Here is a workable technique for consolidating fossils.
Polyurethane will not give the desired penetration of the fossil. This resin is very difficult to remove. Putting polyurethane on a fossil is usually a bad idea.
I recommend against white glue (polyvinyl acetate) as a consolidant because there are better materials available.* (Normal prep lab dilution of white glue is one part water to two parts glue.) Rarely, a specimen cannot be dried without it crumbling, and white glue is the only reasonable answer. In my experience, white glue is messy and never looks good when the specimen is fully-prepared.
A much better material for bone is a polyvinyl butyral plastic such as Butvar B-76, but that material may be hard to find in small quantities. I have used this plastic, dissolved in acetone, for many types of fossils. (I have used it successfully on Silurian-age shales with brachiopods, for example.) It penetrates well, and in the proper dilution it produces a "damp-looking" finish with no gloss.
Butvar B-76 (but not other Butvar varieties) is also soluble in alcohol. (I assume that is denatured alcohol that you can buy in gallon cans.) I have never tried this solution for consolidation. The alcohol takes considerably longer to boil off the treated specimen.
Butvar B-76 and other suitable plastics, such as Vinac, are more frequently available on the Internet these days. But, if you can't find Butvar-76 or Vinac, you may want to fall back on a solution of Duco Cement (clear, like model airplane glue) in acetone. Duco Cement is not a first choice, or even a second; but, it will hold a fossil together while you consider other options.
Dilution? Start with a tube of glue dissolved in about five or six ounces of acetone in a glass jar with a metal screw-top. Shake well.
(From this point, the techniques are the same for any plastic consolidant you choose.) Adjust the dilution with more acetone until, after shaking, the tiniest air bubbles are just slightly retarded in their rise to the surface.
I usually heat specimens with an infra-red lamp to drive off moisture just before dipping the fossil. I do this with all sorts of fossils, and have never had one damaged by the heating. The untreated specimen is always at least as wet at the relative humidity of the air around it, I surmise. (A microwave oven may be as effective, but I've only dried glass beads for my air-abrasive unit.) Residual moisture may cause a white film to develop on the surface of a fossil after dipping in the consolidant. Here's how the white film forms: As the acetone in the consolidant evaporates, the temperature at the surface of the specimen chills abruptly, lowering the dew-point at which ambient water vapor condenses. And, that's my theory -- that the white film has two potential sources: residual interstitial moisture and ambient humidity condensing at the surface chilled by evaporation. Think about a plastic bag of food placed into a freezer, where frost is moisture and bag is the film of consolidant. Frost can form on either or both sizes of the plastic bag, inside frost from moisture in the food and outside frost from atmospheric moisture. My solution is heating the specimen to drive off residual moisture, and consolidating while it is warm to increase the dew-point at the specimen's surface, inhibiting condensation as the acetone boils off.
Do NOT heat the acetone solution directly. The acetone solution will get warm after dipping a number of heated fossils. You must have good ventilation to deal with the fumes!
I use a long-jawed forceps -- ten-inch tweezers, really -- to dip and/or retreive the fossils from the jar.
Ideally, you would submerge the dry specimen in this consolidant for a brief time (say 10-30 seconds, or until the specimen stops fizzing). Set each wet specimen aside to dry on cardboard (I use a beer-flat because that cardboard is absorbant and doesn't readily stick to the fossil).
To avoid pooling of consolidant which may drain from a bone, I rotate the bone once or twice in the first minute or two after placing it on the cardboard. This helps avoid a "drip-bead" of consolidant near the lowest point of the bone.
For a specimen too thick to be submerged, you can use a turkey-baster to flood the difficult areas. I treated an adult mammoth tibia that, because of its size, I dried in the Florida sun, then used the baster to pump consolidant into every opening of the bone.
I use a RubberMaid-type container to hold the consolidant for this basting step - that plastic seems to be impervious to the acetone. Get 'em at your local dollar-store.
Acetone evaporates very quickly. Replenish the consolidant mixture with a bit of acetone if you are using it on many specimens. Store it in a tightly sealed glass jar. Even if some acetone evaporates away between uses (it always does, it seems), you can reconstitute the solution by replacing the acetone.
Acetone is a nasty solvent. The fumes are explosive. The fumes are toxic. The liquid penetrates the skin-blood barrier. It's best to use gloves. Use in a well-ventilated area.
* Here's what 'oilshale' had to say about white glue (wood glue is just another polymer formulation):
"Don't get me wrong - Elmer's White glue is a great stuff for glueing wood and can be also great for "hardening" crumbly fossils!
"But I fully agree with Harry's opinion (even so I am a polymer chemist and my job is to develop white glues and other latices....): I would never use a white glue unless the fossil is wet, crumbly and the substrate is porous and can't be dried before consilidation!
"There is no way to remove this white glue once dried (not even with solvent). It will form a dense polymer layer on the surface without penetrating much into the substrate (white glue are tiny polymer particles dispersed in water with a particle size of around 1Ķm, so the penetration depth won't be much).
"Butvar, a Polyvinyl butyrate (the company I am working in is also producing these polymers, of course different brand names) in this respect is much better (will penetrate better and can easily be removed by solvents).
"I do have a couple of fossil fish which were mistreated by someone else in such a way. Since the substrate was almost nonporous (diatomaceous earth!) and quite soft (and may be also the amount of white glue and concentration used was too high) there is now a thick slightly yellowish polymer film on top. Unfortunately, this is not all: The film shrinks and now peels off (with bones attached to the polymer film of course)!
__________________________________________________ _____________ If you value a vertebrate fossil (I exclude shark teeth here) -- and you want it to last -- consolidate it with a plastic. You cannot reliably judge by eye what will happen to the bone after 2 years, or 5 years, or 15 years in your drawer. Bones with which you could drive nails when first collected may split after years in your drawer. Teeth, when thoroughly dry, may split. These splits cannot be repaired to the original condition because of distortion to the bone or dentin or cementum. This may happen to any bone, so, if you're going to keep the bone, play the probabilities. Consolidate! Impregnation with plastic will prevent many later headaches (I'm not telling you to soak your head in consolidant). I am saying that there is nothing more disheartening to open a drawer and to find a prize specimen tooth split in two. Trust the decades of museum experience.