Clay designs can adhere to glass, but their ability to hold firm depends on the type of clay used and how it's applied. Here's an overview of common clay types and their ability to stick to glass:

1. Air-Dry Clay:

• Adhesion: Air-dry clay can stick to glass surfaces temporarily while it's moist, but once dry, it tends to shrink and pull away from the glass. It’s not the best option for long-term adhesion unless you use additional adhesives like glue.
• Solution: To make air-dry clay adhere more firmly to glass, you can use a clear adhesive or glaze after the clay has dried to help bond it to the glass surface.

2. Polymer Clay:

• Adhesion: Polymer clay can stick well to glass when baked. Before baking, you can press the clay onto the glass surface to create your design. Once baked, the clay hardens and forms a semi-permanent bond with the glass.
• Solution: For even stronger adhesion, lightly sand the glass surface where the clay will be applied to create some texture. This allows the polymer clay to grip better when baked.

3. Epoxy Clay:

• Adhesion: Epoxy clay is excellent for bonding to glass. It hardens to a rock-solid finish and adheres firmly to most surfaces, including glass. Epoxy clay is often used for jewelry making and sculptural work where a strong bond is needed.
• Solution: Simply mold the epoxy clay onto the glass surface, and it will hold once it cures, creating a permanent bond.

4. Ceramic or Pottery Clay:

• Adhesion: Ceramic clay won’t adhere to glass unless it is fired in a kiln along with the glass, which isn’t practical for most applications. Without firing, the clay will not form a permanent bond with the glass.

Tips for Best Results:

• Surface Preparation: Clean the glass surface thoroughly before applying any clay to ensure better adhesion. Removing dirt, oils, or grease helps the clay bond more effectively.
• Adhesives: For added security, you can use clear epoxy, superglue, or other adhesives to ensure the clay sticks well to the glass surface, especially for projects meant to last long-term.

In summary, polymer clay and epoxy clay are the best options for creating designs that will hold firm on glass, especially when baked or cured. For temporary designs, air-dry clay can be used with adhesives to improve its adhesion to glass.