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Plaster mold, 1st time questions

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haleybach
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Plaster mold, 1st time questions

Postby haleybach » Sun May 12, 2013 1:13 pm

My daughter would like to include something made from glass in her 3-d AP art portfolio.

We have been reading here but still have a few questions.

So to practice she wants to start simple. She wants to make a small piece, maybe 5x5x3. She will model a positive out of clay. Put it in a cardboard box and pour plaster over it. To make an open face mold.

Any issues with using clay as her model? Should she seal it or something before she uses it (might it contaminate the plaster)?

She has done some research on types of plaster, and thinks she needs silica flour.

Mix by weight. She does not have anything to mix it with, and was thinking she could just mix it by hand because it is not too large an amount.

Questions:
Does she even need the silica flour at this size and for an open face mold?
Should she prefire the plaster mold?
Any other advice?
Anyone know of a good tutorial on this?

Morganica
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Re: Plaster mold, 1st time questions

Postby Morganica » Sun May 12, 2013 2:47 pm

This is a fairly complicated process so be prepared for some false starts. Best tutorials I've seen outside of books on the subject are at Bullseye on lost wax casting and box casting. Search their education site and you'll find them.

Big thing with using a clay model is that your daughter must be able to easily see and reach all parts of the model from outside the mold (to remove the clay). That means she should use an open-faced mold (i.e., the piece will have a flat back) with minimal or no undercuts and a negative draft.

IOW, have her design something that's solid clay, widest on its flat bottom with no really deep textures or thin, outstretched parts. The closer you get to truncated pyramid shape, the easier it will be to cast. The clay must be wet and soft--if it hardens this method isn't suitable.

The BE guides will explain about plaster and such--you'll probably want to make the base of the clay model thicker than you actually want, to accommodate extra glass. Size-wise, the model needs to be smaller than you might think--molds need at least an inch, top and bottom, for air circulation, the plaster adds another couple of inches and the glass reservoir can need as much as half the height of the model. So if your kiln is, say, 12 inches deep, the model shouldn't be much more than 6 inches tall.

When the model's done you can FIRMLY attach it to the bottom of a box you construct (I make mine out of roofing tarpaper), or one you don't mind sacrificing (I have used shoeboxes lined with plastic wrap, works fine). Spray your model with hairspray, quilt basting spray, or spray adhesive--this helps the plaster to stay put. Draw a line on the box that's at least 3/4 inch above the tallest part of the model. Then put on your safety gear--gloves and a a filter/mask, because plaster/silica mix is nasty stuff to breathe.

It's easiest to sift the mix over the water until you've got the entire weight in the bucket, then let it slake for 5-10 minutes, to absorb the water and start changing. Once it's slaked, it's easier to mix by hand--do it thoroughly for several minutes, make sure all the lumps are gone.

When the mix is ready, pour it into the box, in the lowest corner. DO NOT pour it over the model--that tends to trap air bubbles. Let it rise naturally, engulfing the model, until it reaches the mark you made on the box. Vibrate/jiggle/bump the box gently for a couple of minutes to help the air bubbles rise. (again, just follow the BE directions)

When it's time to remove the clay, use wooden tools if you can (less chance of scraping the plaster). Start from the center and scrape out the clay, bit by bit, being careful not to get all the way down to the plaster. Eventually you'll have hollowed out the model so that you're left with a clay shell. Usually, if you pry up one side of the edge you can roll and fold the entire piece and pull it out without hurting anything.

Clean every single trace of clay out of the mold--it will bisque-fire into your glass, look really bad and potentially cause cracking.

Hope that helps.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
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"I wrote, therefore I was." (me)

haleybach
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Re: Plaster mold, 1st time questions

Postby haleybach » Sun May 12, 2013 3:37 pm

Thank you so much, that is super helpful. She had not considered what shapes might be most successful.

She has the whole summer to work out the process.

Jim Murphy
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Re: Plaster mold, 1st time questions

Postby Jim Murphy » Sun May 12, 2013 4:19 pm

I think Cynthia normally uses water-based clay for her sculpture.

