorate even after very long periods of time and it does not discolor. It has an added advantage that it can be mixed to any consistency desired, so you can make a thin penetrant or a thick glue with it. I have used this material to stabilize stones before cabbing them with success.
Backing: A backing is applied to stones, most commonly turquoise and opal, in order to keep the stone “natural” while holding the fragile material together. Backing is simply applying a thin layer of material to the back of the stone that will help hold the stone together. This mostly helps porous stones stay together and helps to keep stones together that have fractures. Backing material varies widely. Sometimes, a thin piece of stone is adhered to the stone. Then the stone is cut and polished as normal. Backing is not used if the back of the stone may be visible in the finished product. Backing materials include basanite, black jade (typically a dark stone), or synthetic materials. Devcon epoxy, an epoxy with metal in it, is sometimes used. Epoxy 330, a water clear epoxy, is also common. A thin layer of the epoxy is applied and allowed to dry completely before the stone is cut and polished.
Thin cyanoacrylate (“super glue”): Thin super glue can be applied to a stone with fractures or pits. The superglue will fill the fractures and pits, holding them together. While this method may work for fractures and pits, I have not had luck with using it to stabilize a generally fragile stone. Most often the glue just forms a coating on the surface of the stone, which easily peels off during the cutting process.
Butvar - Polyvinyl Acetate
There are a few different types of butvar that have different molecular weights. The two most common are B-72 and B-98. I have used both of these in my museum work. Typically we used B-72 in fossils that were rather wet, but mostly we used B-98. B-72 can be dissolved in alcohol or acetone. B-98 should be dissolved in acetone. Note: Please use caution with these materials. They are not toxic and they will not hurt your skin, but the acetone can. The acetone is a known carcinogen and it is recommended that you wear gloves and eye protection when working with it. Do not get it in your eye – you will glue your contact to your eye and it will scratch the surface of your cornea (sure, ask me how I know!!). For the most part though it is very safe. We use is all the time on fossils in the field and the lab. I will mention a couple of different methods for using butvar to stabilize your rocks.
- Mix a thin consistency of acetone and butvar in a container that has a lid. There should be enough butvar in the container so that it will actually work, but it should still be water thin. So add butvar, and if it starts getting thick, add a little more acetone to thin it back out. The amounts are not critical. You will want a large enough container and enough of the mixture to cover the stone you are working with. Put the stone in the container. Let it sit for several days, 4-7 is recommended. During this time, It is advised to shake or stir the container. I found shaking most useful in order to stir the stone around in the container. One option, which I have not done, but I plan to try at some point, is to put the stone in a container where you can add suction to it. I am thinking like a space bag, where you put things in the bag then use a vacuum to suction it. If you do this, you would want to apply suction to pull the butvar solution up, but NOT suck it completely out of the container. Repeat this several times. The advantage of doing this is that it will force the butvar into the stone and there will be less wait time.
- What I found to be most efficient was to create a thin concentration of butvar in the acetone, it needs to be pretty thin the penetrate the rock. Then I put the rock on a piece of plastic wrap and pour the butvar over it (this is the method we use in the field, especially when the bone is still in the rock and we can’t soak it). I will do this, let the acetone evaporate, and continue doing this until is does not evaporate within about 10-15 seconds. Then I know the butvar has penetrated as much as it can. I turn the rock over and repeat on the other side. Then I let it dry for a few hours. I did not find it necessary, but it was suggested that if you heat the stone before applying the butvar it well help to open the spaces.
I have a new material I collected from North Carolina, which I call Mountain Petronella. It has very soft minerals mixed with hard minerals and they have a lot of planes of weakness in them. I was not able to cab this material but when I stabilized it with butvar, it cabbed great.
I also have a piece of granular peridot, which has individual grains that are very loose. I was able to stabilize and cab this material as well.
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