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:

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.

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.

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. Originally published in September 2006 and modified in April 2012. 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.

This article is provided free of charge for use.

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