hollow plaster cast? prediction of weight?

[Bernard Arnest]

Hi,

For cast plasters, is there a way to make them hollow? I know the
process with wax; if small, you can slush cast; if larger, paint it on
and then weld the sections back up.

Now, plaster probably sets too slowly for slush casting? The wax
freezes right up, the plaster won't. So if I paint it on in layers,
first a couple watery washes to get detail, then with a spatula to
spread on globs to get it 1"-2" thick (for a bust, say)? Then with my
two halves of a bust, rewet the edges and spread on just a paper-thin
layer of plaster to weld them together?


Secondly, predicting weight. If I hypothetically use 60 lbs in
clay, how much plaster will I use, if cast solid?



thanks!
~Bernard Arnest

[Billy Hiebert]

Post by Bernard Arnest
Hi,
For cast plasters, is there a way to make them hollow?

Slush cast or applied or both.

I know the

Post by Bernard Arnest
process with wax; if small, you can slush cast; if larger, paint it on
and then weld the sections back up.
Now, plaster probably sets too slowly for slush casting? The wax
freezes right up, the plaster won't.

You can speed up plaster setting by using warm water, rapid mixing, or
various additives.

So if I paint it on in layers,

Post by Bernard Arnest
first a couple watery washes to get detail, then with a spatula to
spread on globs to get it 1"-2" thick (for a bust, say)? Then with my
two halves of a bust, rewet the edges and spread on just a paper-thin
layer of plaster to weld them together?

Be sure the pieces are completely saturated with water, otherwise the
plaster applied to the seams will begin to set and the pieces will not
join flush.

Post by Bernard Arnest
Secondly, predicting weight. If I hypothetically use 60 lbs in
clay, how much plaster will I use, if cast solid?

Wet clay weighs about the same as wet plaster, about 100 lbs per cubic
foot. Dry plaster weighs about 70 lbs per cubic foot. So 60 lbs of clay
would be 60 lbs of wet plaster and 36 lbs when the plaster dried out.
Each type of clay or plaster would be different, I'm using pottery
plaster as an example.

Post by Bernard Arnest
thanks!
~Bernard Arnest

[Gary Waller]

according to the chart in "Mold Making Casting & Patina for the student
sculptor" by Bruner Felton Barrie (a book you should buy Bernie) 60 lbs of
plaster would be replaced by 50 lbs of plaster, mixed with four gallons of
water. 5 lb of casting plaster =130 cubic inches, 5 lb hydrocal=130 cubic
inches. 5 lb hydrostone=100 cubic inches. Moist clay 1 lb=16cubic inches.

A plaster caster would rinse out the rubber mold with mold soap (lauryl
sulphate detergent - not 'soap') to lessen bubbles. Make a small batch of
accellerated plaster (alum - aluminum sulphate), gently roll it around the
bust, and while still green, would mix fiber and plaster with plaster and
slap it up inside the bust. The fiber is traditionally hemp, it is still
available to the trade, chopped fiberglass is O.K. too. The untraditional
way, which I use, is to fill the bust with a foam filled plaster - but this
requires a special machine, etc. A easy way to accelerate plaster is to use
hot water instead of cold. You have to know the working time of the plaster
you are using and how to control this window. For a lifetime supply of mold
soap, find a store that sells horse supplies, buy a tub of "Orvus" soap - is
cleans the horse hair of dirt but leaves the oil in the hair.

Post by Bernard Arnest
Hi,
For cast plasters, is there a way to make them hollow? I know the
process with wax; if small, you can slush cast; if larger, paint it on
and then weld the sections back up.
Now, plaster probably sets too slowly for slush casting? The wax
freezes right up, the plaster won't. So if I paint it on in layers,
first a couple watery washes to get detail, then with a spatula to
spread on globs to get it 1"-2" thick (for a bust, say)? Then with my
two halves of a bust, rewet the edges and spread on just a paper-thin
layer of plaster to weld them together?
Secondly, predicting weight. If I hypothetically use 60 lbs in
clay, how much plaster will I use, if cast solid?
thanks!
~Bernard Arnest

...

[s***@tfb.com]

The mopdt important caveat of hollow casting is to NOT make mulitple
layers inside the casting.

Plaster expands when it sets... if you slush a thin coat and then
trowel on a thick coat, the thick coat will expand, cracking the
surface of your thin coat.


Paster can be cast hollow in several ways, but the trick is to use just
one mix of plaster.

The easiest way is to simply fill the mold with plaster, wait for the
palster to thicken to a paste consistency, and then simply scoop
plaster out of the middle of the cavity with your hand or a spoon. ( if
you can get a hand or spoon in there )

[Billy Hiebert]

Post by s***@tfb.com
The mopdt important caveat of hollow casting is to NOT make mulitple
layers inside the casting.

Hollow casting in layers is a pretty common practice.

