A decorative effect in ceramic glazes that exploits changes in color and character in certain glazes when their thickness varies. Uneven surfaces can force glazes to dry in different thicknesses. Glazes with melt fluidity often thin on edges of surface contours. Opacified glazes break the least, colored and non-opacified ones the most. Incised or relief decoration is often highlighted with such glazes. The greatest contrast often happens where there is a big difference between the color of the body and glaze. White glazes can break to clear where thin when titanium is employed as an opacifier.
Some glazes look great on red clay and horrible on white
Alberta Slip cone 6 lithium brown (GA6-G1) on a red burning clay (left Plainsman M390) and buff burning (right M340). Obviously this looks better on the former where iron from the underlying body variegates the entire surface and bleeds through on contours where the glaze is thinner, creating a breaking effect.
The classic cone 6 floating blue? No, it is Alberta Slip blue.
And it contains no cobalt! Fairly close in appearance to the classic cone 6 Floating Blue recipe used across North America, this is a variation of the Alberta Slip Rutile Blue glaze (except this adds 1% tin oxide, 1% black copper oxide and 2% ceramic rutile, it is GA6-C1). Because of the melt fluidity, it thins on the edges of contours and breaks to the color of the underlying body. It looks best on dark bodies, but if thick it is OK on light ones also.
A breaking glaze highlights incised decoration
This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain. Notice how the color is white where it thins on contours, this is called "breaking". Thus we say that this glaze "breaks to white". The development of this color needs the right chemistry in the host glaze and it needs depth to work (on the edges the glaze is too thin so there is no color). The breaking phenomenon has many mechanisms, this is just one. Interestingly, this transparent base has more entrained micro-bubbles than a frit-based glaze, these enhance the color effect.
GR10-E 50:50 Alberta Slip:Ravenscrag Slip celadon at cone 10R
On a white stoneware and a porcelain. The glaze is transparent, it has depth and varies in shade according to thickness, breaking to a much lighter shade on the edges of contours.
Tenmokus made from Alberta Slip and Ravenscrag Slip
GR10-K1 Cone 10R Ravenscrag Tenmoku (right) compared to Tenmoku made from Alberta Slip (left, it is 91% Alberta Slip with 5% added calcium carbonate and 2% iron oxide). Left is Plainsman P700 porcelain, right is H570. Tenmokus are popular for the way they break to a crystalline light brown on the edges of contours.
Toilet bowl glaze vs. variegated glaze (at cone 6)
Most artists and potters want some sort of visual variegation in their glazes. The mug on the right demonstrates several types. Opacity variation with thickness: The outer blue varies (breaks) to brown on the edges of contours where the glaze layer is thinner. Phase changes: The rutile blue color swirls within because of phase changes within the glass (zones of differing chemistry). Crystallization: The inside glaze is normally a clear amber transparent, but because these were slow cooling in the firing, iron in the glass has crystallized on the surface. Clay color: The mugs are made from a brown clay, the iron within it is bleeding into the blue and amplifying color change on thin sections.