[NHCOLL-L:2172] plaster casts

[Amber Tarnowski]

Dear Kate,

    There is an easy way to make a cast without cleaning and sealing the
plaster (which may cause more harm than good in the long run). To make a
replicate cast, apply a silicone putty mold to the plaster. Use a thick
putty that picks up excellent detail, such as the two-part
"Knead-A-Mold" available through conservation and restoration suppliers
online. You may want to check around, prices range quite a bit on this
product. Look for the product that is one part pink, one part blue,
because it's not tacky enough to stick permanently to plaster, and the
colors won't transfer onto the plaster. This type of putty does not need
a release layer. This putty is commonly used in loss compensation for
plaster and plaster-like components, such as recasting detailed relief
areas of picture frames. The silicone cures in minutes, and the
resulting semi-permanent mold can be re-used many times to make many
replicate casts.
    I don't recommend sealing the plaster, as my experience with
vertebrate paleontology plaster casts is that the sealants discolour,
crack, and can shrink, damaging the detail of the plaster cast. Cleaning
the plaster with starch may pose a problem if the plaster casts are
stored in an uncontrolled climate, because starch residue is great food
for microbial growth (mold, bacteria, etc.) One technique used in paper
conservation is to brush tiny unsulfonated eraser crumbs around to clean
the surface without getting it wet. The crumbs are small and soft, not
abrasive. From personal experience, I'd say it works pretty well, as you
can focus on individual areas of dirt to obtain an evenly clean surface
in the end. The non-sticky eraser crumbs can be purchased from Talas
conservation supplies:
http://nt.bnt.com/talas/menu.html?category=125&start_index=11 Look for
the product called "Talas ground eraser."

Good luck!

Amber Tarnowski
Kress Foundation Fellow in Art Conservation Biology
Division of Engineering and Applied Sciences
Harvard University
(617) 495-4180
ambert at deas.harvard.edu