Re: ATM Hydrostone tool?

["Bill T." <twentiethwave@hotmail.com>]

Lawrence Sayer wrote:
> Does a Hydrostone tool have to be sealed, or is it water resistant like
> dental stone?
>
> I like the idea of just pouring the stone over the tiles, without
> sealing with a water resistant barrier.
>
I've had great luck with Hydrostone tools both for grinding and polishing --
used about 12 so far.  It is very water resistant, even when it's still wet
from casting!  Never seen any degradation such as crumbling on tools I've
used many times over.

At first I coated the tools with various kinds of paints.  But I soon found
that the paint would start to peel after only a few hours of use.  Now I
just use the tools unpainted for grinding, or give them one quick coat of
old pitch dissolved in turpentine for tools intended for polishing.  Never
had a single scratch I could blame on the tool in either grinding or
polishing.  I never debur tiles, use wax sealers around tiles, or take any
other anti-contamination measures beyond a good wet brushing and rinse
between grit sizes.

I greatly prefer casting the tiles into the stone.  But I've also had good
luck with epoxied tiles.  The unfortunate truth is that it really takes at
least 5 days for the material to set up to the point where you can be
certain of good, long term epoxy adhesion.  I've used cast-in tools as
little as 10 minutes after the material set up without problems -- but you
might want to finish all your grinding in a single session with such a
"green" tool.

The local ceramic supply has demo discs molded with various US Gypsum
products, such as the 'Cals, plasters, dental plasters, etc.  These have
been knocking around for several years.  I can easily separate grits from
the discs with my fingernail, for all the discs except the Hydrostone.
That's why it's my favorite.

On one occasion I ground down through 9 microns with cast in tiles at the
same level as the Hydrostone (at the edges).  Hydrostone was clearly being
removed, and there was surely Hydrostone material in slurry, yet the results
were excellent.  The mirror polished in the expected amount of time out with
no problem areas or pits.  Not recommended, but food for thought on those
maddening occasions when the tool is getting a little spare towards the end
of grinding.

FYI:

weight_of_dry_Hydrosome = surface_area * desired_depth * 0.06.

About 32 lbs. of dry Hydrostone will make a 20" tool about 2" thick.

50 lbs of dry Hydrostone will exactly fill a 5 gallon plastic paint bucket,
for indefinite anhydrous storage if the cover still has a good "o-ring"
seal.  USG recommends maximum in-the-bag storage at 1 year because of
moisture intrusion.

On a 40% coverage porcelain tile tool, where the tiles were cast in contact
with the pre-generated curve, only about 1/32" to 1/16" inch will be removed
from the tiles going from through a sequence of 220 grit, 20 micron, 9
micron.  I removed only about 0.40" during an 7 hour grind on a 40%, 20 inch
f6 tool (starting at 220).

For porcelain tiles, I don't think any more than about 30 to 50% tile
coverage is required for good results.  The big advantage to low coverage is
that your tiles for come in contact much faster than if you use a full
coverage tool -- maybe 20 or 30  minutes vs. a few hours.  Both random and
symmetrical tile arrangements have worked equally well for me.

I've successfuly used porcelain hex tiles from both Willmann-Bell and the
local tile store.  The Willmann-Bell tiles are slightly smaller, about 1"
point to point, and seem to be slightly finer grained.  I prefer W.B.'stiles
mainly because they're such great folks.

Bill T.