Post by s***@tfb.com
Plaster expands when it sets... if you slush a thin coat and then
trowel on a thick coat, the thick coat will expand, cracking the
surface of your thin coat.

The theory is sound but it doesn't seem to happen for me. Maybe it's my
technique, your know, talking nice to the plaster. I think its more
evident in the harder, high expansion materials like White Hydrocal or
Hydrostone. They are more brittle so I suspect they can tolerate less
movement than the soft plasters. I use Ultracal or Hydorcal B-11 when
making strong molds or cast that require multiple layers. Even though
they too are brittle, they have low expansion.

Post by s***@tfb.com
Paster can be cast hollow in several ways, but the trick is to use just
one mix of plaster.
The easiest way is to simply fill the mold with plaster, wait for the
palster to thicken to a paste consistency, and then simply scoop
plaster out of the middle of the cavity with your hand or a spoon. ( if
you can get a hand or spoon in there )

Maybe Tom and Dan will give us some insight from there experience?

[battersby]

Post by Billy Hiebert

Post by s***@tfb.com
The easiest way is to simply fill the mold with plaster, wait for the
palster to thicken to a paste consistency, and then simply scoop
plaster out of the middle of the cavity with your hand or a spoon. ( if
you can get a hand or spoon in there )

Maybe Tom and Dan will give us some insight from there experience?

Christopher's technique is the one I have always used.
Although I don't use spoons, MIT there are probably some good ones readily
available.

Just about every single casting I produce is dug out to some extent..

Post by Billy Hiebert
The theory is sound but it doesn't seem to happen for me. Maybe it's my
technique, your know, talking nice to the plaster. I think its more
evident in the harder, high expansion materials like White Hydrocal or
Hydrostone. They are more brittle so I suspect they can tolerate less
movement than the soft plasters. I use Ultracal or Hydorcal B-11 when
making strong molds or cast that require multiple layers. Even though
they too are brittle, they have low expansion.

First of all you can't talk nice to plaster.
It will then understand that you are weak, and kick your ass.

You must take the bull by the horns, and control the situation at hand.

Expansion rates are an important factor, no doubt, and I do use a lot of
UltraCal,
although never for casting.

Billy, do you manipulate the use consistency (of UltraCal) radically for
casting applications??
Or just wait for the plasticity to stiffen up a bit??
I can't picture a slush casting with UltraCal.
--
Battersby.

T. M. Battersby
www.battersbyornamental.com

Post by Billy Hiebert

Post by s***@tfb.com
The mopdt important caveat of hollow casting is to NOT make mulitple
layers inside the casting.

Hollow casting in layers is a pretty common practice.

Post by s***@tfb.com
Plaster expands when it sets... if you slush a thin coat and then
trowel on a thick coat, the thick coat will expand, cracking the
surface of your thin coat.

The theory is sound but it doesn't seem to happen for me. Maybe it's my
technique, your know, talking nice to the plaster. I think its more
evident in the harder, high expansion materials like White Hydrocal or
Hydrostone. They are more brittle so I suspect they can tolerate less
movement than the soft plasters. I use Ultracal or Hydorcal B-11 when
making strong molds or cast that require multiple layers. Even though
they too are brittle, they have low expansion.

Post by s***@tfb.com
Paster can be cast hollow in several ways, but the trick is to use just
one mix of plaster.
The easiest way is to simply fill the mold with plaster, wait for the
palster to thicken to a paste consistency, and then simply scoop
plaster out of the middle of the cavity with your hand or a spoon. ( if
you can get a hand or spoon in there )

Maybe Tom and Dan will give us some insight from there experience?
--
Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com

...

[Bernard Arnest]

Thanks for the help! With the long answers, I feel obligated to ask
more questions to demonstrate that I digested what was said, but the
fact is that I have no more (yet) :-)


thanks again,
~Bernard Arnest

[Billy Hiebert]

Post by battersby
First of all you can't talk nice to plaster.
It will then understand that you are weak, and kick your ass.
You must take the bull by the horns, and control the situation at hand.

Maybe I should put it another way. I don't get into a fight or contest
with plaster, rather I get to know it and work with it not against it.
For instance, when painting a face coat on a vertical surface, it does
little good to apply it when it is too fluid, it will just run off. But
if you wait until it thickens a little, it will stay where you put it.

Post by battersby
Expansion rates are an important factor, no doubt, and I do use a lot of
UltraCal,
although never for casting.

What do you do with it?

Post by battersby
Billy, do you manipulate the use consistency (of UltraCal) radically for
casting applications??

I go pretty much by USG's directions, maybe a little more or less water
on occasions.

Post by battersby
Or just wait for the plasticity to stiffen up a bit??

Yep, and then be quick. As you know if you alter the ratios the finished
properties will change. I do cheat a little and try not to mix any
longer than necessary so as to extend the plastic state.

Post by battersby
I can't picture a slush casting with UltraCal.

