Re: [ATM] glass or ceramic blank?

[Mark Holm <mdholm@telerama.com>]

Get Richard Schwartz to email you his instructions for a tile in dental 
stone tool.  His instructions are great.  The only problem is the water 
to powder ratio.  I  don't think he mentions that.

You can probably buy dental stone locally from a dentists supply place.  
You want buff labstone or something similar.  There are a whole bunch of 
brands.  Each brand tints theirs differently.  Color isn't a reliable 
indicator of properties.  If you have a choice, pick one with the best 
water resistance, longest setting time, and least shrinkage/expansion 
during setting.  It should set up in 10 - 15 minutes (maybe a bit 
less).  If the dentists supply place doesn't want to deal with you, ask 
your dentist if he/she will order it for you.

Alternatively, get US Gypsum Hydrostone.  It is used for casting 
statuary, architectural details and the like.  I haven't used it, but 
the instructions and propeties sheet (see USG web site) make it sound 
pretty similar to dental stone.

The beauty of Richard's instructions is that the tiles are cast right 
into the dental stone.  It is pretty much a one step operation.  No 
epoxy involved.  Also, the dental stone is sufficiently water resistant 
that you don't need to seal it.  Also, when the tiles wear down so that 
dental stone starts contacting glass, no problem.  Despite it's name, 
dental stone is softer than glass.  No scratches.

With Richard's instructions, the Willman-Bell tiles won't work.  Theirs 
are loose.  You need ones that are joined together into a mat.  You 
might make the W-B tiles work by sticking them to your mirror with a 
thin layer of vaseline, hot wax, or similar then pouring the dental 
stone on top.  Have never tried this, but it is theoretically possible.  
I'd rather use the mat.  Also, there are advanteages to square tiles.  
The W-B ones are hexagonal.  It is easier to dig out partly hardened 
dental stone from between tiles using a carborundum stone when the tiles 
are square.  You don't want to dig it all the way down so that the tiles 
become loose.  Just enough to let some water and grit circulate during 
grinding.  This helps break the suction between tool and mirror.  It 
makes the grinding smoother.  Some folks have dispensed with the 
channeling between the tiles and report good results anyhow.

You want unglazed procelain tiles, or glazed porcelain if that is all 
you can find.  The porcelain part is important.  Only they are hard 
enough.  Usually these are sold as floor tiles.  1 inch square, or hex 
if that is what you can find, in mat form.  Avoid any with anti-skid 
surface.  This might include something sandy to improve traction.  Not 
what you want at all for a grinding tool.

gotgrit.com has had both dental stone and matted tiles for sale.  The 
stone is premeasured.  When I got some a couple of years ago, Richard's 
instructions were included.

I have to tell you, the tile in stone method is really the easy way.  
Except for buying a glass or ceramic, premade tool, tile in stone is the 
easiest.  Ditch that epoxy idea.  It is unnecesary, smelly, tedeous, 
expensive and epoxy exposure can lead to allergic sensitization in some 
people.  Ditch that wood idea.  Wood is not sufficiently dimensionally 
stable.  In particular, if only a little water gets in, warp city.  
Think you can seal wood well enough.  Think again.  It ain't that easy.  
Your tool is going to be quite damp for quite a few hours.

Using the tile in stone method, you can have a usable tool in about one 
hour.  The experienced types get that down to about 1/2 hour.

The first time I did it, I was totally amazed how easy it is.

Mark Holm
mdholm@telerama.com
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