ATM Plaster Demystified?

[schmidt@coyote.csusm.edu (Michael Schmidt)]

As a novice telescope maker about to embark on his first project, I read
with interest the posts here within the last month and searched the
archives for more information regarding the "tile-plaster tool."
Dental plaster seems to be the material of choice,  but it seems not to be
the most readily available. One poster (I forget now the name) said that a
building product, Hydracal, worked fine. Others have swore garlic-breathed
camels at their attempts to waterproof "regular" plaster.

As a chemist, I found this problem of plasters interesting. From the
heat-liberation of dental plaster I guessed that it probably contains a
fair amount of some polymerizable material, probably methacrylate/acrylic
acid. (I know other things dentists have put in my mouth contain these
ingredients.) When I asked my local building supplies center for
"Hydracal," they told me they don't carry it and don't know what it is.
Obviously a brand name. They do, however, carry USG's "Water-Resistant
Compound," which contains vinyl-acrylic copolymer and/or ethyl
acrylate-methacrylic acid copolymer and a bunch of other stuff (and,
incidentally, no "plaster" or gypsum, known to us chemists as calcium
sulfate. Calcium carbonate, or limestone, is used instead as the principal
ingredient). I'm guessing that this compound is the equivalent of
"Hydracal," and I intend to experiment with it for my first project.

I think it would be a great service if those ATMers who have experimented
with various "plasters" could look at the labels of the materials they have
used and report the ingredients listed there, as well as the results
obtained with these materials in terms of shrinkage, durability, adhesion
of tiles with or without epoxies, adhesion of varnishes and other coatings,
and water resistant properties. There are many different products out there
under many different brand names. My survey of 7 "plaster"-type products at
the local building supply house revealed various combinations of over 16
different ingredients, not including all the variations on acrylic-based
polymers. Perhaps if we can start correlating performance with ingredients,
we can come up with better, cheaper, or more convenient alternatives for
our ATM colleagues both present and future.