[ATM] Ultra Thin blanks, Making the mold

[vorblesnak@peak.org]

I had made a couple of slumping molds out of plaster and was prepared to
send those off to various kind souls with kilns who had offered to slump. 
Alas! plaster will not take the heat, as I was warned, and as I proved to
myself.  So it was back to square one.  I opted to make the molds out of
ceramic.  I used a cheap casting slip for the slumping mold, but I am
ahead of myself.

To begin I made two more disks of plaster to create casting molds.  These
were made by pouring plaster into a plastic mold.  The mold was a 'planter
diaper', bought at the local variety store.  They are $1.99 each and are
18 inches in diameter.  I allowed these to dry for a week, then then
turned them out and baked them at 300 degrees F for four hours and allowed
them to cool. Tapped with a hard object they ring.

Now basic r squared over two R.  I wanted a sixteen inch diameter F5
mirror blank.  This would require an 84 inch focal length.  Sagitta is
about three sixteenths of an inch.  I used the plaster disks just as I
would a glass pair and ground out a plaster blank.  If the mirror is three
sixteenths inch deep,  the tool is three sixteenths humped.  Using an old
hole saw I hogged out the middle of the mirror blank and hogged off the
edge of the tool blank.  Now it was just a matter of rubbing the two
together and smoothing the curve.  Once this was done I checked the curve
using a straight edge across the diameter and a caliper. I was within a
sixtyfourth and considered that good enough.

Now I took the mirror blank disk and wrapped a strip of paper around it
and poured in the slip.  This has taken a month to fully dry, though I
took it out of the mold after a week.  Next I used spray adhesive to stick
sixty grit sandpaper to the plaster mold and smoothed the curve of the
cast ceramic slumping mold.

I have made two of these now in case one breaks while firing.  I had an
old mold from Mel Bartels and that one I reduced the curve and slumped the
eighteen inch blank.  So I have one ultra thin blank to grind.

The last word right now is glass.  The last piece of borosilicate, (Pyrex)
I bought cost me $150.00 and I broke it. OUCH!  However, I found old
microwave oven turn tables at Goodwill for $1.99 each and they are in the
twelve to fifteen inch diameter range.  These are Pyrex so thin blanks
might be cheap.

I have three waiting for the molds to fire, so hopefully I will get some
more glass slumped soon.  Grinding is next.

David Davis
Toledo, OR  97391

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