Fusing glass to ceramics is an ancient craft that is becoming popular again. It adds adds depth, texture and color to an object. Melting glass to a glazed surface is the process of merging two separate layers of glass. An ordinary ceramic kiln is adequate to fuse glass to ceramics, but care must be taken with firing and cooling.
Early man-made glass objects are found as far back as 3,500 B.C. and are often earthenware covered with a glass made of silica, a quartz-rich desert sand. Because the glass contained little fluxing agent, the ingredients allowing the silica to melt and flow smoothly, the glass on the beads is often crackled and rough. The blue beads are called Egyptian glass. Egyptian glass is an early form of ceramic glaze. As people learned more about glass and glazes, chemicals were added as colorants, to melt glass at lower temperatures and make it more flexible for more forming techniques.
The heating process
Glass can be fused to glazed ceramic surfaces in a ceramics kiln for contemporary crafts. A glass kiln is preferable for this process as it is insulated for slow cooling. A ceramics kiln can also be converted to a glass kiln.
The best surface for fusing glass is a ceramic object that is already fired and coated with glaze. A tile, plate or any flat shape is easiest to work with. Glass pieces must be glued with bisque repair glue to hold pieces to a vertical surface as the kiln heats. Regular glue loses grip as it heats.
Glass pieces can be arranged on objects and melted together to create a blending of color and patterns. Fusing class is a fun recycling project as glass can come from many sources including broken bottles, vases, and beads.
Kiln shelves should be coated with plenty of kiln wash. As the glass turns to a liquid state, it may overflow the clay surface and stick to uncoated shelves. Kiln wash must dry thoroughly before placing the object on the shelf or it will stick to the object and become part of the fired surface.
Kiln wash is made of equal parts of Alumina Hydrate, Kaolin clay, and fine silica sand.
Any project involving glass should be allowed to heat in the kiln slowly. In fusing glass to ceramics, temperature should reach cone 06 or higher and be sufficient to melt the original glaze surface enough to accept the glass. As the kiln nears cone 6, welding safety glasses can be used to watch the glass melt through the peepholes in the kiln.
A kiln containing glass should never be turned off abruptly. Temperature is reduced slowly by turning off one switch at a time, or if using a computerized kiln, programming it to reduce temperature slowly overnight. Clay, powdered glazes and glass all have different coefficients of expansion, meaning they expand and contract when heated and cooled at different rates. Slow heating and cooling enables the glass to better adhere to the glaze and reduce cracking.
No matter how slowly the kiln is heated and cooled, a craquelure, or a fine crackle pattern, will occur in the glass.
Commercial ceramic glazes contain silica, in addition to a fluxing agent for melting at lower temperatures, a colorant and a refractory agent, enabling glaze to stick to a ceramic surface. Because glaze contains these additional agents, better results occur melting glass to glaze rather than to ceramic bisque. Ceramic bisque is dry fired clay without any glaze coating the surface. Glass adheres best to glaze as glaze is just another form of glass.
A bisque patch product is available at many ceramic stores and online. It allows you to attach glass pieces to upright ceramic surfaces before firing and will hold as kilns reach higher temperatures.
Using a pyrometer, or kiln thermometer, and noting temperatures that yield best results while experimenting with this process, enables crafters to gain control over the process.
Pieces of craquelure glass can detach from ceramic surfaces over time. Tiles made with this process should only be permanently adhered to surfaces that do not need harsh, abrasive cleaning products, such as shower walls.
Fused glass on ceramic is not food safe.
Always wear welding-rated safety glasses when looking into a hot ceramic kiln.