Personally, I'd recommend your daughter use an oil-based clay or plasticine for her model. [I use Poly Plasticine from Polytek myself.]
For oil-based clay, your daughter need not be concerned with clay drying out [cracking] prematurely while sculpting her creation.
Prior to pouring the plaster mold, just spritz the clay model with a 1/2-1% solution of Liquid Ivory Detergent to act as a surfactant and release
any surface air bubbles for a nice smooth bubble-free plaster surface.

As always, good luck!

Jim

Morganica
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Re: Plaster mold, 1st time questions

Postby Morganica » Mon May 13, 2013 11:56 pm

I use both, actually, and I agree that the biggest of several advantages to oil-based clay is that it doesn't dry out. Also that the consistency/workability tends to stay the same as long as you keep it at a warmish room temp, where earthen clay consistency changes almost constantly.

However, it's a heckuva lot more expensive than earthen clay so you'll wince more when you toss out a plaster-contaminated mass of it. It's a bit harder to work and certainly harder to smooth and texture if you're not used to it. It tends to leave a residue in the mold which can leave schmutz on the glass if you don't remove it, and it really stinks if you're working with it in enclosed spaces. It also tends to need a little more equipment to work properly--clay oven to soften it and some way to warm your tools if you're shaping. If you make the mistake of letting it get hot in the mold, good luck digging it out--it can bake in and become cement-like.

It also softens and melts. A friend of mine, a moldmaker, received a 12-foot tall sculpture of a cowboy a couple of Julys ago. It was oil-based clay too tall to go into her regular workroom so she had to leave it in her (unairconditioned) storage room, where it was about 90 degrees. It took lots of dry ice and fans to keep it from sliding down to a puddle.

I tend to use it when I've got a sculpture that will take awhile to build, one that I want to keep coming back to every so often. Otherwise, I prefer earthen clay.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

Jim Murphy
Posts: 28
Joined: Mon Mar 10, 2003 12:39 pm
Location: IL

Re: Plaster mold, 1st time questions

Postby Jim Murphy » Tue May 14, 2013 8:30 am

Morganica wrote:... oil-based clay ... tends to need a little more equipment to work properly--clay oven to soften it and some way to warm your tools if you're shaping. If you make the mistake of letting it get hot in the mold, good luck digging it out--it can bake in and become cement-like.

Hi Cynthia, to soften my Poly Plasticine [from Polytek] I simply heat water to ~110ºF, pour heated water into a Little Playmate [or other lidded insulated container], put
chunks of used clay or new clay bars inside a thin plastic grocery bag [to prevent clay from getting wet/smooshy], then submerge the bag-of-clay into the heated water for ~4 minutes and it's ready to go! Clay stays warm and pliable for ~3-4 hours easily. No special clay oven required. I understand there are many types of oil-based clay out there, but, give it a try.

IF any small bits of clay stick to plaster, it can be easily removed with isopropyl alcohol. I haven't had that problem though when first spritzing the clay model with Liquid Ivory Detergent solution prior to pouring plaster.

Happy sculpting! :)

Jim

Morganica
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Re: Plaster mold, 1st time questions

Postby Morganica » Fri May 17, 2013 12:53 pm

Oil-based clay oven is actually pretty easy to make, and I like it better than the warm water/cooler method because it's not quite as messy, you can deal with relatively large volumes of clay, and you don't have to keep reheating the water for new clay. You get a banker's file box for a couple of bucks and one of those really cheap plug-in clamp lights (or any light with a metal reflector/shade), along with a roll of aluminum foil and ideally some reflective/metal tape.

Cut a hole in the box lid that's the same diameter as the edge of the light shade, and line the entire box with aluminum foil, securing it with the metal tape (you do the inside of the box lid, too). Then tape the light to the hole in the lid.

A couple hours before a sculpting session, plop a as much clay as you're going to need into the box, turn on the light, and close the lid. When you start, the clay will be nice and soft, and will stay that way throughout the session. If you need more clay, just dump it into the box.

I use plastilina for some things (mostly because I once bought a huge batch of it), but I've switched over to Monster Clay, the stuff that they use for special effects prototypes. It can be melted in a crockpot and poured into a silicone mastermold, which is a lot more efficient than pressing the clay into a silicone when you're combining shapes--IOW it's kinda like wax.

