Glaze laydown

Refers to the quality of the dried ceramic glaze layer and how this affects the fired result: e.g. density, hardness, evenness, thickness, freedom from defects, etc.

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In industrial ceramics, the quality of the dried glaze layer, or laydown, is an important part of process control. The objective is a layer of controlled thickness that is even, dense, well bonded and free of cracks, pinholes, crawling precursors and other discontinuities. Industrial glaze slurries, application equipment and drying systems are highly tuned to achieve this. For many manufactured ceramic products, even a single glaze defect can reject an entire item for recycling, refiring or discard.

Potters tend to give glaze laydown much less attention. Since the glaze will melt, there is a natural assumption that irregularities in the dried layer will simply heal during firing. To some extent they do. But a glaze melt can only redistribute material that is present, and it has limited time and fluidity with which to do so. A defect already present in the dried layer can become the starting point for a crawl, pinhole, blister, bare spot or thickness variation in the fired glaze.

Many factors affect laydown quality. A smooth porcelain surface is generally easier to coat evenly than a coarse or heavily grogged body. Moderate, controlled glaze thickness is easier to dry without cracking than an excessively thick layer. A well-suspended, thixotropic dipping slurry can deposit a surprisingly dense and even layer, whereas a poorly suspended one may drain unevenly, settle rapidly or produce thickness variations. Drying should be reasonably quick and uniform, but not so rapid that shrinkage stresses crack or detach the coating.

Glaze recipe and slurry preparation are equally important. Adequate fine clay particles improve suspension, adhesion and cohesion of the dried layer. Recipes having little or no clay can produce fragile, powdery or poorly bonded coatings unless compensated for by gums, binders or other additives. Particle-size distribution also matters: a well-designed mixture of particle sizes can pack more densely and produce a more coherent layer than one dominated by coarse particles.

Gummed slurries can produce excellent laydown because the binder improves adhesion and dry strength. However, they retain water and penetrate porous bisque more slowly, so dipping application can become difficult. Achieving a dense, even layer that dries reasonably quickly requires control of slurry rheology, specific gravity, application and drying. Slow drying on porous bisque can introduce its own problems.

Commercial brushing glazes might appear disadvantaged because brush marks and overlapping strokes make the wet coating visibly uneven. However, their low specific gravity, gum content and requirement for multiple coats work together: successive layers fill and level irregularities and make it possible to build the desired thickness gradually. Brushing also permits deliberate variation of thickness on specific areas of an object.

Spraying likewise has the potential to produce very high-quality laydown. With suitable slurry properties, spray equipment and operator technique, glaze can be deposited gradually and uniformly, even on complex shapes. But poor atomization, excessive overspray, application to an overly dry surface or failure to build sufficient wet cohesion can produce a porous or powdery layer rather than a dense one.

Glaze defects such as crawling, pinholing and unhealed blisters often send potters looking for another recipe. Industry is more likely to regard the glaze recipe as only one part of the system. The slurry rheology, particle-size distribution, additives, application thickness, body surface, bisque condition and drying process are all potential causes. Improving the dried glaze laydown can sometimes solve a fired glaze defect without changing the glaze chemistry at all.