Plaster Of Paris Versus Pottery Plaster – Ultimate Guide

Plaster has versatile uses for almost all potters. If you want to make damp boxes, most probably you will need to use plasters; Plasters are also required for mold making and casting. Yet, there are a lot of different types of plaster. Choosing the right type of plaster can be confusing.  

The main ingredient of pottery plaster is calcium sulfate, which is similar to chalk. Pottery plaster is used to create working molds for the slip casting of sanitaryware, tableware, or refractory castings. It can also be used for the production of working molds for jiggering in tableware or kitchenware. 

As you can see, there are some general differences between different types of pottery plasters. If you are thinking of which plaster to use and can’t choose the right one for you, you have come to the right place. In this article, I will speak about the differences between pottery plaster and regular plaster. We will also discuss the characteristics of the plaster of Paris and its alternatives. Finally, I will tell you all about making plaster of Paris from scratch at home. 

What is The Difference Between Pottery Plaster and Regular Plaster?

There are many different types of plaster. The most widely used plaster in pottery studios around the world is Pottery Plaster. This plaster is ideal for making slip-casting molds and other plaster castings where a high degree of water absorption is required.

Plaster of Paris is yet another popular type of plaster. Plaster of Paris is also easily shaped and formed but is quite weak in comparison to pottery plaster and therefore not a good choice for any working situation in a studio.

Pottery plaster is more solid than the Plaster of Paris. So, it is more suitable for casting molds, that are often exposed to damage that naturally and inevitably occurs as a result of normal wear or aging. 

Because of if’s high durability, pottery plaster is more expensive than other alternatives. For instance, plaster of Paris is usually less expensive and is suitable for low wear and tear items.  

Each type of plaster has its own consistency and this is what needs to be observed. The recommendation to sift the plaster into the water until it mounds on top may produce workable results, but it does not take full advantage of the characteristics of plaster as the ratio of water to plaster is not likely to be exactly right. It is always better to weigh out the water and plaster, after calculating the volume you need to fill. 

Whether it is pottery plaster or plaster of Paris (gypsum plaster, gypsum powder), their base ingredient is calcium sulfate, also known as gypsum. But pottery plaster is stiffer and more expensive than plaster of Paris. Where does this difference come from? 

Pottery plaster is made from alpha gypsum, which has a longer and more organized crystal structure. Therefore, pottery plaster is much stronger than the plaster of Paris, which is often referred to as beta gypsum. The crystal structure plays a major role in hardening the plaster.

Its higher strength makes pottery plaster an ideal choice for creating molds for sanitaryware and general casting applications.

What Is The Difference Between Pottery Plaster And Plaster Of Paris?

The gypsum crystal structure makes them distinct from alpha and beta. In contrast to beta gypsum, alpha gypsum has a longer and better-organized crystal structure. Alpha gypsum is created by heating gypsum (calcination) under pressure and partially dehydrating it. 

A process of heating under atmospheric pressure produces beta gypsum. Its crystals are shorter and less organized. Both processes result in differently-shaped crystals in plaster. The different ratios of alpha and beta gypsum produce different properties.

Therefore, the pottery plaster that is made from alpha gypsum can endure the wear and tear of prolonged use. As a result, you can use a case mold for more cycles of production. Pottery plaster molds can withstand rough handling and cleaning.

What Is The Purpose Of Plaster In Pottery?

For casting and molding, as well as making damp boxes, plaster is helpful in pottery. Plaster molds allow the water to drain from the slip (liquid clay) to facilitate hardening. Both plaster of Paris and pottery plaster are of practical use to potters. The plaster of Paris is easier to find and cheaper. Making molds for slip casting of sanitary ware, tableware, or refractory casting is one of the primary uses of pottery plaster.

What Type Of Plaster Is Used For Pottery?

For pottery, fine pulverized plaster with high absorption and no toxic properties is appropriate. 

As the wheel head turns, you have to flatten the clay onto the convex jigger mold. You use the template to press the clay into the concave jolly mold. To press the clay into them with the template, you have to apply some force. 

To make jiggers and jollys, therefore, it is essential to use tough plaster. In this regard, sturdy pottery plaster made from alpha gypsum is better suited than plaster of Paris.

What Are Plasters And Are There Different Kinds?

Plaster serves as a protective or decorative coating for walls and ceilings. You can also use plaster to make decorative elements by casting them in a mold. The main types of plasters are the clay, gypsum, lime, cement, and heat-resistant plasters.     

Either gypsum, lime, or cement are the main components of the most common plaster types. As a dry powder, you can mix it with water to make a stiff, workable paste. Hydrated plaster hardens as a result of the reaction with water. Plasters vary in their drying time, absorbency, and setting times.  

