John Jensen on wed 26 sep 01
My tried and true method is to weigh and presift 11 pounds of fresh dry
plaster. Then sift it again into one gallon of cool (room temperature)
water. Once all the plaster is sifted into the water I reach in with one
hand and work out the lumps and stir gently to get a smooth even mixture.
As the plaster thickens I test it's readiness by dragging my fingers gently
across the surface. As soon as I can see trails from my fingers remaining
in the surface it is time to pour...wasting no time. I use 11 pounds of dry
plaster per gallon of clean water because it is a convenient amount for me
to handle. Fractions or multiples, of course will work just as well. I
think the proportion of plaster to water can affect to qualities of the
finished plaster and the proportions are somewhere in the middle of the
acceptable range.
Or so I believe. I'm not a plaster expert but the above method has created
a lot of plaster slabs which have been giving reliable service for ten years
or more.
John Jensen, Mudbug Pottery
mudbug@toad.net www.Toadhouse.com
Ron Collins on wed 11 sep 02
if you live in the US, then someone will give you good advice....here, I
can't get decent plaster and have to do without...it matters where you
live....
Janet Starr on sat 24 nov 07
Hello,
It seems that everyone else says that Pottery Plaster #1 is the best
plaster for molds (and it might be for slip casting). But in a tile
press it will eventually crack (at the most inappropriate time - when
you don't have time to make another one and let it set up and dry).
I did a series of tests - basically a line blend with Plaster #1 and
Hydrocal. My engineer husband measured the strength by breaking bars
of them. The result was that a 50/50 mix of Plaster #1 and Hydrocal
was much stronger, did not break in my tile press, and the clay did
not stick to it. (If the mold looks really smooth, I will sand it
lightly after it dries.) You need to carve it on the first day before
it gets too hard. Since I have been doing this, I have not broken a
new mold. My old Pottery Plaster ones keep breaking one by one. So
even if the manufacturer says not to mix their products, it does work.
Saves lots of frustration. I did this after I was just about in
tears with a large tile order and the molds kept breaking.
I was not trained in a ceramics school, so many times I will try
things that others would not do because they were told it would not
work. Sometimes a fresh perspective is good.
'Necessity is the mother of invention.'
Janet
--
Janet Starr
www.craftsmantiles.com
www.featuretile.com
featuretile@gmail.com
Richard Aerni on sun 25 nov 07
Janet,
You're right about necessity being the mother of invention. And the mixture
could in fact be just the right answer for your situation. There are
certainly any number of types of plaster, though, for many different needs.
I use hydrocal in my studio for making master molds, because it is not
absorbent and is very hard, preserving detail so that I can cast many
working molds out of pottery plaster #1, or else moulding plaster in future
times. I wouldn't use it in a working mold, but everyone's situation is
different.
A couple of comments and questions come to mind when reading your posting...
First, have you tried varying the mixing proportions of your plaster and
water when working with pottery plaster #1? The optimum ratio suggested by
US Gypsum for molding applications is 100 parts plaster to 70 parts water
(by weight). Decreasing the quantity of water would theoretically harden
the plaster more (or at least decrease porosity). As a for instance, US
Gypsum's optimum ratio for Hydrocal is 100 parts plaster to 42 parts water
by weight.
Second, pottery plaster's working life has been figured by the techies at US
Gypsum. I don't have their data at hand, and am working from memory, but I
recall they figured that a mold using Pottery Plaster #1 would get 75
pourings (don't know if the number is similar for pressing). But, fact is,
that the plaster loses its edge after numerous usages in a press mould
(softness being an issue) and the plaster loses strength as it absorbs
water. So, perhaps your working PP #1 molds that broke were just at the end
of their useful working life.
Third, in my experience tile-setting (which is analagous to your usage of
the tile press), any tile, no matter what it's modulus of rupture, or fired
strength, will break when weight is applied to it if it is not properly
bedded. Which makes me wonder whether your broken molds in the tile press
might have occurred because there was some discontinuity in the substrate
underneath the mold when the pressure of the press was applied.
Just a few things to think about.
Richard Aerni
Rochester, NY...off to cook a turkey now...today is my Thanksgiving...
