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| Glossary
While industry avoids so-called 'reactive glazes' (for consistency problems) potters avoid the 'porcelain sink' look and constantly seek new ways to get glazes with surface variations in color and texture. Here are some ideas on how to give that drab glaze an exciting variegated or mottled surface.
Highlighting: Employ 'highlights' of the same color but in a different shade. For example, varying the thickness of a translucent (partly opacified) colored glaze will vary the intensity of color (especially where it thins on the edges of sharp contours). Variation can be achieved by pouring, double-dipping, brushing, waxing, and incising techniques.
Physical means: Create variegation by splatter-spraying or sponge-stippling a second layer of glaze with a contrasting color or shade (or the same color in a matter or glossier version). A trigger or pump operated garden sprayer works well for this. Use a fluid glaze over a more stable matte (where spots tend to 'feather' into the underlying layer) or vice-versa (where they tend to sit on top). Remember that while brightly colored variegated glazes look great, subtle variations can also be very effective. A good example is a glossy blue speckle or pattern on a matte blue background.
Surface Crystal Growth: Very fluid glazes are a must in forming crystals on the surface during cooling (low alumina, high flux). TiO2 materials like titanium dioxide and rutile seed crystal networks and encourage their growth. A thin rutile wash applied to a glaze surface can act as a crystal growth catalyst. High calcium and boron encourages the formation of calcium-borate crystals, high zinc glazes also crystallize when they are fluid. The addition of up to 4% tin in such glazes can magnify the effect. Slow cooling greatly enhances crystal growth. Small amounts of lithium (e.g. 1%) can have a remarkable variegating effect on rutile glazes, especially when colorants like iron are present. Industry calls crystal growth on glaze surfaces "devitrification".
Specking Agent: You can add a coloring oxide that contains particulate
matter that speckles the glaze surface. Manganese granular, illmenite, and
granular rutile are examples. However these materials are heavy and tend to
settle in glazes that are too fluid. Ordinary iron oxide and cobalt oxide often
produce small specks in unmilled glazes.
Multilayering: Double layering of different glazes produces variegation
well when the lower layer is more fluid and the upper, stiffer glaze tends to break into islands revealing rivulets of the lower one. Be aware of the problems associated with double layer glazing
(cracking and crawling during drying because either layer shrinks too much, is applied too thick, or onto wet ware).
Use glazes with lower or less plastic clay content for multilayer work and check
out www.ravenscragslip.com.
Phase Differences: The glass matrix in a fired glaze can separate (or fail to mix) during melting forming globules of
different glass chemistry. These reflect light differently and thus variegate. 'Techies'
look for chemistries that encourage phase separation.
Combinations:
Use combinations of the above to variegate surfaces even more. The popular Floating
Blue recipe is an good example. Its color varies with thickness so it
highlights irregularities in the surface. Phase separation in the translucent
matrix makes the color 'swirl' in patterns of blue. Titanium crystals in the
matrix make it sparkle. The growth of calcium-borate crystals on the surface
appear to float over a deep blue background. No wonder this is such a popular
glaze among potters!
Knowing the 'mechanism' of the variegation in your glazes is important to
enhancing and controlling it. Knowing how to variegate won't get you a job in a
porcelain factory, but it will get the admiration of people who see your work.
Revised: 01/12/01
Copyright 1998 Author: Tony Hansen