Incised decoration is carved into leather hard ware. It is often highlighted by cutting through a slip of contrasting color to the body or by glazing the ware using a glaze that changes color or crystallizes on edges of contours (where the glaze layer is thinner). Many tools are available for this.
Incised decoration (left) vs. slip trailed decoration (right)
Both are highlighted by Alberta Slip GA6-A glaze on these cone 6 oxidation fired mugs.
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.
Double-slip layer incised decoration: A challenge in slip-body fitting
An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.
M390 mugs with Alberta Slip based glazes by Tony Hansen
These cone 6 mugs use an 80:20 Alberta Slip:Frit blend inside and out (the outside one has added rutile). Made around 2014. The incised wheat decoration as a dead giveaway that the mug is made by Tony. He has made this type of mug for decades and there is a good reason: continuity of testing. By making the same kind of ware each time he tests a clay, going through the same procedures he has done a thousand times, he can more easily spot differences in the way they perform.
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