Post by AkilliHello,
Is there some material which I can paint onto the surface of a
polyurethane mold, which will hinder the outer layer of a plaster cast from
setting up - hopefully in an uneven fashion - and can be easily washed off upon
de-molding? And, I should add, not degrade the mold at all.
Much Obliged,
Ethan Gross
I've already posted one of my techniques earlier: get some clear, not green,
polyvinyl alcohol mold release (PVA) - this is alcohol and water based, not
stinky or dangerous, brush on to mold surface, get sand and sprinkle onto
the wet, sticky surface. You can let it dry a bit, or even a lot, just be
careful pouring your plaster if it is still wet. Then demold and wash the
surface with a brush and water, and then let it dry. This is for an old
stone finish. I've even used this with polyester resin castings. It works
great and protects the molds too. The same mold can produce fine, medium or
heavy grained surface - depending on the character of the sand/aggregate you
add. I should point out that a lot of the sand remains stuck to the
plaster - it enhances the stone effect, but may not be the look you are
after.
If you want a real 'fractured' look, you will have to pour a 'face mix' in
the mold first - plaster with a retarder, 'soldate' is the very best,
available from Laguna Clay in small quantities. WHen it is starting to set,
pour in your unretarded batch of plaster, and it should set faster than the
retarded face, so the face will look like if you tried to demold before the
plaster set, but the piece will not break, etc. Another thought is to add
lime putty to the face mix, it really slows down the cure, but eventually
cures harder than plaster - you can work the surface while it is still
'green'. Lime and plaster are best friends.
There is another way too - but more for high production environment - where
you include air entrainment chemicals/ nuggets in the facemix, and they fizz
up, creating those travertine marble type effects. There is a company called
formglass who uses a proprietary technique along these lines. Maybe chunks
of 'alka seltzer' or those bath bombs. I made molds for a bath bomb factory
once, I forget the materials but still remember the stench of bargain
basement fragrances. I think bicarbonate of soda? Maybe procedure "A" with
seltzer chunks in place of sand - but it will start bubbling as soon as it
hits the water of the PVA.
I can think of a few more ways too. If I was to do this today, I would cast
in a foamed hydrostone or oxychloride cement, and then work the piece with
files, rasps and chisels.
So tell us Ethan - watchya makin? I still remember your wonderful 'magic
realism' 'surrealistic' painting.