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Basic Plaster Craft Instructions
Part II - Mixing Plaster

By Bob Sherman

The most critical part of plaster casting is mixing the plaster. When mixed properly plaster pours smoothly, hardens rapidly, and attains the maximum possible hardness when dry. When not mixed properly many problems may occur.

Note: This is page 2 of a multi page article. If you arrived here from a direct link please click here to start with part 1 of this series.

PLEASE NOTE!! - Plaster crafting is fairly safe if you observe these safety rules when mixing plaster. Once hardened there is little or no hazard from handling plaster items:

Plaster is very alkaline and and exposure to high dust levels may irritate the skin, eyes, nose, throat, or upper respiratory tract. Wear a dust mask, eye protection, and rubber gloves when mixing plaster.
Do not wear contact lenses when working with plaster.
Plaster generates a lot of heat when setting - never use plaster to make casts of body parts.
Keep away from children or pets.
Other safety precautions may apply depending on the plaster product you use - read the directions.

Using A Scale
Although it adds expense to the startup costs of making plaster crafts, a scale is well worth the investment if you plan to make a lot of castings. By weighing the water and plaster, you will get perfect results every time saving lots of time and money by not making any bad mixes. For those with only a casual interest in plaster craft or looking to make a single project a scale is an unnecessary expense if you are willing to guesstimate the amount of plaster needed.

Working With Plaster
Most plasters mix at the rate of approximately 70 pounds of water to 100 pounds of plaster. There is some variation between manufacturers, however 7 to 10 is an easy ratio to work with and the slight difference should not affect most work. If in doubt, you should follow the manufacturer's directions. Most of us will not be mixing 100 pounds at a time so it is easier to use the same ratio but in ounces - 70 ounces of water to 100 ounces of plaster. That still yields about 7 pounds which is too much for most molds so you will often need to break it down further.

Using A Scale - Determine how much water the mold holds as in step 6 below. Divide the weight of the water by .7 (decimal point seven) to obtain the weight of plaster needed. For example 20 ounces of water divided by .7 = 28.6 ounces of plaster. This could be rounded off to 29 ounces.
Without A Scale - Take the water in your mixing bowl and slowly sprinkle plaster into it until it mounds above the surface. This will take some experimenting but the approximate amount is 1/2 inch mound for small amounts and a 1 1/2 inch mound for larger amounts.

Working With Hydrocal
Most hydrocals mix at the rate of approximately 40 pounds of water to 100 pounds of hydrocal. There is some variation between manufacturers, however 4 to 10 is an easy ratio to work with and the slight difference should not affect most work. If in doubt, you should follow the manufacturer's directions. Most of us will not be mixing 100 pounds at a time so it is easier to use the same ratio but in ounces - 40 ounces of water to 100 ounces of hydrocal. That still yields about 7 pounds which is too much for most molds so you will often need to break it down further.

Using A Scale - Determine how much water the mold holds as in step 6 below. Divide the weight of the water by .4 (decimal point four) to obtain the weight of plaster needed. For example 20 ounces of water divided by .4 = 50 ounces of hydrocal.
Without A Scale - Take the water in your mixing bowl and slowly sprinkle hydrocal into it until most of the water is absorbed. This will take some experimenting but with experience you will get a feel for about how much is right.

Terminology
In the step by step parts of this tutorial the casting material is referred to as plaster for simplicity, although the same steps apply if using hydrocal unless otherwise noted.

Step 6 - How Much Does It Hold
To determine how much plaster your mold will hold, fill it with water. That is the exact amount of water you will need for that mold. It is a good idea to add an extra ounce of water for small molds and several ounces for very large molds. If using a scale, weigh the water. If mixing by guesstimating pour it into your mixing bowl.

Important: Use cold water. Warm water speeds hardening and may actually cause the plaster to harden before you get the mold poured. If you find your hardening time is too slow, experiment with warmer water to speed it up.

Step 7
Plaster should always be added to water - never add water to the plaster. Sprinkle it in slowly to allow it to absorb the water. Never just dump the plaster in - this will inhibit the water absorption.

Step 8
Allow the mixture to sit undisturbed until the plaster has absorbed water and all bubbling has stopped. This should take no more than 2 minutes.

Step 9
If you are coloring your plaster the pigments should be added at this point.

Step 10
Use a blender in a power drill or a potato masher to mix thoroughly for approximately 1 minute. For very small batches you could mix with a stick.

Important: Blending speeds the hardening time so don't overdo it unless you want to speed things up.

Click Here To Continue To Part 3 -Pouring Plaster Molds

Disclaimer: The information presented here is accurate to the best of my knowledge and common plaster crafting practices as of the time of this writing - September 2006. The author and the publisher accept no liability for the use or misuse of any of the information presented in this article. This article is presented for informational purposes and is used at your own risk.

Author: Bob Sherman

Publisher: Bobby's Craft Boutique Inc.

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Basic Plaster Craft Instructions Part II - Mixing Plaster

Basic Plaster Craft Instructions
Part II - Mixing Plaster