Lynne and Bruce Girrell on sun 15 apr 07
First, let me thank all those who wrote to me on list and off for their
assistance with my plaster mixing problem.
Indeed, my plaster was old. But I was able to make the plaster usable again
by calcining.
At first, I was a little shy about how hot I could make the plaster because
USG states that plaster calcines at 170 deg F and that I wanted to drive off
3/4 of the water but not all of it. I wasn't quite sure how I could
determine when I had driven off 3/4 of the water. So I set the oven to 170
and let it sit there for about four hours. That helped, but the set time was
still fast and, at four hours per batch, it was taking a long time.
After checking around some more, I finally found some info in the archives
discussing the upper temperature for calcining. While I never found a hard
number to go with, I found enough to boost my confidence to try the
calcining at 250 deg. I also dropped the time to two hours.
That seemed to do the trick. I kept my mixing time down to three minutes and
the plaster was quite pourable when I was done. Everything went pretty well
after that. The largest air bubbles that I had anywhere were about 2 mm and
I had very few of those, maybe two or three in a 15 inch square form, so I
can't complain.
Thanks all
Bruce Girrell
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Donna Kat on sun 15 apr 07
On Sun, 15 Apr 2007 08:06:08 -0500, Lynne and Bruce Girrell
wrote:
...........
>After checking around some more, I finally found some info in the archives
>discussing the upper temperature for calcining. While I never found a hard
>number to go with, I found enough to boost my confidence to try the
>calcining at 250 deg. I also dropped the time to two hours.
>
>That seemed to do the trick. I kept my mixing time down to three minutes and
>the plaster was quite pourable when I was done. Everything went pretty well
>after that. The largest air bubbles that I had anywhere were about 2 mm and
>I had very few of those, maybe two or three in a 15 inch square form, so I
>can't complain.
>
>Thanks all
>
>Bruce Girrell
One of the hardest things I had to make myself do was to follow the
instructions to soak the plaster before mixing (for the amount I was doing
it was a 5 minute soak). I can't tell you how much smoother and easier it
was to work with having done this. Every other time I had played with
plaster it was in a group setting and everyone seemed to think you had to
mix it up right away or it would set hard before you could mix it. This is
not true. It actually is going to set faster if you start mixing it up
before it has soaked. Donna
http://www.plaster.com/
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