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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.

Cross section view of the inside and outside glazed walls of a porcelain vessel

Cross section view of the inside and outside glazed walls of a porcelain vessel

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.

Solving a difficult engobe flaking problem

Solving a difficult engobe flaking problem

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.

Bi-Clay strips test compatibility between engobe and body

Bi-Clay strips test compatibility between engobe and 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).

A non-vitreous body can have a very poor bond with the glaze

A non-vitreous body can have a very poor bond with the glaze

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.

The green underglaze is failing on impact

The green underglaze is failing on impact

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.

Out Bound Links

  • (Glossary) Crazing

    Crazing refers to small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very s...

  • (Glossary) Engobe

    A white or colored slip applied to clay as a coating. The term "slip" is often used interchangeably with this, but we think of slip more as a decorative material/process. The tile industry uses the largest volumes of engobe by far, these are employed as opaque barriers between less-than-white bodies...

  • (Glossary) Shivering

    A defect in glazed ware where the glaze is compressed too much by the body, the glaze actually peels off the ware on edges to relieve the stress. Shivering is thus the opposite of crazing and is also less common. This problem is potentially dangerous, since the tiny flakes of glaze having razor-shar...

  • (Glossary) Earthenware

    A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). The term earthenware almost alway refers to red burning terra cotta ware (although it is a somewhat more general term referring to a wider range of colors and more primitive forming and firing techniques). E...

  • (Glossary) Porcelain

    Traditional utilitarian porcelains are comparatively white burning and vitreous clay bodies that are made from feldspar, clay and quartz. When fired, the feldspar flows and dissolves many of the other particles into a viscous glassy melt that bonds the quartz particles and, and if temperatures are s...

By Tony Hansen

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