Plaster Notes

Notes on the production of plaster molds for cast ceramic production.

By: Raymond Gonzalez

Step 1 – Gather

When preparing to make plaster, it is important to begin by considering the amount of plaster that you will need. Always check the amount that is on hand (in inventory) prior to beginning your mold.

Things you will need to make your mold:

A completed mold blockwith coddle boards & Clamps
A double ended loop tool
Putty knife
Extra clay
Mold Soap & brush
Two buckets
Scale

Step 2 – Calculate

Begin by measuring the mold. You will want to calculate the cubic inches (in³) of the mold. The formula for cubic inches is L x W x H. You will generally want your mold to be 1 ½ – 2” thick.

Example:

A mold that is 6” x 8” and 2” thick will be 96 in³.

You will then want to calculate the amount of water and plaster needed to cast this mold. We will begin with the in³that we calculated above. The formula for finding how much WATER you will need is as follows: in³/81. This is the amount of water in quarts that you will need. Remember: A quart is ¼ gallon.

Example:

Your mold (above) was 96 in³.

96/81 = 1.19 <- this is the amount of water in QUARTS that you will need.

You then need to use the chart below to find the amount of water in quarts remembering that you will often need to round.

Plaster to Water ratios for Pottery Plaster #1
Water	oz
½ pint	11 oz	312 g.
1 pint	1 lb. 6 oz.	624 g.
1 quart	2 lb. 12 oz.	1,248 g.
1½ quarts	4 lb. 2 oz.	1,872 g.
2 quarts	5 lb. 6 oz.	2,497 g.
2½ quarts	6 lb. 14 oz.	3,021 g.
3 quarts	8 lb. 4 oz.	3,744 g.
3½ quarts	9 lb. 10 oz.	4,388 g.
1 gallon	11 lb.	4,992 g.
1½ gallons	16 lb. 8 oz.	7,488 g.

Example:

Your 1.19 quart mold can be rounded to 1 quart of water. You will need 2 lb. 12 oz. of plaster to cast half of this mold.

Step 3 – Gather

Gather water (room temperature or cooler) in to a bucket.

Hot water will cause plaster to set more quickly.

Gather plaster. Weigh in a separate dry bucket.

Step 4 – Combine

Sift plaster through hands or screen in to water. Do not agitate.

ALWAYS add plaster to water.

NEVER add water to plaster.

Step 5 – Settle

Once all of the plaster is added to the water, allow to settle exactly two minutes.

Step 6 – Agitate

Agitate the plaster with your hand or when mixing large quantities of plaster, a mechanical agitator may be used. You want to agitate until plaster forms an opaque coating on your finger. You do not want it to become firm in your bucket.

Step 7 – Pour

Pour your plaster in to the mold. Pour in one continuous pour from beginning to end. Do not move the plaster stream about your object.

Step 8 – Clean

Clean your bucket. Draw a bit of water in to your bucket and slosh around. Do not pour down the drain as this may clog a drain. Pour in to a designated plaster containment bucket.

Step 9 – Wait

You will need to wait until your plaster has hardened. During this process, a chemical process will happen where the mold will heat up. This is normal. You will need to wait until the mold cools before continuing to Step 10

Step 10 – Turn Mold

At this point you will need to flip the mold and remove the blocking clay. Resist the temptation to remove your object from the mold.

Step 11 – Key & Soap

At this point, you want to use the rounded end of your loop tool to carve keys. Do this by spinning the tool between the palms of your hands and pressing down toward the mold. Once your keys are cut, soap the mold. Use Tincture of Green Soap mixed with water to prevent the two halves of the mold from sticking to one another. Apply several liberal coats to the plaster, allowing each to dry prior to the application of the next. Try to avoid getting soap on your object as this may provide decreased clarity in the reproduction of the mold.

Step 12 – Reconstruct Mold Box

Rebuild mold box with coddle boards as you did when you originally did when making the mold block except this time, you want your coddle boards to surround your hard plaster mold.

Coil plaster and press around coddle boards where they meet the table and anywhere that plaster may leak when pouring.

Step 13 – Mix plaster as in Steps 4-7

Step 14 – Clean as in Step 8

Final Steps

When the mold is firm, remove coddle boards and clean them. Split mold using hands, rubber mallet, or wedges as needed. Remove object from mold. Put the mold back together and store together. Move the mold to a well-ventilated area and prop on boards to allow airflow. Allow to until all water is removed from the plaster. This could take several days to several weeks depending on the amount of relative humidity.

Terminology

Blocking – The act of building up clay that will ultimately block the plaster from coming in contact with the other portion(s) of the plug.

Cheese Hard – the state of plaster where plaster is about as hard as cutting a soft cheese.

Consistency – The ratio of water to plaster will constitute the usability of the mold. Molds that have greater plaster to water ratios will be hard and will therefore absorb less water. Too much water will make a mold that is brittle and unusable. This ratio is generally referred to as “Consistency” and calculated by the ratio of water per 100 parts of plaster.

Cottle /Cottle Board – Boards that are used in the process of making the board that form the outer boundary of the mold.

Key – a notch in a mold that allows mold alignment.

Plastilene/ Plasticine / Kleen Klay – a wax based clay, good for modeling, that has properties that prevent drying.

Plug / Model / Prototype – Often refers to the object being cast.

Surform – A shaver that allows cleaning of mold.

Vent – a hole or tube in the mold that allows either for the exhaustion of air or the exhaustion of slip when slip is dumped.

Step 1 – Gather

Step 2 – Calculate

Step 3 – Gather

Step 4 – Combine

Step 5 – Settle

Step 6 – Agitate

Step 7 – Pour

Step 8 – Clean

Step 9 – Wait

Step 10 – Turn Mold

Step 11 – Key & Soap

Step 12 – Reconstruct Mold Box

Step 13 – Mix plaster as in Steps 4-7

Step 14 – Clean as in Step 8

Final Steps

Terminology

Further Reading