How Do You Make A Ceramic Shell Dish?

Ceramic shell casting is a metal casting technique that has been used for centuries to create detailed metal sculptures, jewelry, and other decorative objects. This process involves making a wax pattern, coating it in liquid ceramic slurry to create a mold, removing the wax, and then pouring molten metal into the hollow ceramic shell to form the final cast metal piece.

The technique dates back over 4,000 years to ancient Chinese and Egyptian civilizations. It allows for intricate, lightweight hollow shapes that would be difficult or impossible to create with other casting methods. The ceramic shell mold can withstand the high temperatures needed to cast reactive metals like titanium or niobium.

Ceramic shell casting is ideal for making jewelry, small sculptures, mechanical parts, aerospace components, medical implants, and other decorative or functional metal items with complex geometries or internal voids. The process enables detailed surface textures and extremely precise reproduction of the original wax pattern design.

Materials Needed

Ceramic shell casting requires several specialized materials including:

• Refractory materials like zircon silicate – This provides the heat resistance needed for the high temperature molten metal.
• Binders like colloidal silica – This acts as the “glue” to hold the refractory particles together in the shell.
• Pattern material like wax – This forms the disposable template that creates the shell cavity.
• Furnace capable of reaching over 1000°C – Required to melt out the wax, fire the ceramic shell, and melt the casting metal.

Having the proper materials is critical for successfully making a ceramic shell that can withstand the high heat and pressures of metal casting.

Making the Wax Pattern

The wax pattern is the foundation of the ceramic shell mold. It is typically made from casting wax using one of several techniques.

Some key design considerations when making the wax pattern:

• The pattern should be an exact replica of the final metal piece, accounting for shrinkage.
• Include sprues and gates in the design to allow molten metal to flow into all parts of the mold.
• The wax should be strong enough to retain its shape during handling.
• Supporting structures may be needed for overhangs or large areas.
• Avoid thin or fragile wax sections that may break or deform.

Sculpting the wax pattern can be done by hand or using specialty tools. Hand tools like wax carving knives allow cutting, smoothing and detailing the wax. Wax may also be shaped using heated metal tools. Rubber molds can be made to produce identical wax copies.

Sprues and gates are wax channels attached to the main pattern to direct the flow of metal into the mold. Sprues are the main channels, while gates are smaller branches. The wax pattern is attached to a central sprue to form a wax tree before applying the ceramic shell layers.

Applying Refractory Coats

Once the wax pattern is completed, the next step is to apply refractory coats over it. This process helps create the ceramic shell mold. Here are the steps for applying the refractory coats:

First, dip the wax pattern into a zircon-based slurry. This provides the base layer. After dipping, gently shake off any excess slurry.

Next, stucco or sprinkle zircon silicate sand over the slurry-coated pattern. The sand grains will stick to the wet slurry. This provides the first layer of the ceramic shell mold.

Allow the slurry and sand layer to dry completely. Then repeat the process by dipping again in the slurry and stuccoing with more sand. Build up several layers this way, drying each layer thoroughly before applying the next.

Applying multiple coats and stucco layers helps create a strong ceramic shell that can withstand the heat and pressures of metal casting. Drying each layer properly is also important for shell strength. Follow these refractory coat steps carefully for best results.

Dewaxing

The next step in the ceramic shell process is to remove the wax pattern from inside the ceramic shell through a process called dewaxing. This is a crucial step because any wax left inside the shell will burn out in the firing process, leaving voids and compromising the casting.

Dewaxing is typically done in one of two ways:

Steam Autoclave: The ceramic shell is placed in a steam autoclave, where steam is injected at high pressures. The hot steam melts out the wax pattern, allowing it to drain out of the shell. The high pressure helps force the melted wax out of even the smallest passages.

Flash Firing: The shell is placed in a furnace and quickly heated to over 1000°F. This flash fire melts out the wax rapidly. The shell is then removed from the furnace and the melted wax poured out. Flash firing is faster than autoclaving but risks damaging the ceramic shell if not done properly.

Regardless of method, it is imperative that all wax is fully drained and removed from the ceramic shell prior to firing. Any voids or hollows left from unmelted wax will negatively impact the casting quality. Dewaxing should be done slowly and completely to ensure the wax pattern is eliminated in its entirety from the now hollow ceramic shell.

Firing the Ceramic Shell

Once the ceramic shell has been built up through repeated dipping and stuccoing, it must be fired in a kiln to harden it and prepare it for casting. Firing the ceramic shell involves slowly heating it up to over 1000°C, holding it at that peak temperature for a period of time, then allowing it to cool back down in a controlled manner.

