This piece was slipcast using Ashraf Hanna stonware clay and after bisque firing was sprayed with the two different coloured crackle glazes and fired in a raku kiln. The top was wheel thrown and also given the same treatment as the main body.
 
What is Raku Crackle Glaze?
 
Raku crackle glaze is a type of raku glaze known for its characteristic crackled surface. The cracks form due to the rapid cooling of the pottery, which causes the glaze to contract more quickly than the clay body underneath. This difference in contraction rates creates a web-like pattern of cracks.
 
The Process of Creating Raku Crackle Glaze
 
The glaze used for crackle effects typically contains silica and other ingredients that promote cracking during the cooling phase. Common ingredients might include feldspar, kaolin, and various fluxes to adjust the melting point and behaviour of the glaze. The glaze is mixed to the desired consistency and applied to bisque-fired pottery through dipping, brushing, or spraying.
 
Firing
 
The glazed pottery is placed in a raku kiln and rapidly heated to a temperature range of approximately 1,650 to 1,850 degrees Fahrenheit (900 to 1,010 degrees Celsius). The firing process is quick, usually taking 30 to 45 minutes, allowing the glaze to melt and form a smooth surface.
 
Reduction and Cooling
 
After reaching the peak temperature, the red-hot pottery is removed from the kiln using long tongs and placed into a reduction chamber, usually a dustbin! The chamber can be filled with a variety of combustible materials such as sawdust, newspaper, or leaves). The intense heat ignites the combustibles creating lots of smoke! The chamber is then sealed and the smoke penetrates the cracks in the glaze 'smoking' the clay surface turning the cracks black.  The pottery is then often quenched in water or allowed to cool in the air. 
 
Characteristics of Raku Crackle Glaze
 
The crackle pattern is random and unique to each piece. The size and distribution of the cracks can vary based on factors such as glaze composition, application thickness, and the cooling process. As the crackle pattern is somewhat unpredictable, achieving a specific look can be challenging and each piece will have a unique crackle pattern.
 
Durability 
 
Raku-fired pottery, including crackle glaze pieces, is generally more porous and less durable than high-fired ceramics. These pieces are often used for decorative purposes rather than functional ware.