Important Notes Before You Begin

1. Glass-on-ceramic is not food safe - All pieces created using this technique should be for decorative use only. Do not use for serving or storing food or drink, as the cracks and crevices in the melted glass can harbor food particles and bacteria, making the surface unsafe for contact with consumables.
   
   
2. Fire to Cone 6 (approx. 2232°F / 1222°C) - This temperature is ideal for melting glass while maintaining the integrity of the ceramic body and glaze.
   
   
3. Always test and document - Use test tiles to try different glass types and glaze combinations before applying to final pieces. This helps avoid unwanted reactions and ensures repeatable results.

With experimentation and the right technique, fusing and melting glass with pottery can open up an exciting new chapter in your ceramic journey.

What Kind of Glass Should You Use on Pottery?

Glass varies widely in melting temperature and chemical composition, which can significantly affect your final results. (See Appendix A and Appendix B below for detailed information.)

At Lakeside Pottery Studio, we've tested a range of glass types, including broken bottles, sea glass, recycled window glass, and mosaic pieces, with mixed outcomes. One key lesson we learned is that appearance is not a reliable indicator: two pieces of glass that look identical can behave very differently in the kiln. For instance, one may retain its original green color while another turns white after firing, simply because they came from different sources.

We also found that sea glass, despite its higher cost, produced the same results as ordinary broken glass, making it a less practical choice for this technique.

Glass and Glaze Additives:
Color Effects and Health Considerations

Many types of glass and glaze contain metal oxides or additives that affect both color development and safety. Some of these may release harmful fumes or residues when fired. For this reason, it's essential to test each specific glass source in your kiln atmosphere.

To achieve consistent, safe, and beautiful results:

• Document your test results carefully
• Use the same glass source and glaze combination moving forward
• Fire at Cone 5–6 in the same kiln to replicate conditions

Refer to Appendix A for color outcomes related to different glass compositions, and consult Appendix B for melting points and compatibility considerations.

Note: Red glass can be unpredictable during firing. It often does not remain red, instead, it may turn brown or black, especially when combined with glaze or other glass colors. To increase the chances of achieving a true, vibrant red after firing, use red glass alone, and apply it in a thicker layer. Thin applications or mixing with other materials tend to compromise the red color during the melting process.

Where to Get Glass for Pottery Fusing?

We source our glass from American Specialty Glass (ASG), a reputable supplier based in Utah offering a wide selection of colors and chip sizes, perfect for fusing on pottery. All of ASG's products are made from recycled glass, aligning with eco-friendly practices.

You can explore their full range and purchase sample boxes to experiment with a variety of colors and textures before committing to larger quantities.

Visit their website at: www.americanspecialtyglass.com

Trying sample kits is highly recommended to find the perfect glass options that suit your artistic vision and firing setup.

See list below which reflects we have been using for several years from ASG:

1) Turquoise Terrazzo size 3 (the most used with the best and consistent results with all of our glazes)
2) Yellow Chunky Terrazzo size 3 (second best with consistent results with all of our glazes)
3) Dark Blue Terrazzo size 3
4) Purple Terrazzo size 3
5) Light Green Terrazzo size 3
6) Red Chunky Terrazzo size 2

Firing Temperature and Important Firing Tips for Glass on Pottery

How to fire glass with pottery? At Lakeside Pottery Studio, we fire our ceramic pieces in an electric kiln to Cone 6 (approximately 2223°F / 1215°C) after bisque firing the pots first to Cone 06.

We experimented with firing glass at lower temperatures, such as Cone 5, with American Specialty Glass and other glass types, but in all cases, the glass did not melt fully or fuse properly. For detailed melting temperature information, see Appendix B below.

Key Considerations When Placing Glass on Pottery Before Firing

It’s important to anticipate how the glass will behave in its molten (liquid) state during firing. Think of the glass placement like placing an ice cube that will melt and spread. As the glass melts, it will flow outward and increase in width.

• Make sure your kiln shelf is perfectly level to prevent the molten glass from moving toward the lowest point.
  
  
• Avoid placing glass on vertical or sloping surfaces, as gravity will cause the melted glass to run off.
  
