Testing Properties of Preservatives in the Preparation Lab

Scott Madsen

JVP 16(3) Sept. 1996- Abstracts.  

Preparator Session talk, AMNH, 1996

To anyone who reads this- these notes are transcribed pretty much verbatim from my notes for the talk.  They are obviously meant to go along with some slides, which unfortunately are not digitized.  When I can remember, and as it is helpful, I will try to describe what the viewer was seeing on the screen (in bold).

Disclaimer- these experiments were literally done in my garage at home, since my supervisor (a paleontologist at the time) would not let me do them at work.  Nevertheless, I did them as carefully as possible and weights and measures were done with a triple-beam balance.  Ratios are given as percentages by weight of solids and liquids. 

The sand concept for comparing consolidant properties was suggested to me by Bill Ameral at MCZ-Harvard.  The sand should not necessarily by thought of as a bone-mimic, but rather as a simple handy medium with relatively consistent physical properties for comparative purposes.  For what it’s worth, the sand was taken from a sand table in my lab and its origin was a beach on the Green River, UT just outside the lab.  It was fairly well sorted medium-fine grained sand, mostly silica with minor carbonate and other unidentified junk.  I sifted it to get a fairly regular consistency.

While sand is not fossil bone, the idea here was to look at the properties of the consolidant itself when tested against a consistent matrix.

-SKM Dec 2005    

Talk-

This talk is just what the title says.  I want to offer a few results from some simple experiments I ran on resins and solvents.

My focus was to figure out what resins mixed with which solvents, and then sees if I could determine how well they worked as consolidants.

This picture (a huge white blob in a beaker) is a reenactment of the situation that got me into this testing in the first place.  For several years I heard recurring stories of “the resin that would not dissolve”.  Some of these were resins that I had recommended to people, others were resins people found sitting on shelves with the simple label- “Butvar”.

In addition, Ann and I get a lot of requests to recommend consolidants for the field and lab.

And finally, the growing awareness of a conservation minded approach to preparation was even creeping into the labs of those of us living in backwaters like Vernal, Utah.

The first thing I did was a lot of homework, finding out what I could from conservation texts and literature, product info sheets and MSDSs.

I talked to other preparators.  Unfortunately, when I asked WHY someone used a particular product, all too often I heard something like- “It was on the shelf when I got here”.

Manufacturers were helpful providing information.  Unfortunately, most of them don’t deal with rock and bone.

The trick is to sort through all this information and translate it into something that helps you in your particular situation.

I decided it was time to experiment.  My lab doesn’t have a lot of fancy analytical equipment; in fact I couldn’t use my lab at all, so I chose the next best thing- my home workshop.  It looked like a meth lab by the time I got it set up (shot of my garage with beakers and acetone cans, etc).

But these simple qualitative “kitchen” experiments don’t take much material.  Some bottles, beakers, a gram scale and you’re ready to go.

I borrowed some sorted sand from a local quarry, and then compacted it in some sand tables to produce my “test matrix”.

Next I got a bunch of resins together, Butvar B-76 and B-98, some Air Products “Vinac” Poly Vinyl Acetate known as B-15, and 2 Acryloids, B-72 and B-67 (shot of various dry bead or powder resins). 

The first thing I wanted to do was see how well the various resins dissolved in a given solvent.

For solvents I chose Acetone and pure anhydrous ethyl alcohol, mainly because we’ve got a lot of them around the lab.  (The solvents were reagent grade from Fisher).

But before I got to mixing, I wanted to find out how fast this stuff evaporates, a bit of information that I think affects a lot of the results to come (shot of open containers).

This was simple- acetone evaporated twice as fast as the alcohol.

Rather than bore you with a bunch of slides of half-dissolved resins, I’m going to cut to the results now.

I measured out all of the resins and solvents by weight making a 10% concentration, usually weighing out 380 grams of solvent (about 500ml) and added 38 grams of solids.

What I found was that ALL of these resins dissolved in Acetone with various amounts of stirring, usually within 3-10 minutes, with 1 exception.

Monsanto says B-98 (Butvar) “swells” in acetone.  It does!  At 10% solution I got this jello-like blob in about 20 seconds. 

But, what I also found was that you can add about 8% water (as Monsanto advises, I took mine out of the tap) to this solution and it will dissolve.  First you get this spaghetti-like mess, but in a few minutes it all goes into solution.  But you must remember to first mix the water with the acetone, and then add the solids.

The ethyl alcohol wasn’t as versatile as a solvent.  As the Rohm and Hass literature said, Acryloid B-67 and B-72 wouldn’t mix at all with alcohol, even when I tried a kicker of 8% water. 

Also (as the product literature said), the PVA B-15 required adding 5-10% water to get it to dissolve in ethyl alcohol.

