The white slip on the left is an adjustment to the popular Fish Sauce slip (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right (L3685C with 15% frit) is becoming translucent, obviously it will have a higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). The slip is basically a very plastic white body. Since these are not nearly as vitreous as red ones at low fire they need help to mature and a frit is the natural answer. With the right amount the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body.
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).
These are thermal expansion curves for body, engobe and glaze (from a dilatometer, a device that measures it against increasing temperature). The upper line is the body. The center line is the engobe. The lower line is the glaze. The ceramic tile industry is very conscious, not only of glaze-fit but also engobe-fit. Engobes (slips) are employed to cover brown or red burning bodies so they glaze like a porcelain. Typically technicians tune the formulation of the engobe to have an expansion between the body and glaze. The body is highest so that during cooling, as it contracts, it puts a squeeze on the engobe (the engobe thus never finds itself under tension). The glaze has the lowest expansion, it is under a state of compression by the engobe (and slightly more by the body). This equilibrium enables the tile to wear for many years without crazing or shivering. Chart courtesy of Mohamed Abdelmagid.
An example of an engobe (slip) applied to dry ware and then fired at cone 2. The one of the left has been poured, the right sprayed. Control of the thickness of engobes is important, thus the specific gravity and viscosity of the slurries are very important. Engobes are invaluable since a red or brown burning body can be made to fire white like porcelain (enabling much better glaze surface quality).
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.
Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them.
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.
L3685U - Cone 03 White Stoneware/Engobe
A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on a matching red stoneware.