This demonstrates the difficulty you can encounter when trying to get an engobe working with a clay body. Here the slip/glaze is flaking off the rim of pieces at cone 04 (does not happen at 06). The front bi-clay bar demonstrates the white and red clays dry well together (the slight curve happened on the drying). They also fire well together (the curvature did not change on firing). The back two thin bars seem to demonstrate thermal expansion compatibility: a thick layer of glaze is not under enough compression to curve either bar during firing. While the white clay contains 15% frit and forms a good bond with the red body, that bond is not nearly as good as the one between the glaze and the white slip. Yet it is still flaking off the rim at the slip/body interface. Why? At first it seemed that failure was happening at quartz inversion (because the body had less quartz than the white slip). However now it appears that the combination of compressions of the slip and glaze are sufficient to break the slip-body bond on convex contours. The compression of the slip and glaze likely did not demonstrate well on the bars because at this low a temperature they are not vitreous enough to be easily curled.
This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).
Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrinkage at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they do fit well (either during the firing or after the customer stresses your product). The engobe also needs to shrink with the body during drying. How can you measure compatibility? Bi-body strips. First I prepare a plastic sample of the engobe. Then I roll 4 mm thick slabs of it and the body, lay them face-to-face and roll that down to 4 mm again. I cut 2.5x12 cm bars and dry and fire them. The curling indicates misfit. This engobe needs more plastic clay (so it dry-shrinks more) and less frit (to shrink less on firing).
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
Ask the right questions to analyse the real cause of glaze shivering. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
|Tests||Engobe Body Compatibility Test|
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