  
• Do not place glass over cracks or scratches on pottery with the expectation that the melted glass will fill or hide imperfections. The glass will most likely leak through these cracks, drip onto the kiln shelf, and could even fall onto other pieces on shelves below, potentially damaging them.

Following these firing guidelines and placement tips will help you achieve smooth, beautiful glass fusions on your pottery while avoiding common pitfalls.

How to Apply Glass to Pottery

There are a couple of effective methods for applying glass to your pottery pieces, each producing different visual results.

• Glass on top of glaze: You can place glass chips directly over a glazed surface. During firing, the melting glass will interact with the glaze, often producing interesting flow, pooling, and color effects.
  
  
• Glass on bare bisque: For a more controlled, you can scrape off the glaze in the area where you plan to place the glass, applying it directly to the bisqueware. This prevents interaction between the glass and glaze and allows the color and texture of the melted glass to stand on its own.

See the process below to understand how to place and secure the glass for best results during firing.

Before Scraping	Scraping glaze	Glass oover scraped glaze
Glass over glaze	Removing scraped glaze	Lighter color where glaze was removed

How Much Glass Should You Use on Pottery?

Determining how much glass to apply depends on both the effect you want to achieve and the physical limitations caused by differences in material properties. Glass and clay have different thermal expansion rates, meaning they expand and contract at different rates during firing and cooling. This mismatch often results in crazing (crackling) of the glass surface, depending on the type of glass, clay body, and the quantity used.

Guidelines and Cautions:

• Test first on sample tiles. Before applying glass to a finished piece, always experiment with test tiles (see example below). This helps you understand how a particular clay and glaze combination reacts with the glass during firing.
  
  
• Less is often more. Overusing glass can create stress and structural problems due to the tension between the shrinking clay and the more rigid, melted glass surface.

Potential Problems with Too Much Glass:

1. Sharp, uneven cracking:
   If too much glass is used, the crackled surface may not settle evenly. Some cracks might be raised, leaving sharp edges that can pose a risk of injury.
   
   
2. Structural failure on handbuilt forms:
   On larger or handbuilt items like trays, excess glass can create tension that leads to delayed cracking. For example, a tray may appear fine when removed from the kiln, only to develop cracks or break at the corners a few hours later as internal stress is released.

To achieve safe, aesthetically pleasing results, start small, document your tests, and adjust the glass volume based on how your materials respond in the kiln.

Too much glass on corner can break the ceramic

How Glass Interacts with Glazes on Pottery

When working with pottery, it's helpful to think of glass as another type of glaze, because that's essentially how it behaves in the kiln. This means that each glass and glaze combination can produce different visual and chemical results.

It's crucial to test each combination individually, even when using glass of the same color. For instance, red glass may remain vibrant red with one glaze but turn brown or black when used with another. These chemical interactions are often unpredictable and can only be understood through testing.

From our experience at Lakeside Pottery Studio, the most consistent results across various glazes come from yellow and light blue glass (sourced from American Specialty Glass), which tend to retain their color more reliably regardless of the underlying glaze.

Is Glass on Pottery Food Safe?

No - glass on pottery is not food safe.

Fired glass surfaces naturally develop cracks and crazing during the cooling process. These tiny fissures can trap food particles, dirt, and bacteria, making the surface unsafe for food contact.

Pieces using glass should be considered decorative only and never used for serving or storing food or beverages.

Why Is Crackled Glaze Used on Functional Pottery in the Far East, But Glass on Pottery in the USA and Elsewhere Isn’t Considered Food Safe?

See answer in Appendix C below

More Examples of Melted Glass on Pottery

Kids projects where the melted glass appears like water

Appendix B - Glass Melting Point

Glass has different melting points depending on what the composition of the glass actually is. Standard soda lime Silica (the most common kind of natural glass) melts at something on the order of 1500 °C, or about 2700 °F. Pure silica needs fluxes to melt at lower temperatures. Glass is made from a variety of substances, depending on the intent of use. Mostly sand, lime and soda are what most glasses are made of. There are many types of glasses ex, bulletproof, tempered, tinted, stained, etc.