stephani stephenson on tue 7 aug 07
hi Taylor
on the run here ,your post had lots of great
observations and notations
one thing about the standing water on top of poured
plaster
my hunch it is when there is excess water in the mix.
the plaster seems to expel it. or slough it up to the
surface as it atarts to 'go off'
i haven't seen it referred to in writing, just
something that occasionally happens , especially when
you eyeball the mixing proportions
especially if there is a little bit of water , just
blot the water off with paper towels once the plaster
begins to set
of you are way off, it may affect the plaster strength
at least in my observation.
when i weigh ingredients according to the proper
consistency of the plaster that doesn't happen
yeah and also you are right about catching the plaster
in time with regard to the soft clumps versus hard
bits!
i mix for 4 minutes with pottery type plasters and 8
or more minutes for gypsum cements. also it is
important to let pottery type plaster slake for 4
minutes before mixing, and gypsum cements slake for 8
minutes...that means after you combine plaster and
water , don't touch it for that time before you
mix...it seems to make for that creamy deluxe, no
standing water, smooth smooth plaster
oh, gotta go..yes webbing ok too
sounds like you did great..
Stepahni
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Taylor Hendrix on tue 7 aug 07
Thanks Stephani,
Yep about the standing water. I confirmed it yesterday with my further
reading. It does reduce the strength but increases the absorption...up
to a point I reckon.
My next project may be a commemorative tile that will be molded for a
run of 7 only. I have no plasticine, but that shouldn't be a problem.
Sculpting the relief will!
Dave F. suggested like you to mix mechanically and that has done
wonders for my plaster work. My last pour was perfect. I'm calling for
a plaster calculator from USG today. :)
Hope you're having fun.
Taylor
On 8/7/07, stephani stephenson wrote:
...
> one thing about the standing water on top of poured
> plaster
> my hunch it is when there is excess water in the mix.
> the plaster seems to expel it. or slough it up to the
> surface as it atarts to 'go off'
...
John Rodgers on thu 9 aug 07
Stephani,
An addendum to your comments --
Plaster has a crystalline structure that is exacting in its formation.
During the crystal formation, heat of hydration is released. Excess
water in the mix gets squeezed out as the crystals form because it is
excessive, and is not used in the crystal lattice formation. Once the
crystal lattice is formed, the excess water that has been squeezed out -
and usually floats on the surface - is re-absorbed, but this excess does
not become part of the crystal lattice, but rather is entrained in the
interstices between the lattice crystals. This free water eventually
migrates to the surface by way of capillary action and evaporates into
the air as the mold dries out. But the water that becomes part of the
crystal lattice remains as an integral part of the crystal structure.
When the dried plaster is heated sufficiently, the water in the crystal
formation can be driven off, breaking down the crystal lattice, and the
mold will collapse to powder form again. Might be a bit lumpy, but
theoretically, one could pulverize it again and re-use it. Not a very
practical thing, however.
Regards,
John Rodgers
stephani stephenson wrote:
> hi Taylor
> on the run here ,your post had lots of great
> observations and notations
>
> one thing about the standing water on top of poured
> plaster
> my hunch it is when there is excess water in the mix.
> the plaster seems to expel it. or slough it up to the
> surface as it atarts to 'go off'
>
> i haven't seen it referred to in writing, just
> something that occasionally happens , especially when
> you eyeball the mixing proportions
>
> especially if there is a little bit of water , just
> blot the water off with paper towels once the plaster
> begins to set
>
> of you are way off, it may affect the plaster strength
> at least in my observation.
>
> when i weigh ingredients according to the proper
> consistency of the plaster that doesn't happen
>
> yeah and also you are right about catching the plaster
> in time with regard to the soft clumps versus hard
> bits!
>
> i mix for 4 minutes with pottery type plasters and 8
> or more minutes for gypsum cements. also it is
> important to let pottery type plaster slake for 4
> minutes before mixing, and gypsum cements slake for 8
> minutes...that means after you combine plaster and
> water , don't touch it for that time before you
> mix...it seems to make for that creamy deluxe, no
> standing water, smooth smooth plaster
> oh, gotta go..yes webbing ok too
> sounds like you did great..
> Stepahni
>
>
>
>
>
>
> ____________________________________________________________________________________
> Be a better Globetrotter. Get better travel answers from someone who knows. Yahoo! Answers - Check it out.
> http://answers.yahoo.com/dir/?link=list&sid=396545469
>
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