Jeff Longtin on sat 14 sep 02
Now from the:
MORE INFORMATION ABOUT PLASTER THAN YOU EVER REALLY WANTED TO KNOW dept:
Ditmar makes a good point;
>As an addenda to Jeff's post....
>Age, AND how / where it's stored makes a big difference. A 6 month old bag I
>store in Hawaii will have different setting times from a bag stored in
>Arizona. Test first to be sure. .....Moisture / humidity is the culprit.
There are two types of plaster available to me up here in Minnesota, United
States Gypsum and Georgia Pacific. USG sells plaster in 100 lbs. bags and GP
sells plaster in 50 lbs. bags.
Due to a sore wrist and sore knee I prefer the GP plaster. Ever try to gently
set a 100 lbs. bag of plaster on a work surface? (my poor back!) Also, for
many years I had to climb two flights of stairs to reach my studio. (my poor
knees!)
USG bags have a plastic liner which protects the plaster much better than the
GP bags. The GP bags do not have plastic liners. Therefore the issue of age,
and moisture exposure, is much more critical with GP products than with USG
products. IMHO that is. This is also true if your supplier stores their
plaster in a cement floored room. Hopefully they're not putting the plaster
directly on the floor and hopefully your not the sap who just got the last
bags at the bottom of the pile. Then its almost a guarantee that your plaster
has already been exposed to moisture and will set very very quickly.
As Ditmar points out, however, sometimes you can use this to your advantage.
If you do run across a bag that is especially old, or exposed, you can
anticipate quick set times and use it accordingly.
On the flip side, once you pop open a bag it doesn't really matter if the bag
has a plastic liner or not. You've exposed the bag to air moisture anyway so
the clock is ticking regardless.
An aside. I've been making molds now for about 18 years. Ten years ago the
clay and ceramics shops in the area starting sending folks my way if they had
either moldmaking or slip-casting questions. (Seems they thought I had some
expertise in this area? hehehe)
About once a month, or sometimes once a week, I'll get a call from someone
who is having some sort of molding problem.
("Gee I just added the plaster to the water, mixed it up, poured it, and now
my mold is falling apart. What did I do wrong?")
Usually I'll start off asking a few questions "What is the temperature of
your water?" ("I don't know?"). What proportion of plaster to water did you
use?"("I don't know that either?") "How old is your plaster?" ("Don't know
that either?") and finally "How long did you mix the plaster?" (Don't know
that either?").
Usually by the end of this question period they realise that maybe if they
knew some of these things they wouldn't be having as many problems as they're
having. Other times it sounds like too much of a hassle so they hire me to
make their molds. ("You want how much money to make a mold!?")
What Ron/John say about glazes also applies to plaster: measure, measure,
measure. It really will guarantee you success nearly everytime.
Jeff Longtin
Complex Molds Made Easy
btw- CMME is not the name of my mold making company. Its just a phrase I came
up with to distinguish my mold making skills/service from the other mold
makers in mpls/st.paul. Seems the other guys (and gals) made really cheap
molds really fast. When their customers would call the ceramic/clay shops in
town to complain the shops would send the disgruntled customer my way. No, I
do not offer my services cheaply, but then I've also never had a customer
complain either.
Generally speaking though molding problems are not caused by the ingredients,
usually they're caused by poor mold design. That's one that took me 18 years
to perfect and that's the one I really can't teach people. (God knows I've
tried!)
Seeing the world in positive/negative is easy for some people, for some
people it isn't.
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