• Gerstley Borate 65
• Nepheline Syenite 20
• EPK (Kaolin) 5
• Flint (Silica) 10
• Copper Carbonate 10

You want to apply this glaze fairly thick and it needs to be fired a little hotter than some Raku glazes – somewhere around 1850F-1900F. The piece should be pulled from the kiln and then held in the air for about 15 seconds and then placed in a reduction chamber with combustibles. If it is reduced too much it will be mainly copper. If it is lightly reduced it will mainly be dark green with crackle. If it is reduced “just right” you will get green crackle with copper flashing.

Lidded, Double-walled pot with Gary’s Green Crackle by Gary R. Ferguson

Formula (by Volume)

• Gerstley Borate 8 cups
• Bone Ash 2 cups
• Copper Carbonate .5 cup
• Cobalt Oxide .25cup
• Optional: Cobalt Carbonate 2%

How Robert Piepenburg fires this glaze –

Apply the glaze using almost any method: spray, pour, brush, or dip. Fire in oxidation or reduction to approximately 1875F. Pull piece and place in a bed of straw in a sand pit. Then cover the piece with a metal container pushing the rim into the sand and wait 3 minutes. Burp the can for ½ a second, then wait until piece cools (15 to 45 minutes). Feeze the colore and finish cooling by spraying with or dunking in water.