Question:

How do I make molds?

Parabolic surfaced molds/wheel

Cool Trick! You can make a nice slump mold by centering a tub of plaster in its liquid stage on your wheel and spinning it at constant speed till it sets up. Spinning a liquid in a tub produces a parabolic surface and the faster it spins, the deeper the curvature is. If the speed is fast enough for the edge of the parabola to climb over the edge it will spill plaster all over the place! Try this with a tub of water until you are confident you won't spill it over the edges.

Mixing airless plaster

Mix. You want to make sure that you aren't adding air to your plaster, or you will get bubbles which will cause problems later. So if you use an electric mixer make sure to keep the blade deep in the plaster. Or, just take your hand (a glove is useful) and place it at the bottom of the bucket, and slowly move your hand back and forth across the bottom of the bucket. This slowly wets all the particles. When you can draw a line in the plaster and it doesn't immediately flatten back out, the plaster is ready to pour.

3-D Casting and Tooling

3-D Casting and Tooling

Moulding Plaster (Casting Plaster) - Often referred to as "casting plaster," moulding plaster is a good general purpose plaster where expansion control, hardness and strength are not of major importance.

White Art Plaster - Similar in working properties to Casting Plaster but with a surface hardening agent that reduces paint absorption.

Tuf Cal Plaster - A unique polymer-modified high early-strength fibered plaster. Provides higher green strength and greater chip and impact resistance than White Art or White Hydrocal. Ideal for hollow cast products.

Drystone - A fast setting gypsum cement that provides strong, durable casts without the need to dry the cast pieces. Drystone offers an environmentally sound alternative to resin-based products and casts with reduced breakage compared to conventional plaster and gypsum cement.

Fast Cast - Exterior gypsum cement. Fast setting cement formulated for exterior decorative statuary that can offer increased casting production for more than 300% versus regular Portland cement. Must be used with sand aggregate.

Gardencast - Designed for decorative outdoor statuary. It features low absorption, high strength and efficient casting characteristics.

Hydrocal White - A basic hydrocal gypsum cement used for statuary, glass casting and model making. It can be carved or added to.

Hydrostone - Used extensively for very durable statuary and 3-D casting. Especially suitable where high strength and resistance to water absorption are necessary. Also used for ram press molds.

Ultracal - A low absorption gypsum cement for case molds. Specially made for close tolerance tooling. Provides the greatest hardness, accuracy and freedom from efflorescence of any gypsum cement on the market.

Adding plaster(end)

Keep adding plaster, sprinkling it in the same manner. After a while the plaster will be visible for a few seconds before it disappears into the water. You are getting close... Keep adding plaster. When you add plaster and can count to 5 before it disappears, you have enough. Let it sit for 1-2 minutes to wet the plaster particles (this helps reduce air bubbles).

Water - Plaster

Add water to a bucket. It should be between cool and room temperature. You will learn through trial and error how much water to use, but most people tend to underestimate the amount of water needed. Two thirds water to one third plaster is a general rule of thumb. If you want to avoid cleanup, line your bucket with a plastic bag, then discard the whole thing at the end.

Canvas Molds/adding feet

You can cut holes in the canvas where you want the feet to go. That way you can attach the feet while the clay is still wet, and not have to worry about getting it off the sling at exactly the right time, when it is hard enough to hold the shape but still soft enough to add the feet!

Newspaper Molds

Newspaper
Another thing you might consider is making a form out of loosely crumpled and dampened newspapers. This allows a more loose look. Shape the damp, crumpled newspaper into the form you like, and cover it with plastic. The newspaper will dry and this hump mold will likely be useable for a while. If it starts to come apart you can squirt the newspaper with water and re-shape.

Larger molds

If you are pouring a very large mold, or for some reason didn't mix enough plaster in the first batch, immediately start mixing the second batch. When it is ready you can pour it over the existing plaster. Scratch crisscross lines into the top of the first layer before pouring the second layer to help them adhere together.

Plaster types

Glossary of Plaster Casting Materials

Ceramical Plasters

No. 1 Pottery Plaster - Formulated to provide stronger, longer-lasting ceramic slip molds. Standard of the industry.

Duramold Pottery - Compared to No. Pottery Plaster: 10% longer mold life, 28% higher wet compressive strength, 66% stronger dry compressive strength; makes more durable working molds, resulting in lower costs per unit cast.

Puritan Pottery - Slightly denser, longer-wearing mold material. Recommended for jiggering applications.

Ceramical - Low consistency, smooth-wearing mold material for use in ram pressing clay ware. Characteristics include low absorption, high strength, ability to purge easily, resistance to abrasion and wear.

Hydrostone ) - Often substituted for Ceramical in ram pressing applications. For more information see below.

Alternative

You don't have to mess with plaster to make molds. You can make your own from bisque. Fire the bisque at a high enough temperature to give it strength, but low enough that it is still porous so the clay dries and does not stick. To make a bowl mold, throw a solid piece on the wheel, smoothing the edges well with ribs, etc. When it is leather hard hollow out the inside until your edges are about 1 inch thick. Fire.

Test tiles DIY

You can make your own test tile chart just like your local studio by using small inexpensive molds and pouring your own tiles.
Or
Roll a slab and cut 2"x4" rectangles. Mark the rectangles with grooves, this gives you a guide to the glaze run. Cut a small hole in the corner, to make hanging easy.
Bisque.

Plaster additives

Additives

Nylon Fiber - May be added for strength.

Sodate Retarder - May be added in small amounts to delay the setting time up to an hour.

Dextrine - The addition of 2% to 4% will increase the surface strength and aid in resistance to abrasion of finished pieces.