Hi Charles
If I made a hand impression in plaster: can I make a cast of pewter
in that? Do I have to do anything to the plaster to get a good
release? Should I still dry the plaster or is waiting a few days good
enough? If I attached a nut to the top of the pewter while still hot
will it stick?
Thanks
Simone
If I made a hand impression in plaster: can I make a cast of pewter in that? Do I have to do anything to the plaster to get a good release? Should I still dry the plaster or is waiting a few days good enough? If I attached a nut to the top of the pewter while still hot will it stick?
I am going to put my sculptor hat for a minute.
If you want to use plaster of paris as a mold:
prepare a model without undercuts if you want to get away with one
piece mold, or things will get very complicated.
mix plaster to consistency of heavy cream. Minimum agitation
required or bubbles will result
First apply thin layer of plaster with a brush, and only then
pour the rest of it.
Let it set and remove your model.
Take your negative and soak in the water until no bubbles are
seen.
Mix another batch of plaster and in the same way, first brushing
thin layer and then pouring, cast another positive from your soaked
negative. No release required.
Refine you positive and when satisfied take another negative.
That is your production mold. For release you can use silicon
formulation, or even simple shampoo. Production mold must be
absolutely dry. Good variation is to add table salt to water while
mixing plaster to increase strength. tea spoon per quart should work
fine.
back to goldsmithing
Leonid Surpin
Hi Simone,
If I made a hand impression in plaster: can I make a cast of pewter in that? Do I have to do anything to the plaster to get a good release? Should I still dry the plaster or is waiting a few days good enough? If I attached a nut to the top of the pewter while still hot will it stick?
Yep you can, but if the mould is large I’d let it dry for a week,
then put it in the oven to get rid of any moisture therein.
To keep the mould stable I would put the mould in a bucket of dry
sand. This no only gives you some stability when pouring, but if your
mould cracks the pewter isn’t going to spill on the floor, it will
stop at the sand.
I’m thinking that a (steel or galvanised?) nut will not stick to the
top of your pewter. A better approach, imo, would be to drill and tap
a thread into the piece, then put in a machine thread with a little
Loctite.
Regards Charles A.
Should I still dry the plaster or is waiting a few days good enough?
It must be absolutely dry, you must heat it to 300 degrees F for an
hour or so to drive off the water. If you do not do this you can end
up with a face full of molten pewter. It is very dangerous to pour
any molten metal into a moist mold.
James Binnion
James Binnion Metal Arts
Ok. So what I’m hearing/reading IS…
If I make a mold of plaster…press a hand into it to make an
impression with NO undercuts…dry a few days THEN dry in oven to
350F for 1 hour…use a sand sink…I should be able to make a 1
off print in pewter…
Is that correct? …and thank you for the advice
Simone in Ottawa…pretty tropical here these days. Hard to believe
that it goes to -40C in winter when its +33C today and sunny
Since I cast both small (a ring, for example) and large pieces into
plaster molds compounded with sand, I have become aware that the
amount of time and the temperature used in burnout (for a wax model)
must relate to the size of the mold. A ring or pendant mold is
heated to better than 1000F. for three hours before pouring sillver
in my studio. A large mold for bronze may use 100 lb plaster, 50 lb
of fine sand and an appropriate amount of water and is heated to
1000F. for three days. As James Brinnon points out, pouring hot metal
into a mold that is not adequately dried is a very bad idea. At the
very least, you might expect to lose the mold and your work. I am
sending this out because I would not guarantee that 3 hours at 300F.
is long enough, depending upon the mass of your mold.
The rest of this is a bit off the thread but you might note that it
is better to pour your metal into a “hot” mold than of a cold one for
obvious reasons. Athough it works for pewter and lead, simple plaster
doesn’t well stand the time and temps I use. The fine sand, added to
my mix as a refractory material, makes the mold both mechanically
stronger and more resistant to thermal stress. I noted a series of
comments on Ganoksin a while back that said plaster of paris cannot
be used in jewelry casting because it is too weak and will lead to
breakage and personal injury. The statem ent was made that only the
commercial investment materials were safe. This is most definately
not true. If the waxes are properly prepped before investment, the
plaster and sand (or other refractory material) properly mixed and
their resultant molds properly fired, the molds are safde and give
good resolution (perhaps some of the commercial products give better,
but I have not seen this). In practicing this process since 1963,. I
have never had a jewelry mold break in burnout or in the centrifuge.
Large molds for bronze or aluminum sometimes form cracks no matter
what system you use, Runout of metal from the mold is controlled by
the metal flask into which the investment is poured and by partly
burying the mold in sand as has already been suggested. The slivers
and flashing formed as metal enters the cracks are easily removed
from the cast piece once the mold is opened.
As a friend who uses Ganoksin often notes, this is way too much
On the other hand, is most of what I own
these days.
I hope your foray into metal casting is successful. Pewter is easily
cast and you should be alright unless your mold is much larger than I
think it to be.
Gerald Vaughan
If I make a mold of plaster....press a hand into it to make an impression with NO undercuts....dry a few days THEN dry in oven to 350F for 1 hour....use a sand sink...I should be able to make a 1 off print in pewter.... Is that correct?...and thank you for the advice
Can’t see a problem with that. The only issue would be with surface
tension, and only if you’re using an open mould.
Regards Charles A.
Hi Charles
Can't see a problem with that. The only issue would be with surface tension, and only if you're using an open mould.
What do you mean by surface tension? …and what problems do you
foresee with an open mold?