If the mold is not accessible with a brush, then you have to rotate, and
the first coat is the most difficult. A non oily release will of coarse
keep roll off to a minimum.
I've done quite a few molds for plastic casting where the molds had to
be completely dry before use. I would use Ultracal for it's strength and
low expansion. In order to keep the drying time to a minimum, I would
make the molds 1/8 to 3/8 thick max. This would be done with one mix.
Most of these molds were small, less than two feet square. Any larger
would take multiple mixes, and would need a little reinforcement.

[Dan S]

What he's saying, I bet, is that UltraCal, like HydroStone, remains a fluid
until it takes a snap set. It is (it seems) unsuitable for slushing.
The great difficulty with joining halves is how tiny is the time window from
when the plaster stays where you plop it to when it won't readily squish
together/out of the way when halves collide. I prefer dumping+slushing to
join halves.
Dan

Organization: SBC http://yahoo.sbc.com
Newsgroups: alt.sculpture
Date: Mon, 28 Aug 2006 16:02:48 GMT
Subject: Re: hollow plaster cast? prediction of weight?

Post by battersby
First of all you can't talk nice to plaster.
It will then understand that you are weak, and kick your ass.
You must take the bull by the horns, and control the situation at hand.

Maybe I should put it another way. I don't get into a fight or contest
with plaster, rather I get to know it and work with it not against it.
For instance, when painting a face coat on a vertical surface, it does
little good to apply it when it is too fluid, it will just run off. But
if you wait until it thickens a little, it will stay where you put it.

Post by battersby
Expansion rates are an important factor, no doubt, and I do use a lot of
UltraCal,
although never for casting.

What do you do with it?

Post by battersby
Billy, do you manipulate the use consistency (of UltraCal) radically for
casting applications??

I go pretty much by USG's directions, maybe a little more or less water
on occasions.

Post by battersby
Or just wait for the plasticity to stiffen up a bit??

Yep, and then be quick. As you know if you alter the ratios the finished
properties will change. I do cheat a little and try not to mix any
longer than necessary so as to extend the plastic state.

Post by battersby
I can't picture a slush casting with UltraCal.

If the mold is not accessible with a brush, then you have to rotate, and
the first coat is the most difficult. A non oily release will of coarse
keep roll off to a minimum.
I've done quite a few molds for plastic casting where the molds had to
be completely dry before use. I would use Ultracal for it's strength and
low expansion. In order to keep the drying time to a minimum, I would
make the molds 1/8 to 3/8 thick max. This would be done with one mix.
Most of these molds were small, less than two feet square. Any larger
would take multiple mixes, and would need a little reinforcement.
--
Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com

...

[Billy Hiebert]

Post by Dan S
What he's saying, I bet, is that UltraCal, like HydroStone, remains a fluid
until it takes a snap set. It is (it seems) unsuitable for slushing.
The great difficulty with joining halves is how tiny is the time window from
when the plaster stays where you plop it to when it won't readily squish
together/out of the way when halves collide. I prefer dumping+slushing to
join halves.
Dan

I guess the size of the cast or mold has a lot to do with it. In may own
situation, my projects have been rather small, a few square feet at the
most, so the tiny window has not been a huge problem. It's been a while
since I've done more than one square foot, and I'm probably not quick
enough these days for anything much larger. I've never been fond of the
"squish" method for plaster or plastic. The extra thickness at the
parting plane, and mismatch are a problem for accurate work. It gets
even worse with complex parting planes and multi part molds.

[s***@tfb.com]

If you are casting in ultracal- then you are getting away with it
because Ultracal has the lowest co-efficient of expansion of any
plaster media. Its specifically formulated for low expansion.

For casting large plaster items hollow, I always make the mold with a
removable hinge pin ( usually galvanized electrical conduit)

Then lay the mold open and cast each half with a fractioned mix of
plaster so that I can create a putty coat on the surface, and back that
with a half inch coat of sisal re-inforced plaster with the second part
of the fractioned mix. ( there might be some wire in there as well if
needed)

Then I just clean and prep the separation lines for the two halves,
making sure I expose a good bit of sisal on both sides.. and seal them
with shellac.

Flop a fresh mix of plaster in its thickened state along that
separation line and then swing the mold shut on the hinge and clamp it
tight.

If there is an opening anywhere, I can reach in and with the same mix
of plaster I spread along the separation line I can re-inforce across
the joint with a narrow band of sisal.

Other places sometimes require "stapling" after de-molding... that is,
after demolding, cutting a deep groove into the surface perpendicular
to the flashing line, and dilling into the plaster/sisal mass on
either end of that groove.

I then bend a stiff galvanized section of wire into a "staple" the same
length and fit it into the groove so that I can patch plaster over the
now embedded wire.

This makes for a very durable and stong tie between separately cast
plaster halves.


but the cool thing is the hinged casing....
With a properly designed hinged mold, and a chain hoist, One man can
easily cast something the size of a horse.