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Body glaze Interface

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.

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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 the 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). This mechanism 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 that spans across the boundary in both directions, creating a interfacial zone of changing microstructure and chemistry. Needless to say, this bond can develop to be 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, slips and underglazes. The integrity of the bond they form is not like glazes, there is 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.

The infacial zone between body and glaze can enable the exchange of oxides that modify the character of the glaze. For example, metal oxides like iron or manganese in the body will typically migrate into the glaze, changing its color, sometimes profoundly. Porcelain bodies fluxed using nepheline syenite or feldspar have plenty of Na₂O that can fuse into glazes (depending on their melt fluidity) and affect the development of surface crystalization and gloss. Glaze melt fluidity itself can really be affected by the body (subdued in non-vitreous bodies and greatly enhanced in vitreous bodies).

Related Information

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

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 concave 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

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

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.

Super vitreous body glosses a normally matte glaze

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.

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.

Melt flow test demonstrates glaze:clay interaction

The melts being compared here are our code number 6998, a production run of Alberta Slip. The same sample batch and ball weight is being compared in these two flow testers fired side by side in a cone 10R kiln. Why are the flows behaving so differently? It is the clay from which the flow testers were cast. The one on the left is made from L4404A, a highly refractory casting slip. The one on the right is M370, a medium-temperature porcelain (it survives pretty well to cone 10 but is obviously very vitreous). The difference in the flows (the width and length) is a product of the interaction with the material being tested and the tester itself. On the M370 tester the flow is adhering to the clay surface so well that it has spread and thinned enough so that few bubble-breaks are visible. This interaction has even slowed the flow. But the L4404A flow tester is clearly better, minimizing interaction and better revealing the fluidity of the melt.

Same iron red glaze on black stoneware and on porcelain

The glaze is G3948A iron red fired at cone 6 using the C6DHSC schedule. The bodies are Plainsman Coffee Clay and Polar Ice (the insides are different glazes). They were in the same kiln. These mugs demonstrate how much reactive glazes can interact with the body beneath and how much that affects their fired properties, especially when they have high melt fluidity like this one. On the left the glaze is drawing color out of the body. The porcelain on the right has no color to give but it does have sodium - and it is supplying enough to act as a catalyst to the creation of the iron crystals.

This pottery glaze is not flaking off - the underglaze is.

Pure ceramic stain powders don't melt at typical pottery temperatures so they cannot bond with clay body surfaces. They don't suspend in water and don't harden on drying. Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a 'base medium', designed to impart the missing properties. Unfortunately, consider a problem: Blue stains are fluxes and potent at low percentages. Orange stains (and others) are refractory and require much higher percentages. But for convenience underglaze manufacturers will want to use the same medium and percentage pigment to make an entire line of underglazes. That causes the problem seen here - flaking is happening at the interface between the body and orange underglaze. This underglaze recipe needs more frit (or a more potent one) so that enough melting occurs to create a better bond. For some colors it can thus be an advantage to make your own underglazes.

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