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Mold Making Video: Making a One-Piece Plaster Mold

Posted By Holly Goring On September 26, 2008 @ 10:36 am In Daily,Features,Making Ceramic Molds,Video | 9 Comments

 

Hey everyone, Sherman Hall here (I’m the guy on the left at the top of the page). I have three things to announce today; First, our managing editor, Jennifer Harnetty (the one in the middle), just went on maternity leave, so a big CONGRATULATIONS goes out to her and hubby and the new one.

 

This brings us to the second announcement, which is that you’re stuck with me and Bill (you guessed it—he’s the one on the right) for a few months. Don’t worry, this scares us as much as it scares you, so we came up with announcement number three: our invaluable assistant editor, and resident plaster whiz kid, Holly Goring, made a video on making a one-piece plaster mold, which will surely save us—and you—from video withdrawal.

 

Holly demonstrates how to make a simple one-piece plaster mold using a thrown clay tumbler. Of course, you could make all sorts of things besides tumblers, but it’s a nice basic shape to start with, and once you make one, the possibilities will become endless. If you’ve never made a mold, this is a great way to get started. We’ve included instructions below for mixing plaster, as well as a plaster mixing chart, at the bottom of this feature. But first, here are the step-by-step instructions from Holly’s video for planning, preparing, pouring and finishing a one-piece plaster mold:

 


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1. Make a form with no undercuts.


Make a simple form out of clay with thicker walls than a finished piece would. The extra mass will help the form stay put when plaster is poured in and will allow it so soak up some of the water in the plaster without collapsing. Your form must have no undercuts. An easy way to check for undercuts is to place the form upside down on a flat surface, look at it from above and run a finger up the side. If at any point the tip of your finger goes behind part of the form, it is undercut and will not release from a mold. Let the form get leather hard so it will be sturdy enough to have a mold made from it.

2. Prepare the mold on a smooth surface.


A laminate (Formica) counter top provides a nice smooth surface for pouring plaster molds. It makes the top of the mold nice and even, and it releases from the plaster without the need for mold soap.


3. Prepare the form and cottle boards.


Wet the lip of your piece and adhere it upside down to the laminate work surface, then clamp the cottle boards together around the form, leaving about 1 inch on all sides between the form and the boards. Seal all joints with clay coils and press firmly to ensure that no plaster will leak out of the form. Seal all porous surfaces with mold soap (you do not have to soap the clay positive.

 

 

4. Mix the plaster.


See detailed instructions below, or go to the video archives to check out Holly’s “Plaster Mixing 101″ video.

5. Pour the plaster.


After mixing, tap the bucket on a hard surface to release trapped air. Pour the plaster carefully. Wherever possible, pour plaster carefuly into the deepest area so the slurry flows evenly across the surface of the mold.

 

6. Let the mold cure and dry.


When plaster sets, it heats up because of a chemical reaction. When it has cooled, it is safe to remove the cottles or forms—about 45 minutes to an hour after pouring. Molds must be dry before use. Drying molds properly promotes good strength development, uniform absorption and reduced efflorescence. Dry molds evenly. Don’t set them near a kiln where one side is exposed to excessive heat or the relative humidity is near zero. Place them on racks in a relatively dry location away from drafts.

Mixing the Plaster

Use fresh water.


The mixing water you use should be at room temperature or 70°F (21°C). If the water is too warm, the plaster will set too fast and vice versa. Use only clean, drinkable tap water or distilled water. Metallic salts, such as aluminum sulfate, can accelerate the setting time, and soluble salts can cause efflorescence on the mold surface.

Use fresh plaster.


Plaster is calcined, meaning chemically bound water has been driven off through heating. If the plaster has been sitting around in a damp environment, it will have lumps in it, in which case it is no longer usable. Pitch it. Use plaster that has been stored dry and is lump free.

Weigh out materials.


Do not guess about the amounts of plaster and water you’ll need. Once you start the mixing process, you do not want to go back and adjust quantities. To determine the amount you need, estimate the volume in cubic inches then divide by 231 to give gallons or by 58 to give quarts. Deduct 20% to allow for the volume of plaster, then refer to the table.

Mix the plaster.


Small batches of plaster can be mixed by hand. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream. Breakdown lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates—longer mixing times produce tighter and less-absorptive molds.

Add plaster to water.


Slowly sift the plaster onto the surface of the water. Do not dump the plaster or toss it in by handfuls. Adding the plaster shouldn’t take more than 3 minutes.

Soak the plaster.


Allow the plaster to soak for 1–2 minutes maximum. The soaking allows each plaster crystal to be completely surrounded by water and it removes air from the mix. Small batches require less soaking than large batches. If the soaking time is too short, it may contribute to pinholes; and if it is too long, it will contribute to fast set times, early stiffening and gritty mold surfaces.

Mix the plaster.


Small batches of plaster can be mixed by hand. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream. Breakdown lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates—longer mixing times produce tighter and less-absorptive molds.

 

Water to Plaster Mixing Chart

1 quart 2 lbs 14 oz. (1293 grams)
1 1/2 quarts 4 lbs. 4 oz. (1,937 grams)
2 quarts 5 lbs. 11 oz. (2,585 grams)
2 1/2 quarts 7 lbs. 2 oz. (3,230 grams)
3 quarts 8 lbs. 9 oz. (3,878 grams)
3 1/2 quarts 10 lbs. (4,522 grams)
1 gallon 11 lbs. 6 oz. (5,171 grams)
1 1/2 gallons 17 lbs. 2 oz. (7,756 grams)
2 gallons 22 lbs. 13 oz. (10,337 grams)
2 1/2 gallons 28 lbs. 8 oz. (12,923 grams)
3 gallons

34 lbs. 3 oz. (15,508 grams)

 
The above table is based on USG® No. 1 Pottery Plaster mixed to a consistency of 73 (73 parts plaster to 100 parts water) recommended for most studio applications. Excessive water yields a more porous but more brittle mold, and less water means a very dense, hard mold that will not absorb water.
 
Sources: United States Gypsum (USG) Company and Clay: A Studio Handbook, by Vince Pitelka, published by The American Ceramic Society, 2001.
 
For great mold making techniques, be sure to download your free copy of Ceramic Mold Making Techniques: Tips for Making Plaster Molds and Slip Casting Clay, Volume II.

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