|Monthly Tech-Tip |
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.
In ceramics, the zone of adherence between glaze and the underlying body is called the interface. The integrity of body-glaze interface is important to strength and functionality of ware. In porcelains it is much more highly developed than in earthenwares. In the latter, the glaze, although melted well, is in contact with a body that is developing no glass and has little strength. Thus the mechanism of the bond is simply the roughness of the body surface (where the glaze can hang on) and the collective strength of that roughness. With porcelain, the glaze is able to form a bond the spans across the boundary in both directions, creating a interfacial zone of changing microstructure. Needless to say, this bond is almost unbreakable. Other types of body/glaze combinations form bonds that are somewhere between these two extremes.
Because higher temperature and/or more vitreous bodies form a better glaze bond ware can withstand greater differences in thermal expansion between glaze and body without crazing or shivering. By contrast, earthenwares shiver and craze easily.
Interface is also an issue with engobes and slips. The integrity of the bond they form is analogous to glazes, but with much less glass development. Porcelains, by nature, do not need an engobe, whereas lower temperature bodies whose surface needs to be color-lightened for glazing commonly employ engobe coverings (especially in the tile industry). However, making the engobe fit the body can be much more difficult. Its drying shrinkage, firing shrinkage, quartz and cristobalite reaction and thermal expansion all need to be matched to the body.
Porcelains look much more glassy and melted than you might expect when viewed close up (this is cone 6 Polar Ice from Planisman Clays). The development of the glassy phase within the body creates a very good bond with the glaze. Actually it is a bonding zone where the glaze has melted into the body enough to create a transition rather than just a point of contact. The degree to which this transition develops determines the integrity of the bond. Of course, with porcelains it is far better developed than with stonewares and terra cottas.
This glaze is on very thick. That gives it the power to impose its thermal expansion (which is different that the body) to the point where it literally flakes off. The problem is worsened when the glaze and body lack fluxes, that means they do not interact, no glassy interface is formed.
This is a low fire fritted stoneware fired to cone 03. But it still has about 4% porosity. The green underglaze is not developing enough glass to bond well with the body surface. Repeated blows to the surface by a hammer are chipping off chunks of glaze/underglaze at the bond with the body. This is not happening with the other underglazes. The green underglaze is obviously more refractory than the others and should be reformulated.
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 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.
The same glaze on both pieces. Fired in the same kiln at cone 6. Both are porcelains, but the one on the right is far more vitreous (it fires off-white so coloured glazes lose some of their brightness). The body is so vitreous that there is considerable interaction between it and the glaze, enough diffusion occurs to affect the surface character.
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
What is the difference between earthenware and a regular stoneware body? Earthenwares lack the glass development to fill voids and glue particles.
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 and fire opaque. They are used for functional or decorative purposes.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.