Sunday, October 31, 2010

Or you could just follow the directions

For a field of study purporting to be a science, it is amazing how much of paleontology remains blissfully unaware of how its methods actually work. As a prime example I submit: The Mixing of Plaster.

My ire was raised again on this topic while unpacking some field jackets collected earlier this summer by another field crew. Instead of gleaming white plaster, or at least mudcaked but well-set plaster, we found fuzzy green and brown, soft, powdery, damp field jackets. There was so much moisture in the incompletely cured jackets that they had molded. I've dealt with so many floppy, improperly cured jackets over the years that I just don't think I can take it anymore.

This topic will follow the model of the SVPOW folks and their MYDD campaign. Thus I say, Follow The Damn Directions (FTDD)! [Sidebar: This commentary will eventually be followed by posts, talks, and papers of expanded theme, Publish Your Damn Preparation Methods (PYDPM)]

Despite the fact that almost every paleontologist who has ever lived has at some point in their lives mixed plaster, even the Society of Vertebrate Paleontology website says this:
"Is there a proper way to mix plaster?
A2: The Simple Answer:
Slowly pour plaster into 1-2 inches of water until little islands of plaster form. Let sit as is for 1-2 minutes before mixing, mix to a creamy smooth consistency. After mixing is complete you’re good to go."

Let's take this opportunity to throw any (quite actually widespread and sometimes militant) fairytale notions of island building right out the window. Now, I don't mean to imply that the collective experience of so many field workers should be enough for this to not be a hotly debated topic. I mean to explicitly state that the setting of plaster is a chemical reaction, governed by the properties of the reactants with very predictable results. Manufacturers ALWAYS have recommended mix ratios on the packaging that will yield the optimal results. The shotgun approach answer is akin to dealing with the question "What glue should I use on fossils." First of all, the formulation of plaster that you buy at Home Depot is very likely quite different from the material that comes from Ace Hardware. Take a look at the number of plasters and gypsum cement available from United States Gypsum alone. Almost every one of them requires a different ratio of powder to water. Some of these ratios vary by as much as 45%. Plaster is a generic term that is absolutely meaningless in a technical discussion about how it works, again, just like "glue". First of all, identify the compound that you are talking about, the functional differences between USG Moulding Plaster and Hydrocal White Gypsum Cement are night and day. There are some excellent instructions for mixing provided at their website, but I'll quote as an introduction this:

"Successful mixing of industrial plasters requires strict adherence to specific standards and procedures. The improvements in plaster formulations made in recent years have resulted in more uniform products, but to obtain the full benefit of these improved products, shop procedures must be standardized."

When I was first instructed in the ways of plaster, I was told "This is just like cooking, keep adding ingredients till it tastes right." Which incidentally also ignores the chemical foundations of cooking. Especially while jacketing, I have often seen workers mix a batch, decide that it is too thin, and then keep adding powder to the mixture until it is deemed "thick enough" to bandage. This is almost always wrong. The whole purpose of making a field jacket is to create a rigid enclosure that will protect the fossils riding inside for not only the trip back to the museum, but also the subsequent days or decades until the block is finally opened. I have opened many field jackets from the first 50 years of the 20th Century where the bones were as well protected and intact as the day they were collected. I've opened many from the last 15 years where the ill-fitting, sometimes aluminum-foil-lined, soft and floppy jackets have allowed the contents to shift around and grind the enclosed bones against one another until rubble remained. 

First identifying the material you are using, become familiar with how it actually works, then understand how you can manipulate the properties of the material to suit your needs. Don't cut corners or guess. Measuring quantities is not difficult. Even for field work. I like to carry plaster in large Ziploc baggies. Depending on the material, I know how many liters of water it takes to properly set that volume of powder, and can accurately (and even more easily) get the results I'm looking for every time. I can't imagine a situation where a technician in a DNA sequencing lab would prepare a gel by adding a pinch of agarose, a teaspoon of buffer solution, then put it on the stove for a while till it looks right. This is science. Follow The Damn Directions.

1 comment:

Sarah Werning said...

Well, maybe not agarose, but in molecular labs everyone uses their own master mix recipe for PCR (kind of a DNA soup with the building materials for PCR), and the only thing anyone seems to agree on is that the amount MgCl2 is important and can really screw you up if you're off. It was really surprising when I started doing molecular work how much variability there is. I guess the difference is that the recipe is at least known, followed, and recorded in lab notebooks.

It's worth mentioning that the ratios are likely more important in the field given that the water purity is questionable and that mixing times are less likely to be followed correctly. I posted recipes in my lab to remind people.