The plaster is more suitable for finishing than for load-bearing works. It is used in dentistry to make dental impressions and dentures (false teeth). Lime plaster is the traditional substrate for fresco painting. Ancient cultures created large figurative reliefs on walls with plaster. 

What Are The General Applications Of Plasters?

Detailing in the form of geometric and naturalistic demands on plaster for room interiors. False ceilings are available by converting the powder into a sheet to attach to the basic ceiling. For murals, plasters (stucco) serve as a supporting structure.  

Mortuaries and funeral directors use plaster to recreate tissue, fix severed limbs to corpses, and fill wounds. There is a vast use of plasters in medicine, including dentistry and radiotherapy. 

How Did We Come To Call It The Plaster Of Paris?

In Paris, there was an abundance of gypsum. During the seventeenth century, King Louis XIV of France ordered that buildings of Paris be clad in gypsum to prevent them from catching fire. They heated gypsum to make this plaster. This became the historical reason for its name. 

What Are The Other Characteristics Of Plaster Of Paris?

Gypsum contains calcium sulfate dihydrates, whereas plaster of Paris contains calcium sulfate hemihydrates (bassanite). Natural gypsum is a mineral that occurs on the earth. The heating process draws water from the mineral.

If you moisten the plaster of Paris, you can form it into different shapes. Gypsum does not have this property. Moisture can retard the setting of plaster of Paris by causing its hydration. As a result, it will become futile after a while. To prevent this, store them in moisture-proof containers.

When you mix dry plaster of Paris powder with water, it rehydrates into gypsum. The setting of the plaster slurry begins about ten minutes after mixing and takes about forty-five minutes. You can observe a slight expansion of the volume during this process. 

Plaster Of Paris, What Are Its Uses?

Architectural designers use it to create ornamental patterns for ceilings and walls. Before painting, it makes surfaces smooth. Making statue castings, decorative materials, toys, cosmetics, blackboards, and chalks are among the many uses of this material.

Some use it as a fire-proofing material. Hospitals find them helpful when setting fractured bones in the correct position to ensure proper healing. It is useful for creating impressions of the patient’s limb and a positive model. 

What Is The Likelihood Of Plaster Of Paris Cracking?

As a general rule, plaster of Paris does not shrink or crack when dry. When making plaster of Paris, some manufacturers use a mixture of alpha and beta gypsum. In such instances, their strength becomes almost as strong as pottery plaster.

Are There Any Drawbacks To Alpha Gypsum?

Alpha gypsum plaster is strong, but it is less porous. Therefore it takes a long time to absorb liquid from clay. Mold has difficulty drying out after use too. This is a disadvantage in the production process. To avoid this, manufacturers make high-grade pottery plaster with special ingredients. They combine calcium sulfate hemihydrate with potassium sodium tartrate and potassium sulfate in a particular proportion. This makes pottery plaster both strong and absorbent at the same time.

What is an Alternative to Plaster of Paris?

Plaster of Paris, a cement made with gypsum can neither be sanded nor is waterproof. Because of those traits, or better said, lack of those traits, you may be looking for an alternative to plaster of Paris. 

Because of its rigidity and the fact that it does not shrink, plaster of Paris has long been used for casts for broken limbs. However, substitutes such as fiberglass, resin, and preformed plastic components often replace plaster of Paris. 

In addition, alternatives may include chalk and water, lime and water, soy powder and water, acrylic undercoat from the hardware store, matte medium, or gelatin.

Cement plaster is yet another alternative to a regular plaster of Paris.

Cement plaster is a mixture of suitable plaster, sand, Portland cement, and water, which is normally applied to masonry internal and external to achieve a smooth surface.

How Can I Make my Own Plaster of Paris?

Plaster of Paris is very easy to make at home. It has a simple formula, it’s literally flour, water, and salt. 

You should start by combining the water with 2 cups of flour in a mixing bowl. Pour the water over the flour as evenly as you can. Once all the water is added, start stirring the mix with a spoon or spatula. 

Stir the mixture well with a spatula. You know when it’s done when there will be no lumps left. The final product should be very thick, white paste. If the substance is so thick that it is difficult for you to stir, just add some more water and mix again. If it’s too runny, try adding more flour. 

After you get to the right consistency, work with plaster for about 10 to 15 minutes. You’ll notice the plaster mixture will start to become more solid as you work with it. Pour the mixture into your molds as soon as you’ve finished stirring it for the best results.

When you are done mixing, allow the plaster to dry. The mixture will start to set quickly, usually just in a few minutes. You will also notice that this plaster gives off heat. It typically takes about half an hour or even less for the plaster of Paris to set.

Let the plaster set and air-dry for 48 or 72 hours. It may be ready sooner, however, I recommend waiting 2 or 3 days just to be sure. You won’t need to put it in the fridge, room temperature is more than okay for the plaster to dry.