Engobes must be drying, firing and thermal expansion compatible with the body they are on. Engobe are body-like, they do not melt, they are not glazes. That means they have a less developed interface with the body. Thus it is doubly important to fit them as well as possible to avoid flake-off in response to mechanical or thermal stress (this is serious if it happens on functional ware having a glaze over the engobe, sharp flakes of glaze could enter food or drink). Thousands of tons of engobes are used in the tile industry and it is also vital that they fit properly.
This test gives the engobe much more thickness than it would normally have to amplify differences between it and the body. The test is done by preparing, drying and firing multiple bi-clay strips (thin sandwiches of body and engobe) and observing the way they bend during the processes. Extreme bends indicate incompatibility. When fit to a plastic clay body is being tested, the engobe must be dewatered and brought to the same stiffness as the body. Then they are individually rolled to slabs of 1/8" thickness, sandwiched and cut into and 8x5 inch size. That slab is then rolled back down to 1/8" thickness. To avoid one side being thinner than the other, the slab should be lightly rolled on one side, turned over and rolled again. Do this repeatedly until the 1/8" thickness is achieved.
Finally, cut bars four bars to 1 x 5.5". Lay two of them with engobe side up and two with body side up. Dry them and measure the amount of warp of each to get an average. Record that. Fire them and repeat the measurements and record.
Ideally, the bar should bend slightly toward the body, putting the engobe under a little compression.
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).
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).
The front bar was dried with the white engobe upward, the rear one with it downward (an EBCT test). While both have curled toward the red body, the front one has done so less. To enter the results we will average the amount of bend. But there is another problem. During rolling, the white body thinned more and is thus not as thick as the red. Thus there is question about whether the bend is because of this or because of higher shrinkage of the red.
This is Odyssey slip, a engobe recipe that is trafficked on the web. It is recommended for low, medium and high fire ware. It is 30% Ferro Frit 3110 and 70% ball clay. This is a bi-clay strip, a sandwich of two plastic clays rolled into a thin slab and cut into a bar (to make the bar the Odyssey slip was dewatered to typical pottery clay stiffness). We are looking at the engobe side of an EBCT test (the other side is Plainsman M390). During the latter stages of the firing the engobe has begun to melt and blister and darken in color (which it should not be doing). During earlier stages of firing this engobe would certainly have had a higher shrinkage and would have bent the bar its way. But it is now bent the other way. That means the engobe could well be under compression (having a lower thermal expansion than the body). Or the body could simply have pulled it the other way when the engobe lost its rigidity. Either way, the engobe does not fit this body at this temperature.
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 how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. But during firing the porcelain has a much higher fired shrinkage and it pulls the bar toward itself. During cooling, the sculpture clay has a higher thermal expansion and it pushes from its side bending the bar further. This bar is a bomb of internal stresses, just waiting for a mechanical or thermal stress to bust it into a hundred pieces. Admittedly, putting a thin layer of this onto a piece of heavy ware is not going to bend it. But will it flake off when exposed to stresses (like freeze thaw, being put in an oven, having a hot liquid poured into it, being bumped).
The EBCT test bars (engobe compatibility) in the foreground demonstrate the issue (they sandwich the engobe and body as a bi-clay strip). After firing at cone 10R they have curled toward the engobe side, indicating that it is shrinking more. On the mug the engobe has done likewise, shrinking more than the body and creating a crack pattern.
L3685U slurry was applied to the insides of both of these mugs. But on the left it is a "slip", on the right an "engobe". Why? The left mug only has a thin layer, applied by painting a gummed version on (at leather hard stage). On the right a gelled slurry was poured into the leather hard piece, poured out and the rim dipped (creating a much thicker layer with more power to impose its own drying and firing shrinkage). So it is much more important that the latter be compatible with the underlying body. The EBCT test is used to measure how compatible the body and engobe are.
BNDR - Bend Dry mm/Toward (V)
Indicate direction of bend and whether toward A or B- Material B (V)
BNFR - Bend Fired mm/Toward (V)
Indicate direction of bend and whether toward A or BBCON - Cone (V)
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.