I'd rather not have the clay stick in the first place--it's too easy to miss something with all the detail I tend to put into my molds. I also tend to do a lot of silicones/poylurethanes, so I automatically coat my models with lacquer, and sometimes also with a release.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

Jim Murphy
Posts: 28
Joined: Mon Mar 10, 2003 12:39 pm
Location: IL

Re: Plaster mold, 1st time questions

Postby Jim Murphy » Fri May 17, 2013 7:54 pm

Morganica wrote:Oil-based clay oven is actually pretty easy to make, and I like it better than the warm water/cooler method because it's not quite as messy, you can deal with relatively large volumes of clay, and you don't have to keep reheating the water for new clay. You get a banker's file box for a couple of bucks and one of those really cheap plug-in clamp lights (or any light with a metal reflector/shade), along with a roll of aluminum foil and ideally some reflective/metal tape.

Cut a hole in the box lid that's the same diameter as the edge of the light shade, and line the entire box with aluminum foil, securing it with the metal tape (you do the inside of the box lid, too). Then tape the light to the hole in the lid.

A couple hours before a sculpting session, plop a as much clay as you're going to need into the box, turn on the light, and close the lid. When you start, the clay will be nice and soft, and will stay that way throughout the session. If you need more clay, just dump it into the box.

I understand artists are a free-spirited bunch, but, as an Electrical Engineer who worked many years for Underwriters Labs [UL] I feel compelled to explain that even IF the plug-in
clamp light [or other light fixture] is UL Listed, it most certainly wasn't investigated and tested for safety while installed in an enclosed metal box where the light fixture and wiring may be exposed to temperatures beyond their ratings and intended use ... possibly leading to loss of life, property, etc. :shock:

Be safe out there! Pay attention to all those WARNING/CAUTION markings and safety instructions provided with Listed lamp fixtures.

Jim

Morganica
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Re: Plaster mold, 1st time questions

Postby Morganica » Fri May 17, 2013 8:51 pm

Well...possibly, but it's not IN an enclosed metal box. It's outside the box, looking in, and the box is foil-lined cardboard. And you have it on while you're working the clay, not all the time (i.e., unsupervised).
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

Jim Murphy
Posts: 28
Joined: Mon Mar 10, 2003 12:39 pm
Location: IL

Re: Plaster mold, 1st time questions

Postby Jim Murphy » Sat May 18, 2013 9:13 am

Morganica wrote:Well...possibly, but it's not IN an enclosed metal box. It's outside the box, looking in, and the box is foil-lined cardboard. And you have it on while you're working the clay, not all the time (i.e., unsupervised).

Well that's a little bit different. Just read and understand those lamp fixture WARNING/CAUTION marking labels and IMPORTANT SAFETY INSTRUCTIONS.

Good luck,

Jim

haleybach
Posts: 75
Joined: Tue Sep 04, 2012 1:59 pm
Location: Austin TX

Re: Plaster mold, 1st time questions

Postby haleybach » Sat May 18, 2013 1:05 pm

Ok, I'm not going to tell my daughter about Monster Clay just yet.
We have some modeling clay supplied by her art teacher, and I have some water based clay right now too.

Her early tests are looking awesome.

She has also tested making texture tiles out of clay, letting them get bone dry, and then firing the glass on them without first firing the clay. No seporator, just glass on bone dry mold. Worked perfectly, no sticking.

So. How much is 50# of plaster in terms of storage space? She wants a specific plaster that our local ceramic supply store sells only in 50# increments. I'm thinking plaster is pretty heavy so it will not be too bad.

Morganica
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Re: Plaster mold, 1st time questions

Postby Morganica » Sat May 18, 2013 1:30 pm

A 50-lb bag of plaster is about the size of a sofa cushion or very large pillow.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)

haleybach
Posts: 75
Joined: Tue Sep 04, 2012 1:59 pm
Location: Austin TX

Re: Plaster mold, 1st time questions

Postby haleybach » Sat May 18, 2013 5:29 pm

Morganica wrote:A 50-lb bag of plaster is about the size of a sofa cushion or very large pillow.


Excellent. Thanks.

S.TImmerman
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Re: Plaster mold, 1st time questions

Postby S.TImmerman » Mon Feb 02, 2015 10:50 pm

I just love this board , I googled my question and such wonderful artists chime in. Thank you !


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