Re: ATM 18" mirror Progress and Dental Stone question

["David Harbour" <scarab@peakonline.com>]

Damien-

I have ground many large mirrors with portland/sand/ceramic tile tools. I
have never, ever, ever scratched a mirror for having a grain of sand fall
out, and I have never, ever, ever scratched or gouged a mirror for having a
chip of hex tile break off. I plan to write an article about how I make
these, but I'm having some health difficulties just now that are keeping me
from pursuing this hobby that I love. But these portland sand/ceramic tile
tools are easy to make, work wonderfully well, and are dimensionally stable,
and portland is available everywhere in the world.

For large ones I make a circular, double retaining ring to hold the
posterboard dam truly circular. This double ring is made of a couple of
pieces of quarter inch plywood with spacers between them seperating them by
about the thickness of the planned tool. Then I cut out the strip of
posterboard that I am going to make the coffer dam for the portland sand
mix, and seal it with that popular clear shipping tape that everyone uses to
tape up boxes for sending things UPS, parcel post- the clear, wide stuff- it
is not expensive.

Then, lay out a large piece of plastic trashbag and set the mirror on it.
After you pour the sand/portland mix on top of the tiles which you will lay
out on the nmirror, you will wrap the whole shebang up in this to get a "wet
cure". Then, wet the face of the mirror all over and put another piece of
this black plastic mylar across it, and squeegie out the water.  Tape its
edges down along the sides of the mirror. Don't forget- black plastic goes
on mirror's face first, then the tiles.

Then, I set the coffer dam with its double ring on the mirror, and lay the
little one inch "Winburn Hex cushion edge" tiles out on the black plastic
covering the face of the mirror, spacing them with little wooden spacers cut
out from kitchen matches. These wooden matches are the ideal spacers to use
for these little white hex tiles- they allow enough portland/sand mix to
fill in between the tiles to hold them securely, but not too deeply, so you
have enough channels after you remove the little matchstick spacers after
separating tool and mirror. Cut a bunch of these to length ahead of time
with a pair of toenail clippers.  And they give an almost ideal spacing,
about three thirty secondths of an inch. After all your tiles with
matchstick spacers are laid out, mix up the portland/sand mix (about one to
four or one to three) and spoon in the first layer gently, or you can tip
some tiles up and out of their positions. Then, put in the rest of the
stuff. Then, after screeding off the top of the mix flat (this is where an
accurately cut coffer dam comes in handy- it will guide your straightedge as
you screed off the top of the mix) lay a water soaked (but not dripping wet-
wring it slightly) towel on top of the mix, and wrap it with the large
trashbag you laid the mirror on, and tie it up- this way the moisture stays
in and you get a complete cure. Five days is pretty good. Go seven or eight
if you want to.

Slide mirror and tool apart- then, finish the tool this way- wrap the back
and sides with about five layers of the same clear packing tape that you
used to line the posterboard coffer dam with. Turn tool over, pull out
little matchstick spacers. Wax the channels to full depth. Wax in any little
vacant spaces near the edge of the tool.

After the wax is set, cool, scrape all the excess you can off from the face
of the tool.

Next, excavate a little of the wax out from the channels- (use a little
screwdriver whose blade is the right size for this- sorry, the tiles will
eat up the blades of several of your little screwdrivers) only enough to get
the channels' depth just below the level of the tiles.

Tool is now ready for use- your first few wets will grind off the rest of
the wax from the faces of the tiels that you weren't able to scrape off.
With only shallow channels, even when you might have the mirror on top, the
tool will not swallow up your grit and let it run out the sides. Every now
and then, you will have to re-excavate some wax out of the channels. I have
made at least a dozen large tools this way- I have never had a mirror get
scratched from a particle of sand falling out of the tool and onto the
mirror. It is the mirror maker's responsibility to ascertain, take pains, to
keep the tool sealed front and back at all times- front, with wax, back and
sides, shipping tape. The wax also adds strength to the tiles in terms of
their resistance to falling off of the tool. I have never, ever, had one
fall off.

The sand in the mix is the key to the tool's dimensional stability. Try it,
you will like it. The largest tool I ever made this way was a fourteen and a
quarter inch tool. For a big tool like this, I will lay an "X" of steel
inside after I get about half of the mix in, and then pour the rest of the
mix on top (three eighths or half inch hex cap bolts).

I've worked many large mirrors mostly face up with 12 and 13 inch tools made
this way. They give sterling results with safety.

Well, I guess I don't have to write the article, now. Use dental stone if
you want to, but Portland is easy, cheap, and works. ("Works for me"- never
had a disaster with the dozen or so large tools I made and used this way).

I'd be pleased to help you further if you had any questions, needed any
further tips, if you or anyone else wants to try one of these
portland/sand/hex tile tools. "Winburn Hex" (with cushion edge) tiles seem
almost as if they were designed for mirror makers. I believe any tile
retailer can get them- I think they are made in Little Rock, Arkansas. By
the way, a friend of mine struggled over and over again to make roughing out
tools (flat, tiles epoxied to substrate) with one inch Winburn Hex tiles,
and constantly had the tiles lose chips from their edges and gouge out a
gash on the mirror. He shunned channeled ceramic tools after these bad
experiences afterwards. Then, one day, he told me how he separated them from
their paper matrix that they come glued to- with a small shop torch!!! Ha!
Mystery solved- he was devitrifying them with the heat before he ever got to
use them. He loves tile/portland/sand mix tools, now, won't use anything
else. Neither will I- neither will you, if you try one.

I hope this rather longish post will be found of value to some of you out
there- this kind of tool has worked so well for me, so I wonder why everyone
is chasing after dental stone. I only use it to pack a central plug's (for
Cass. Primary) biscuit cut groove.

Dave


----- Original Message -----
From: "Damian Kruger" <djkruger@mpx.com.au>
To: "ATM LIST" <atm@shore.net>
Sent: Tuesday, July 10, 2001 6:37 AM
Subject: ATM 18" mirror Progress and Dental Stone question


>
> Hi List,
> Does anyone out there have a source of dental stone in Australia? I'm
about
> to pour a 18" tool after hogging out my 18 " blank to a depth of 4mm (F 7)
> by hand with a 5kg barbell weight, and 80 grit.  It's taken almost exactly
4
> hours (yes four) to get there, quite quick compared to my 2 previous
mirrors
> with glass tools.  The desired depth is 5.71mm (F 5) and so to obtain it
I'm
> going to switch to a dental stone tool so I can get the curve to the edge
> (currently 2-3" short) and get it spherical.  John Murray if your out
there
> I believe you had a source a while back.  For those who are about to
suggest
> seeing a dentist, has anyone had any luck with a sourcing from dentists?
> Price?
>
> Previous posts have indicated Dentists reluctant to sell it to people of
the
> street.  Currently I'm not chummy with any either!   For those interested
in
> the grinding logs for big mirrors I will post mine to the group (and put
it
> on a web page) at a later date so those who are contenplating grinding a
big
> one by hand  can see what they're in for.
> Regards,
>             Damian Kruger
>             Cairns   Australia
>
>
>