Re: [ATM] start...tool....

[Mark Holm <mdholm@telerama.com>]

Bob May wrote:
> WoW!  Considering that the stone product that I use takes about
> 1/2 hour from the first mix time to turn hard enough to handle
> and grind with, speeding it up from there is merely silly.

Having done this, and having been completely amazed at how easy and fast
it is, I keep harping on this point whenever the choice of tool
materials comes up on the list.  The tile in dental stone scheme is
easier and faster than any other and no more expensive than using
expensive epoxy with cheaper base materials.  Bob's statement is just
more support.  Tile in cement is probably similar in ease, though
considerably slower.

I am not pushing this idea because I invented it.  I only found out
about it through this list.  I worked from instructions by Richard
Schwartz and materials obtained from Tom Moulton at gotgrit.  I push the
idea because it works!  It works very well.

A probable advantage of dental stone is it is probably less likely to
have sand or other gritty stuff that might scratch glass.  It is soft
enough that you don't need to worry if it comes in contact with the
glass during grinding.  By that, I don't mean whacking the glass with
the tool, but the tool wearing down so that dental stone is being
abraded by the grit slurry against the glass.  This won't produce
scratches.  At least one list member has written that there really isn't
any need to recess the channels between tiles, since the grinding action
will slightly recess the softer dental stone automatically.  (Despite
it's name, dental stone is softer than both mirror glass, and the
porcelain tiles you should be using for your tool.)


> Temperature does indeed make the stone set faster but even making
> a tool with 50deg. water doesn't extend the time by very much,
> making the setting time about 40 minutes or so compared to a
> 70deg. water.

This could be important for somebody trying to make a larger tool.
Starting with cold water could give more working time.  Even for an 8
inch tool, you have to mix pretty quickly to get the stuff well mixed in
time to get it in the mold before it starts to set up.  Don't worry, the
 setting reactions are exothermic.  The dental stone will get plenty
warm enough as setting proceeds.  An 8 inch by about 1.2 inch tool gets
hot enough to be a bit uncomfortable to hold.

As far as putting it in an oven goes, keep the temperatures down.
Calcium sulfate hydrate (the major component in cured gypsum products)
loses water and turns back to powdery anhydrous calcium sulfate at
rather low temperature.  If you are making a grinding tool, don't bother
putting it in an oven at all.  Drying it out doesn't improve the
strength much.  The curing reaction requires water.  If you are making a
lap base to stick pitch to, then it does need to be dried.  Just set the
oven at the lowest setting, or, if the weather is warm and sunny, set it
out, on top of a towel in the sun (omit the Coppertone).  The towel will
wick some of the excess water out and the warmth of the sun will help
with the rest, while not risking getting too hot.

Mark Holm
mdholm@telerama.com

_______________________________________________
ATM mailing list http://www.atmlist.net/