From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


Here is a link to US Gypsum's product literature for Hydro-Stone and
related products.  For Hydro-Stone, it looks like the setting time is
roughly comparable
to dental stone.  It appears to be possible to slow it down some by
starting with cold water.  Under Hydro-Stone, the Photo and Data Sheet link
and the second Technical Data link have what looks to be the most pertinant
information for the tool/lap base application.  Intriguingly, the Technical
Data link indicates it is possible to retard setting time with an additive.

Also, one might want to look at the Tuff-Stone product which is Hydro-Stone with
some sort of fibers added.  They claim improved impact resistance.  This
probably also implies higher tensile strength, although they don't say so.
Might have some benefit with larger tools.  I don't know if tensile modulus
is increased, that would probably be the key to thinner tools.

http://www.usg.com/Product_Index/_product_index.asp?vProdCat=75&vFamily=6&vGree
nLink=75


This page has a link to USG Retarder information.  This is a retarder
mentioned in the Hydro-Stone technical information for slowing down the
setting time. Not
a lot about it, but enough to get started.  It does warn that you don't
want to go overboard with retarder, as cured strength can be impaired, but
doesn't give any quantitative guidance.  From the chart in the Hydro-Stone
information, it looks like you can probably get useful working time
increase without going near the limit of the data on the chart.  Presumably
they wouldn't have charted conditions that would give rise to radical
strength impairment.

http://www.usg.com/Product_Index/_product_index.asp?vFamily=1&vBrand=0&vProdCat
=9&typ=products&sp=true

On the subject of thinner and lighter tools, for those large tools where
weight gets to be a problem, I have thought of, but never tried this idea. 
When making
a tile-in-stone tool, cut a circle of chicken wire, or perhaps better,
hardware cloth (a sort of very coarse galvanized steel screen) on top of
the tiles before
pouring on the liquid stone, cement or whatever.  Then imbed a circle just under
the surface of the freshly poured material.  This should have the effect of
raising both the tensile strength, and, more importantly, the tensile
modulus of
the disc, allowing one to get away with a thinner and lighter tool.

Another approach would be to mix in some perlite (light stuff that mixes
well with plaster) and keep the thickness the same.  A tricky approach
might be to pour a thin layer of "pure" stone on the tiles, then
a thick layer, well lightned with perlite, then another thin
"pure" layer.  One would have to work very fast to get the mixing
and pouring done within setting time.  Retarder would probably be a
necessity here.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP