How Do You Slip Casting Mold?
Slip casting is a ceramic forming technique where liquid clay slip is poured into a plaster mold and allowed to harden. As the slip contacts the mold, the plaster absorbs water from the slip, forming a layer of solid clay on the surface of the mold. Once the layer reaches the desired thickness, any excess slip is poured out of the mold. The remaining clay is allowed to dry before being removed from the mold.
Slip casting originated in China as early as the 14th century and was brought to Europe in the 18th century. It enabled the production of complex forms and thin wares not possible with other techniques. It is now one of the most common forming techniques used in commercial ceramic production and by studio artists.
The basic steps in the slip casting process are:
- Making a plaster mold
- Pouring a liquid clay slip into the mold
- Allowing it to harden as water is absorbed by the plaster
- Dumping out the excess slip
- Allowing the clay layer to dry and harden further
- Removing the final cast piece from the mold
This technique allows for the efficient and precise production of intricately detailed forms with thin, uniform walls not easily achieved with other ceramic forming methods. It is particularly suited for repetition and mass production.
Materials Needed
The main materials needed for slip casting molds are plaster, clay slip, and mold release.[1] Plaster is used to create the actual mold. Standard pottery plaster that sets in about 30-45 minutes works well. The plaster mold absorbs the water from the liquid clay slip over time, pulling the clay particles out of the slip and onto the mold surface.
The clay slip consists of clay particles dispersed in water, creating a pourable liquid. Any clay body can be used to make slip, but higher plasticity clays work best to avoid defects. Clays like porcelain, stoneware, and earthenware are commonly used. The slip should be screened through a sieve to catch any large particles. A creamy slip consistency, similar to a milkshake, provides ideal absorption and drying times.
Mold release is brushed onto the plaster mold before pouring the slip. This allows the dried clay to release cleanly from the mold surface. Vegetable oil or petroleum-based mold release work well. The mold needs to be watertight, so it may require several coats of release.
In terms of equipment, pottery wheels allow trimming and refining the bottom of slip cast pieces after removal from the molds. Basic studio tools like ribs, loop tools, sponges, and knives help with cleanup and surface treatment. Kilns are needed for bisque and glaze firing. Optional accessories include slip trailers, vibrators, and vacuums.
Making the Plaster Mold
The first step in slip casting is creating a plaster mold. The design of the mold will depend on the desired shape of the final piece. Simple geometrical shapes like cylinders and cubes can be made with a basic two-piece mold. More complex forms require a multi-part mold.
To make the mold, start by sculpting a prototype of the desired shape out of clay or another material. This is your model and will form the negative space that liquid clay is poured into. Coat the model in a release agent like petroleum jelly so it can be easily removed later.
Next, mix up a batch of plaster according to the manufacturer’s instructions. Carefully pour the plaster over the model to encase it. Let the plaster harden completely before removing the prototype. This leaves behind a hollow plaster mold.
Allow the plaster mold to dry thoroughly for 24-48 hours. Any moisture left in the mold can cause cracking when hot liquid clay is poured in. The mold also needs to be totally rigid and strong to withstand the pressures of slip casting without deforming.
Registration keys, like extra plaster pieces that stick out, can be added to help align multi-piece molds (https://ceramicartsnetwork.org/daily/article/How-to-Make-a-Plaster-Mold-for-Slip-Casting). Proper drying is crucial for creating a high quality plaster mold ready for the slip casting process.
Preparing the Clay Slip
When slip casting, it’s important to start with a properly prepared clay slip. The clay slip is the suspension of clay particles in water that will be poured into the plaster mold.
The most common types of clay used for slip casting are porcelain, stoneware, and earthenware clays. Porcelain clays make very smooth casting slips and are often preferred. However, stoneware and earthenware can also be used 1.
To prepare the slip, start by wedging your clay thoroughly to remove any air pockets. Then slowly mix the wedged clay with water in a large container or bucket. Use an electric drill with a paint mixer attachment to blend the clay and water together into a smooth suspension. The slip should have the consistency of heavy cream.
Add a deflocculant such as sodium silicate to help disperse the clay particles evenly in the water. Test the specific gravity of the slip using a hydrometer and aim for 1.8-2.0. Adding more deflocculant or water can help adjust the viscosity as needed.
Once mixed to the proper consistency, let the slip hydrate for 6-12 hours before use. Then pour the slip through a fine mesh sieve to remove any remaining clumps or debris.
Pouring the Slip
Once the plaster mold has dried completely, the next step is to pour the prepared clay slip into the mold. First, the inside of the mold needs to be coated with a thin layer of mold release, like mineral oil or petroleum jelly, to prevent the dried slip from sticking to the plaster (The Crucible, 2019). Apply the release agent by brushing or wiping it over the entire interior surface of the mold.
Next, slowly pour the liquid clay slip into the opening at the top of the mold, being careful not to trap any air bubbles. Fill the mold up to the top edge. Then allow time for the plaster to absorb moisture from the slip – usually 15-25 minutes. The plaster will draw the water out of the slip, causing the clay to deposit in a thin, even layer on the interior walls of the mold (The Crucible, 2019).
Absorption time can vary based on factors like slip thickness, plaster porosity, and mold depth. For thicker molds, it may take up to an hour or more for the slip to reach the proper consistency. Check the slip periodically by gently touching the surface. When it has firmed up and lost its wet sheen, it is ready for the next step.
Drying and Removal
Once the slip has been poured into the mold, it’s important to wait for the right amount of time for the clay to dry before attempting to remove it. According to this source, you want to wait until the clay has reached a “leather-hard” state. This is when the clay has hardened enough that it can be handled without deforming, but still retains some moisture content. Waiting too long can cause the clay to crack as it dries and shrinks. Waiting too little can cause the clay to tear or distort when removed.
When the casting has reached the ideal leather-hard state, it’s time for removal. Gently pry the two halves of the plaster mold apart and carefully peel the excess clay away from the casting. Remove any keys, pins or rubber bands used to hold the mold together. Be sure to clean any remaining slip out of the mold promptly after each use. Once the cast ceramic piece has been removed, it is ready for trimming and finishing work before drying and firing.
Trimming and Cleaning
After the slip cast piece has adequately dried and hardened, the next step is to trim and refine the shape using various trimming tools. Trimming involves removing the sprue and seam line from where the two halves of the plaster mold met. This helps create a smooth, finished surface. Trimming tools like loop tools, rib tools, and trimming knives can remove excess clay and shape details like the foot, lip, and handle (The Art of Slip Casting).
During the trimming process, it’s important to periodically stop and inspect the piece from all angles to identify any uneven surfaces or problem spots. Refining the shape and surface should be done carefully and methodically for best results. Gentle scraping, thinning, and smoothing will remove rough edges and create a polished look. Once satisfied with the form, a damp sponge can wipe away any loose debris or dust before the final drying and firing stages.
Drying and Firing
After the slip has been poured and fully absorbed by the plaster mold, the cast piece needs to dry completely before removing it from the mold. Allow the cast piece to sit undisturbed as it dries for 12-24 hours. Test the piece by gently touching it to see if any moisture comes off on your finger. If the piece feels dry, it is ready to be removed from the mold.
Carefully separate the two halves of the plaster mold and gently remove the cast piece. Let the piece continue air drying for another day or two until it is bone dry. As it dries, some minor warping or cracking may occur which can be fixed later.
Once fully dry, the piece is ready to be fired in a kiln. There are multiple firing methods that can be used. A once-fire approach involves slowly raising the temperature to around cone 04-06 to fully vitrify the piece. For more complex shapes, a twice-fired method may work better. First fire to cone 08-04 to harden the piece, then fire a second time to cone 06 for full vitrification. Kiln temperatures and ramp speeds will vary depending on the clay body used. Follow manufacturer recommendations for best results.[1]
Applying Glazes
Applying glaze is an important step in finishing slip cast pieces. There are several techniques for applying glaze including brushing, dipping, pouring, and spraying.
When choosing a glaze, consider the clay body, desired visual effects, and firing method. Glazes come in various colors and finishes from matte to glossy. Test different glazes on test tiles to ensure compatibility with the clay body and to achieve the desired results.
For hand application, use a soft brush to gently coat the bisque ware. Apply in thin, even layers and avoid over-brushing to prevent glaze dripping. Allow the piece to dry completely before firing. Dipping involves submerging the object into a bucket or vat of glaze. Maintain a consistent dipping speed and angle to control glaze thickness. Drain excess glaze before drying. Spraying glaze requires specific equipment but allows for a very even application. Pour off excess glaze from the piece after application.
Always follow the glaze manufacturer’s instructions for application methods, thickness, firing temperature/speed, and number of coats. Test glaze combinations on test tiles before applying to finished wares. Firing will melt and permanently affix the glaze to the clay surface.
Conclusion
Slip casting is a versatile and useful process for creating ceramic objects in large quantities or complex shapes. It involves making a plaster mold, pouring liquid clay slip into the mold, allowing it to harden to the shape of the mold, and then removing the clay casting.
Throughout this process, we covered the key steps involved, including:
- Making a two-part plaster mold from a prototype model
- Mixing and pouring a liquid clay slip into the mold
- Allowing the slip to sit in the mold until it thickens and holds shape
- Carefully removing the clay casting from the mold
- Cleaning, trimming, drying and firing the cast piece
- Optionally glazing the final fired object
The benefits of slip casting include the ability to precisely reproduce complex shapes, produce multiple copies efficiently, and create pieces with very thin walls. Challenges can include avoiding flaws, controlling shrinkage, and managing the drying and firing process. Overall, with practice slip casting allows ceramic artists to create detailed and refined objects in larger quantities than handbuilding or wheel throwing alone.