Simone
Hi Simone,
Can’t see a problem with that. The only issue would be with surface
tension, and only if you’re using an open mould.
What do you mean by surface tension?..and what problems do you foresee with an open mold?
Metal in a molten state is a liquid, and liquids are subject to
surface tension. If you fill a glass to the brim with water, you can
go a little further due to surface tension, and the water will form a
minscus.
Metal is no different, when you melt a metal it will form a rounded
shape, until there is more volume to overcome surface tension,then
it flows to a point where surface tension comes back into effect. You
can demonstrate this by pouring pewter onto a firebrick, a small
amount will form balls, the more metal you add the ball will flatten
until it becomes an irregular shape with rounded edges.
When you are casting any metal it is still subject to surface
tension, and with an open mould (a one piece flat mould for example)
the back edges will rounded.
If you want sharp edges all around, then you make a two piece mould
that has a button, or a feeder. This button/feeder applies head
pressure to the casting, forcing the liquid metal into the corners of
your mould, and you will have quite sharp corners.
Of course if you do use a two part mould you will need vents (maybe
risers also) to allow the escape of air within the mould.
This may at first seem complicated, but I can assure you that it’s
not. I can either direct you to places on the web that outline these
processes and techniques, contact a friend in your local area to
help out (we are global… have mad scientist will travel;-) ), or
give you a book list.
Regards Charles A.
I am sending this out because I would not guarantee that 3 hours at 300F. is long enough, depending upon the mass of your mold.
I was thinking about small jewelry scale molds not larger items. The
key is that the mold reaches 300F as then the water will be gone. As
long as there is water present in the mold it will not reach 300F as
the escaping steam will continue to cool it till the water is all
gone.
The rest of this is a bit off the thread but you might note that it is better to pour your metal into a "hot" mold than of a cold one for obvious reasons. Athough it works for pewter and lead, simple plaster doesn't well stand the time and temps I use. The fine sand, added to my mix as a refractory material, makes the mold both mechanically stronger and more resistant to thermal stress. I noted a series of comments on Ganoksin a while back that said plaster of paris cannot be used in jewelry casting because it is too weak and will lead to breakage and personal injury.
When I was taught to cast in 1971 we mixed plaster wit silica flour
(very finely ground pure sand) and cristobalite powder ( a slightly
different form of finely ground silica) to make jewelry investment.
It worked fine, the first time I came upon premixed investment I
thought what a great idea but it was quite expensive compared to the
mix it yourself version. The commercial investments are mixed to
give very precise expansion while heating, repeatable setting times
and for consistency batch to batch. They also add wetting agents to
reduce bubbles on the waxes but they still are plaster and silica.
So mixing your own from plaster and sand will work, the finer the
sand the better the surface finish. We used about 30% plaster by
weight. How does that compare with what you are using? Too much
plaster will cause more cracking of the mold at typical silver or
gold casting temperatures.
James Binnion
James Binnion Metal Arts
Ah. I understand. I thought that it was another problem that I had
not anticipated.
Thanks so much. Will let you know how this goes.
Simone
When I was taught to cast in 1971 we mixed plaster wit silica flour (very finely ground pure sand) and cristobalite powder ( a slightly different form of finely ground silica) to make jewelry investment.
An obvious health warning here silicates in your lungs are not good,
so take precautions.
Regards Charles A.
An obvious health warning here silicates in your lungs are not good, so take precautions.
You are so right. Back in the early 70’s there was virtually no
occupational health procedures in place in the school art classes. We
worked with a lot of things that with good reason would never be
allowed into a school classroom today. This is possibly the biggest
hazard in casting, standard investment is about 70% silica and is way
too often ignored as a health hazard because of the years long delay
in seeing the negative effects of breathing it.
James Binnion
James Binnion Metal Arts
A few years ago i did a series of experiments on different mixtures
for in vestment of sculptlure. I was taught to mix plaster by adding
water until the mix was about as thick as heavy cream. In general, it
worked. When I had students in the studio, their cows and goats
apparently give vastly different thickness of cream compared to
mine. Weighing works better. By weight, I use one part water to two
parts plaster. When that is smoothly mixed, I add one part fine sand
and some perlite while still mixing.
The perlite is not weighed but an amount is added that is judged to
be 3/4 the VOLUME of the sand used. The plaster I use is Hydrocal, a
bit s ronger than the usual plaster of paris.
For jewelry casting I use about the same mix without the perlite. I
have had to increase the amount of water by a millitad (judgement
call on each batch) to make the mix flow better and slow the
setting. You are correct to note that finer sand gives bretter
detail. I sift my sand throuigh fine steel mesh to get a fine
fraction for the jewelry investment. It also helps to “paint” the wax
model with diluted dish detergent which is allowed to dry before
investment. This aids wetting of the wax which increases accuracy. I
am sorry that I can’t give tighter measurements, but, after all, this
is art and a bit of experience and judgement are always valuable. My
casting and investing shed is unheated except for the various
furnaces. Temperature (of the water) often has an impact upon the
investment materials as to setting time and perhaps viscosity and
flow (this is just my subjective opinion–no measurements)
I was also taught to ramp the temperature in burnout. On large
pieces wherein I had placed type K thermocouples I found as you
stated that the investment temp did not increase greatly w.hether I
ramped it or not until most of the H20 had left the mold. Then temp
went up to the point where organics (wax) were ashed. As a
consequence, I punted the ramping and proceeded to burn at full
tilt. Works just fine. Also works with the jewelry molds. Doesn’t
seem to compromise mold integrity as far as my experience goes.
Good luck–Gerald Vaughan