Saturday, August 15, 2009

Let me stand next to your fire....

In tribute to the 40th Anniversary of the Woodstock Music and Arts Fair, and by request of Casey Holliday, this post will deal with a method developed by Holliday and Brown in 1999 at the Walt Disney World Animal Kingdom Fossil Preparation Field Station. The "Pyro Preparation" method is specifically used to facilitate safe removal of plaster and burlap field jackets from very delicate specimens. The technique was created to deal with vertebrae of Rapetosaurus krausei where much of the matrix had been removed in the field, followed by application of a thick (~2-3 cm) and tightly conforming plaster jacket. Removal of the field jacket in the lab by traditional means was impossible without significant damage to or destruction of the fossil material, as the vertebrae were poorly mineralized and subject to substantial weathering before discovery. This condition resulted in extremely crumbly bone that was very difficult to consolidate with the jacket in place. The field jacket was tightly wrapped around the neural spines and left transverse processes of several articulated vertebra, with little to no matrix buffer between the bone and jacket. The mechanical lock created by the conforming plaster exerted considerable leverage on the bases of the spines, and attempts at removing the plaster with a razor blade and Stryker cast cutting saw resulted in much breakage and grinding of elements. In desperation we decided to try fire as an option. First, the plaster is gently scored with a razor blade to expose the underlying burlap, then a volatile solvent (in this case acetone) is applied to the burlap. When ignited, most or all of the burlap burns up, allowing the next layer of plaster to be crumbled and scratched away (Fig 1). Before being applied to the fossil, this technique was attempted with the experiementers hand slipped between the jacket and matrix, to ensure that temperatures inside the block would not be high enough to cause damage through thermal shock or scortching. After several layers of burlap were burned away, the jacket was pliable enough to remove by slowly peeling it away, while applying consolidant to the freshly exposed and friable fossil surfaces.

4 comments:

ReBecca Foster said...

Awesome! Happy Woodstock Days! Janis Joplin was just finishing her set up 40 years ago now!

saurian said...

Coincidentally, a couple of us were out prospecting on Sunday when this very subject came up. I'm familiar with the more traditional forms of plaster and cast removal but this is fascinating. Certainly you need an experienced hand in these cases!

Casey said...

And the fire technique was also excellent at burning back the roots (1-2mm diameter or larger in some cases) that had grown through the fossils...where pulling/cutting/sawing/vibration often also wiggled or damaged the fossils more than one would like.

220mya said...

Reminds me of a field technique for consolidating fossils in wet environments:

Baird, D. 1978. The burnt dope technique and other intertidal ploys. The Chiseler 1(2):16-17.