Casting Plaster — Pottery Plaster for Moulds, Models & Positives

Primary uses

• Absorbent mould making
• Positive casting
• Support shell and pattern making

Applications

• Slip-casting moulds for ceramics and sanitaryware
• Positives cast from alginate and rubber moulds
• Mother moulds and support shells
• Patterns, models and studio casting

Advantages

• Absorbency makes it the default mould material for slip casting, where a non-porous mould simply will not work
• Low material cost per mould, which suits studios that replace moulds on a cycle
• Mixes and pours with basic equipment, so no reactive chemistry handling is required

Limitations to consider

• Moulds wear, soften and lose absorbency over a production life, so slip casters plan replacement as routine
• Cannot capture deep undercuts the way a flexible rubber can, which forces multi-part mould design
• Consistency, absorbency and hardness behaviour differ by supplier product and are stated only in their data
• Mix ratio, pot life, working time, demould time, full-cure schedule, hardness, tear strength, elongation, shrinkage, viscosity and temperature resistance are properties of one specific supplier system. They are never quoted here from memory or by analogy with a similar product; they are read from the supplier data sheet for the exact material offered and confirmed at quotation.

How to choose the right grade

• Start from the casting material and the number of parts the mould has to produce, because durability and detail pull in opposite directions
• Describe the pattern itself — its material, porosity, undercuts and whether it can be destroyed — since that decides the mould strategy before any product is chosen
• State the shop conditions the work happens in, including temperature, ventilation and whether the job is poured in a box or brushed on in place
• Separate absorbency-led work such as slip casting from hardness-led work such as master patterns, because they pull toward different products
• State how many pieces the mould has to produce before it is scrapped, since that decides whether alpha gypsum is the better buy
• Mould-making and casting behaviour depends on the pattern, the shop temperature and humidity, the mixing method and the operator. A production-representative trial on your own pattern and in your own shop conditions is the only honest way to qualify a material before a run is committed.
• Exact specification is confirmed against supplier documentation at quotation.

Typical grades

• Pottery plaster products intended for slip-casting moulds
• General moulding and studio casting products
• Products the supplier distinguishes on absorbency and consistency behaviour
• Buyers search this category by grade name, and those names describe what the workshop needs rather than what is asserted here. Which grade a specific offer corresponds to, and what the supplier declares for it, is read from the technical documentation issued for the exact material quoted.
• Materials in this category interact: rubbers, releases, sealers, resins, pigments and fillers can inhibit cure, stain a surface or shorten mould life when they are combined outside the combinations the supplier has tested. State every material already in the process at enquiry so compatibility is checked against supplier documentation rather than discovered on the shop floor.
• Reactive materials age in the container, and moisture-sensitive systems age faster once opened. Order against real consumption rather than against a discount, and confirm the shelf life the supplier states for the exact product and pack.