On Sat, 24 Nov 2007 14:54:03 -0800, Janet Starr wrote:
>Hello,
>
>It seems that everyone else says that Pottery Plaster #1 is the best
>plaster for molds (and it might be for slip casting). But in a tile
>press it will eventually crack (at the most inappropriate time - when
>you don't have time to make another one and let it set up and dry).
>
>I did a series of tests - basically a line blend with Plaster #1 and
>Hydrocal. My engineer husband measured the strength by breaking bars
>of them. The result was that a 50/50 mix of Plaster #1 and Hydrocal
>was much stronger, did not break in my tile press, and the clay did
>not stick to it. (If the mold looks really smooth, I will sand it
>lightly after it dries.) You need to carve it on the first day before
>it gets too hard. Since I have been doing this, I have not broken a
>new mold. My old Pottery Plaster ones keep breaking one by one. So
>even if the manufacturer says not to mix their products, it does work.
> Saves lots of frustration. I did this after I was just about in
>tears with a large tile order and the molds kept breaking.
>
>I was not trained in a ceramics school, so many times I will try
>things that others would not do because they were told it would not
>work. Sometimes a fresh perspective is good.
>
Lynne and Bruce Girrell on sun 25 nov 07
Janet -
What water ratio did you use? PP #1 uses 70% water and Hydrocal calls for
45%. What did you end up using?
(and why not Hydro-stone or Tuf-Stone with their 10000 psi compressive
strength?)
> I did a series of tests - basically a line blend with Plaster #1 and
> Hydrocal.
Bruce Girrell
Ron Roy on sun 25 nov 07
Hi Janet,
How did he test for strength?
RR
>Hello,
>
>It seems that everyone else says that Pottery Plaster #1 is the best
>plaster for molds (and it might be for slip casting). But in a tile
>press it will eventually crack (at the most inappropriate time - when
>you don't have time to make another one and let it set up and dry).
>
>I did a series of tests - basically a line blend with Plaster #1 and
>Hydrocal. My engineer husband measured the strength by breaking bars
>of them. The result was that a 50/50 mix of Plaster #1 and Hydrocal
>was much stronger, did not break in my tile press, and the clay did
>not stick to it. (If the mold looks really smooth, I will sand it
>lightly after it dries.) You need to carve it on the first day before
>it gets too hard. Since I have been doing this, I have not broken a
>new mold. My old Pottery Plaster ones keep breaking one by one. So
>even if the manufacturer says not to mix their products, it does work.
> Saves lots of frustration. I did this after I was just about in
>tears with a large tile order and the molds kept breaking.
>
>I was not trained in a ceramics school, so many times I will try
>things that others would not do because they were told it would not
>work. Sometimes a fresh perspective is good.
>
>'Necessity is the mother of invention.'
>
>Janet
>
>--
>Janet Starr
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Janet Starr on sun 25 nov 07
How we tested plaster strength:
We made plaster bars 1/2" thick, 1" wide and 6" long. I made a 5 step
line blend of Hydrocal and Plaster #1. We put each of the five
plaster blocks on two end supports on a scale and broke the piece with
a hammer in the center and saw how many pounds the scale read at the
time of the break. I then did some carving in each type of
hydrocal/plaster ratio and saw which one would still release the clay.
The 50/50 mix was the best (and that makes it very simple).
Water Ratio:
I must admit to being a Neanderthal in plaster mixing and prefer the
right quantity of water for the size of the mold, one scoop of plaster
alternating with one scoop of hydrocal until it will not absorb any
more plaster. So the different amount of water needed by plaster or
hydrocal wouldn't matter to me. Just the look and the feel. It might
not be 100% accurate.
It is possible that I had reached the life of the plaster molds and
that they were going to break eventually.
There was nothing under the substrate of the mold to break the plaster
when pressing.
I have noticed a difference if the clay to be pressed was on the soft
side or the hard side. I won't use harder clay (rewedged) with my old
plaster molds anymore.
Since I have been mixing the plaster and hydrocal, I haven't had any
broken molds.
Janet
--
Janet Starr
www.craftsmantiles.com
www.featuretile.com
featuretile@gmail.com
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