|Monthly Tech-Tip |
Classic terra cotta bodies are not vitreous, so engobes used on them need to have similar low fired shrinkage. But when terra cottas are fired above cone 04 they start to mature and fired shrinkage increases quickly, flaking off engobes that do not have sufficient added frit (to increase their shrinkage). Even if an under-compression engobe can hang on, the extra stresses of an overglaze of lower thermal expansion can compromise the engobe:body bond. That has happened on the center mug. That engobe has less frit (10% vs. 15% for the others). The clear glaze on the left has high thermal expansion and is crazing, while the engobe:body bond can tolerate that it, is obviously not desirable.
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).
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
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.