I keep having small chips appear in plaster cast components. Aside
from the obvious stop being so careless, I have tried preparing with
thinned-down blacks of various flavours but it does not seem to
penetrate the surface sufficiently. Has anyone experimented with
ink or anything else to see if it penetrates better?
Maybe too late for plaster already laid down, but I have a distant
memory of a friend adding some potassium permanganate into his plaster
mix to turn it a brown colour and prevent the white chip problem.
That's from forty years or so ago and I'm not sure how easy it is to
get small amounts of potassium permanganate these days, or whether
Health and Safety would even allow it.
Just found this
and it is good for athlete's foot as well :-)
On Mon, 6 Apr 2009 09:31:43 +0100, "Nigel Cliffe"
said in :
Yes, I thought about that, but I don't know what colouring will
avoid weakening the cast, and if you use acrylics the colours are
too washed out. I wonder if you can use clothing dyes in the mix?
Try googling on "plaster coloring materials" and "... colouring...". I
found a good hit first try, generally you should use earth colours
(water soluble formulations, I gather), as these are "lime proof." These
also happen to be the colours we need most for rock and masonry
castings, and such -- bonus!
You're right, additives of any kind affect the strength of the plaster,
but they also affect the setting time. Eg, from
Interesting question, and not one often asked! The hydration
reaction that results in the hardening of Plaster of Paris
proceeds very rapidly as your question infers. This reaction
can be slowed somewhat by the addition of several common
organic acids. Tartaric and citric acids (tartaric acid in
particular) have been used for this purpose, and are known as
Clothing dyes are generally organic salts, so they would slow the
setting time - don't know, never tried it.
IMO, the best bet to prevent chipping is to paint the casting with
acrylic paint, a thin wash to penetrate, followed by a creamy mix to
form a (pretty tough) skin on the plaster. That's what's worked for me,
anyhow. OTOH, I don't subject plaster bits to much handling. Water
colours and inks will also soak in, so you could first colour the
plaster with these, then add the acrylic paint.
Another consideration is the toughness of the plaster itself. Ordinary
plaster of Paris is pretty skimpy stuff. It crumbles easily, often
because it had insufficient water in the mix. PoP should be quite
shloopy, about the consistency of thick cream. I think people are often
afraid of making the mix too wet, under the impression that PoP dries -
it doesn't, it sets, and it needs plenty of water to do so. Dental
plasters, while pricey, are better materials for making plaster
castings. Over here, Hydrocal (tm), originally made for the plaster
molding trade, is a harder and stronger version of plaster, that
modellers use extensively for those reasons. I'm pretty sure that an
analogous product is available in the UK. Consult as plasterer.
I add dry powdered poster paint to my plaster mix. Here in New Zealand
the paint is sold by educational supply shops for use in primary schools=
It's very cheap. Only problem is it takes a lot of mixing to get it righ=
through.It takes only a very small amount. Haven't noticed any fading.