|Monthly Tech-Tip |
This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).
This is the L3954B engobe. 15% Mason 6600 black body stain has been added (instead of the normal 10% Zircopax for white). Of course, a cover glaze is needed for a functional surface, thickly applied engobes must have the same fired maturity as the body for good fit. Lots of information is available on using L3954B (including mixing and adjustment instructions). Engobes are tricky to use, follow the links below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay (it is not a porcelain but has adjustable maturity to be fitted to them if needed). These bodies dry better than porcelains and are much less expensive, so coating them with an engobe to get a surface like this makes a lot of sense.
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 concave 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.
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).
L3685X white slip (left mug) has 5% more frit than Y (right). The frit is a melter, creating more glass bonds to adhere it to the body (it also hardens it and darkens the color a little). But the frit also increases firing shrinkage, 'stretching' the white layer on the body as the kiln cools (the slightly curled bi-clay bar demonstrates that). However the glaze, G2931G, is under some compression (to prevent crazing), it is therefore 'pushing back' on the white slip. This creates a state of equilibrium. The Y slip on the right is outside the equilibrium, it flakes off at the rim because the bond is not good enough. Adding more frit, the other side of the balance, would put the slip under excessive tension, reducing ware strength and increasing failure on exposure to thermal shock (the very curled bi-clay bar in the front, not this clay/slip demonstrates the tension a poorly fitted slip could impose).
The term Slip can have various meanings in traditional ceramics.
Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
Engobe Body Compatibility Test
A bi-clay test strip curls to demonstrate the drying and fired compatibility or fit of an engobe (or slip) on a ceramic body or of two bodies for marbling.