The lesson here is to know exactly what it is you are trying to mix.  Just to know “Butvar” isn’t enough.

The next thing I wanted to know was how these resins behaved as a consolidant. 

How well will they hold together a loose, unconsolidated lump of rock and bone (or in this case, sand)?

How deep will they penetrate (in this sand)?

How long will it take to be effective (consolidate)?

Also, I wanted to know if I could mix these solvents to come up with a move effective consolidant for my work.

To keep it simple, I mixed a 50-50% solution of the acetone and alcohol mixtures I had already prepared (B-76 @ 50-50, B-15 @ 50-50, etc.).

The sand table experiment, a concept I got from Bill Ameral years ago, is pretty crude but simple.

You just take a dropper and drip about 20 drops of resin mixture onto a spot at a fairly consistent rate.

I ran about a dozen samples for each consolidant (shot of rows of wet spots on sand table).

I left the samples alone for about 3 hours, then came back to test the samples for hardness and consolidation using the most precise tool at my disposal-

My fingers!  I snapped the damn things with my fingers to see what it felt like!  One after another…  Just TRY and get this published!

Actually, you don’t have to break too many of the wafers before some patterns emerge, and I did get some pretty clear results.  (Shot of dozens of sandy wafers lined up with respective resins- the wafers are very roughly 1 ½ inches wide and thickness (depth) varies quite a bit). 

First I wanted to get a handle on the penetration and lateral spread of the consolidant.  Discounting the real anomalous blobs, you didn’t really even need the calipers to see the results.

In these tests the Acetone solutions always penetrated deeper into the sand than the Alcohol solutions, usually about twice as deep.

The pure alcohol solutions had a greater lateral spread.

The 50-50% (acetone-alcohol) solutions fell somewhere in the middle.

I assume these results are mainly a function of viscosity of the fluids.

Next, I wanted to get a sense of the general hardness and overall consolidation of the wafers. 

Were they solid through and through like rocks, or did they just have a solid crust, or were they loose and sandy inside? 

Did they produce a clean break or a ragged edge, or did they crush when I pinched them?

So I would try to compare the hardness and consolidation of B-98 in Acetone vs. B-98 in Alcohol, etc.

…and then I would compare one resin against another, PVA in alcohol vs. B-76 in Acetone, and so on.

Again, I admit this is very subjective, but here were my results-

First, in these tests, the Acryloids in Acetone were the clear “losers”.  Though they penetrated and spread well (better than the Butvars and PVA), they were crumbly as hell, thought they did have pretty tough crusts.  It should be noted though, that even at a 10% concentration the Acryloid solutions were extremely watery- given a higher concentration of solids they could perform better.

Of the Butvar and PVA samples, the Alcohol solutions were the clear “losers”.  They were always weaker than the acetone samples, sometimes almost flexible at 3 hours.  They crushed easily when pinched and tended to crumble with ragged breaks. 

In contrast, the Acetone solutions of Butvar and PVA always had tough crusts and tended to break cleanly, though they often had some evenly consolidated or sandy interiors which crumbled.  This may have to do with the rapid evaporation of the acetone. 

On the other hand, the 50-50% mixes (acetone-alcohol with solids) seemed to take the best of both worlds.  Thought not as strong as the resins in pure acetone, they were tough and usually better consolidated throughout.

Comparing resins, the PVA B-15 in acetone or a PVA 50-50% mix performed pretty well here and in similar tests I’ve done, but I’ve got somewhat inconsistent results.  In general it formed tough crusts and was pretty well consolidated, but overall did not perform as consistently as the Butvars. 

Between the Butvars, B-76 and B-98 both performed well in Acetone and 50-50 mixes, with clean breaks, tough crusts and good consolidation, though B-76 edged out B-98 in overall toughness and consistency. 

These results changed little over the next 2 days, in other words, in 2 days none of the wafers were a whole lot harder than they were in 3 hours.

In addition, I ran a few quick tests to see what worked best in the short term.  I ran some spot tests and checked results at 15 minutes and 1 hour.  At 15 minutes I pressed on the wafers and scratched the surface with a fingernail.  All of the acetone mixes had a good crust already, while the alcohol solutions were still soft.  I assume this is a function of faster evaporation of the acetone from the matrix.

Incidentally, some Acrysol WS-24 I tested at full strength out of the bottle was still very soft at 3 hours, and the ethyl cyanoacrylate was hard as nails.

So what is the bottom line?  There is none! 

Basically, it all depends on what you want in a consolidant.  The point is I think we have a lot of tools out there to come up with the “best” solution for our particular situation.

There is a lot of information out there and materials to experiment with.  We can do a lot better than I did with this crude experiment.  Hopefully this meeting today will generate some new ideas and enthusiasm.  Controlled quantitative experimentation is needed. 

Finally, I hope we can all find the institutional support we need for this work.

Thanks.