Working with Clay 5: Glazing (Glost-Firing)

Ceramic glaze is a liquid you use to coat bisque ware to decorate it, and possibly to make it more waterproof and functional (depending on the glaze). Ceramic glazes look like paint, but glaze is not paint, the ingredients are different. Ceramic glaze consists of three basic ingredients:

1. Silica – this melts the glaze into a glass.
2. Flux – this lowers the melting point of the silica so that it’s compatible with ceramics. Otherwise, the Silica’s melting temperature is way too high (3,100°F!)
3. Refractory (Alumina) – this makes the glaze sticky, so it doesn’t simply melt, drip down off the vessel, and pool in the floor of the kiln.

In addition, small amounts of metal oxides can change the color of a glaze.

NOTE: don’t apply a glaze until the piece has already been fired! Otherwise it can peel away and drip off the piece, and cause it to blister and crack. It could also cause an explosion, as it traps hair trying to escape the clay. The only way to successfully “single-fire” a piece with glaze, is to use special glazes with 20-30% ball clay so that its shrinkage rate matches that of the vessel, glaze the inside first, allowing it to thoroughly dry, and then fire it extremely slowly.

PRO TIP: If two ceramic pieces come apart in the kiln during bisque firing, you can “reglue” them with glaze.

 Choosing the Right Glaze

The glaze you choose for your piece should depend on aesthetics, but also on function (how you plan to use the piece). Some glazes have a glossy finish, appropriate for holding liquids - great for the kitchen and bathroom. Other glazes are metallic or have a matte finish - they may look prettier, but are less functional. Metallic glazes are not microwave safe, and matte finishes are more porous, they can get moldy when wet. Some glazes contain dangerous chemicals, like lead. Check the label on a jar of glaze to see if there’s a knife & fork symbol, or if it says “dinnerware safe”.

It’s also important to remember that the colors of the glazes before firing are not necessarily the colors they will look like after firing. Your studio should have a glazing reference chart to consult.

Crazing is when the glaze cools in such a manner that it weakens the structure of the vessel underneath, pulling it apart so that it’s very weak and wants to shatter when cooled. It might look sturdy but it’s not. Crazing happens when you mis-match a particular glaze with the wrong kind of clay. Just as there are low-fire and high-fire clays, there are also low-fire and high-fire glazes. Make sure your glazes match the kind of clay you're using!

Crackle glaze

This is the same as crazing, only it’s intentional. You can alter the glaze recipe. A popular recipe at cone 5 or 6 is:

                        70% Nephline Syenite

                        15% Gillespie Borate or Gerstley Borate

                        15% EPK Kaolin

 

The thicker you apply the glaze, the more dramatic the cracks. Professionals recommend to not use too much, nor too little. You can then stain the piece in ink, black tea, or acrylic wash to get in between the glass pieces.

Faience: This is also a tin-glaze, similar to Maiolica, but more common in France and northern Europe. It makes blue-and-white dishes that imitate Chinese porcelain. Examples include Delftware, which is a kind of faience pottery. It commonly shows windmills, sailing ships and flowers.

Leopard Skin Glaze

There are four different techniques to achieve this effect.

1.      Runny Base & Stable Spots – Dip your brush in a runny base glaze (glossy white or transparent amber). Once dry, flick or paint dots of a stiff, stable glaze (satin-matte black or dark brown). Fire it.

2.      Reticulation Glaze (Chemical Crawling) – First coat the vessel in underglaze (black or iron red) or a highly fluxed slip. Apply a high-magnesium reticulation glaze on top. Fire it.

3.      Gas & Trap Firing – Apply a high-soda ash Shino glaze thickly. During a gas reduction firing, introduce heavy smoke, very early in the cycle.

4.      Precise Hand-Painting Underglaze

Majolica (Maiolica): an earthenware vessel coated in a white, opaque tin glaze. Artists paint bright, intricate designs while the glaze is wet, before firing. It’s popular in Italy and Spain.

Painting your Glaze

NOTE: Be very careful not to glaze the bottom of your vessel! If you do, it will glue to the inside of your kiln, and there's no way to remove it without chiseling it off - destroying your vessel in the process. Check the bottom of every single piece before you put it in the kiln, and wipe away any excess glaze that you see.

Sticker Resists

You can put down temporary stickers (or cut tape to shape) and glaze over it, then pull the stickers off when it’s dry before firing, to get various shapes.

Resist Slip & Glaze

You can paint wax over a piece so that the glaze won’t adhere to those parts of the vessel. Then, it burns away during firing. First you burnish the vessel and bisque fire it. Then you use the resist slip and a “clever glaze”. First you apply the resist slip a temporary barrier protecting the burnished surface. Then, you add a paper stencil. Whatever is exposed will be black.

Firing Glaze Ware

Depending on the kind of clay and glaze you use, you may choose a low-fire or high-fire program. FCPS recommends the following settings:

Low Fire Glaze

Stage        Ramp            Temp        Hold

1                9999 (full)    1200°F    10 minutes

2                9999 (full)    1850°F    10 minutes

High Fire Glaze

Stage        Ramp            Temp        Hold

1                 9999 (full)    1200°F    10 minutes

2                 9999 (full)    2150°F    10 minutes

These settings are faster than bisque firing, often taking about 3 hours to get up to the highest temperature.