The firing schedule typically involves ramping the kiln temperature up at a rate of around 80-150°C per hour. This gradual increase in temperature allows the ceramic slurry coats to pyrolize and burn off any remaining wax in the shell. Once the kiln reaches the peak temperature, usually between 1000-1200°C, the shells are held or “soaked” at that temperature for anywhere from 2-8 hours. This soaking time ensures the ceramic has fully vitrified and any chemical bonds have completed.

After soaking, the firing schedule will slowly cool the kiln at a controlled rate around 80-150°C per hour. This slow, controlled cooling is important to prevent cracks and stresses in the ceramic material. Slow cooling allows the ceramic particles to align and form a strong, hardened shell ready for pouring molten metal into.

The fully fired ceramic shell should be rigid and able to withstand the high temperatures and pressure from the molten casting metal without cracking or breaking apart.

Casting the Metal

Once the ceramic shell has been fired and hardened, it is ready for the molten metal to be poured in. This step requires a crucible and furnace capable of generating enough heat to melt the metal. Common metals used in lost-wax casting include gold, silver, brass, bronze, and stainless steel. Each has its own specific melting point that the furnace must reach.

When the metal has fully liquefied, the crucible is carefully carried to the ceramic shell and the molten metal is poured inside through an opening at the top or bottom. Enough metal must be melted to fully fill the cavity left by the burned out wax pattern. The ceramic shell is typically heated before pouring to prevent thermal shock and cracking when the hot metal makes contact.

After pouring, the metal is left to cool and solidify fully into its final shape inside the shell. This cooling time can range from minutes to hours depending on the size and thickness of the metal cast. Once fully solidified, the ceramic shell can be broken away in a process called divesting to reveal the completed metal casting.

Divesting

Once the ceramic shell mold has cooled after firing, the next step is to divest or remove the ceramic shell to reveal the finished metal casting inside. This is done using high-pressure water jetting or media blasting with materials like glass beads or walnut shells.

The ceramic shell needs to be completely removed without damaging the delicate casting inside. High pressure water between 10,000-15,000 psi is directed at the shell to gradually wash it away. Media blasting uses abrasive particles propelled by compressed air or water to knock off the ceramic coating. The material used needs to be soft enough not to harm the metal.

As the shell is removed, the finished casting is slowly revealed. Any remaining bits of ceramic are cleaned off to expose the final casting. At this point, the casting will be examined for any flaws before moving to finishing steps like polishing or applying patinas. A successful divesting process will leave a smoothly cast metal duplicate of the original wax pattern.

Finishing the Piece

Once the metal has been cast and the ceramic shell divested, there are a few more steps to complete the ceramic shell casting process. Finishing the piece involves removing any unwanted metal bits and then smoothing and decorating the final product.

First, you’ll need to cut off the sprues and gates – the channels that allowed the molten metal to flow into the mold. Use a jeweler’s saw or rotary tool with a cut-off wheel to neatly cut away the sprues and gates, getting as close to the casting as possible. Take care not to cut or damage the finished piece.

Next, sand and polish the casting using increasingly finer grit sandpaper and buffing wheels. Start with a coarse grit like 80 or 120 to remove tool marks, seam lines, and surface irregularities. Then switch to finer grits like 220, 400, 600 to achieve a smooth sanded surface. Finally, use buffing wheels charged with polishing compounds to bring the metal to a brilliant shine.

As a finishing touch, certain patinas or chemical finishes can be applied to add color and decorative effects. Patinas work to oxidize the top layer of metal, producing hues like blue, green, brown or black. Other decorative finishes like liver of sulfur can achieve antiquing effects. The options are endless for personalizing the look and feel of your final ceramic shell casting.

Summary

Ceramic shell casting is a complex yet rewarding process for creating detailed metal castings. We started by creating a wax pattern, either carved by hand or made using special wax printers. This pattern forms the basis for the final metal piece.

The wax goes through several steps of dipping in liquid ceramic slurry, stuccoing with coarse ceramic powders, and drying, to build up a hard ceramic shell mould around it. Once the mould is ready, the wax is melted out in a process called dewaxing, leaving a hollow negative space for the metal to fill.

The ceramic shell is fired in a kiln to harden it and prepare it for receiving the molten metal. The hot liquid metal is poured into the shell to fill the cavity, then allowed to cool and solidify into the final casting.

This technique is ideal for detailed castings with complex shapes, undercuts and fine features. The ceramic shell can capture details and shapes that would be difficult or impossible with sand casting. From artistic sculptures to jewelry, many creatives use ceramic shell casting. The main benefits are ability to capture fine detail, smooth surface finish, and casting of reactive metals like titanium and zirconium.

Overall, ceramic shell casting allows artists and designers to translate their visions into metal reality. With care and practice, intricate objects can be formed using this unique process.