A pinhole marring a fired glaze is possibly the most troublesome of all glaze faults, disrupting an otherwise smooth surface. Slow firing at peak temperature helps heal pinholes, but preventing them in the first place is the holy grail of a smooth glaze.
Defining the Terms
Pinhole: A glaze flaw produced by a bubble bursting at the glaze surface, revealing the fired clay body beneath.
Viscosity: The ease or difficulty with which a glaze will flow when subjected to a given force.
Surface Tension: Internal cohesive force of a liquid that, as it increases, causes the liquid to resist wetting a surface of another material. A high surface tension glaze forms a bead on a clay body surface rather than spreading onto the body.
The Root of the Problem
The root cause of any pinhole is a large glaze bubble. If no bubbles form in the glaze, no pinhole faults will blemish the glaze.
A pinhole starts as a bubble at the contact point between a glaze and the surface it’s being fired on. During the firing, the bubble grows so large its diameter is greater than the thickness of the glaze and the bubble bursts at the surface of the glaze. At that point, either glaze flows into the crater left behind by the bursting bubble, or it doesn’t. The latter case produces a pinhole.
The gases that form bubbles, leading to pinholes, have many potential sources. Decomposition of oxides in the body or glaze and gases trapped in pore spaces in both are obvious culprits. Because bubbles can come from many sources, most efforts to prevent pinholing focus on making glazes runny enough, and with low enough surface tension, so molten glaze fills the craters left by bursting glaze bubbles. To make runny, low-surface-tension glazes requires managing glaze composition.
Adjusting the proportion of clay in a glaze is the most common starting point. Alumina atoms raise glaze viscosity and thus also raise the surface tension. Adding clay, an alumina silicate mineral, will make a molten glaze “thicker,” or more viscous. Reducing clay content thus makes the glaze runnier or less viscous.
An increase in the proportion of the flux element magnesium also raises glaze surface tension. In high quantities, magnesium makes a glaze with an excessively high surface tension (for example, crawl glazes). Flux, colorant, and glass-former atoms all influence glaze viscosity and surface tension to some degree. Given the virtually infinite number of possible combinations of these elements in a glaze, testing is necessary to balance glaze fluidity and surface tension for a given application.
In general, adding any flux element or colorant (with the exception of some commercial stains) will reduce glaze viscosity and surface tension. Since colorants are ordinarily used in small quantities, their effect is usually small. Cobalt may become an exception. When used to make an extremely dark blue, cobalt can make a glaze quite runny.
Increasing the proportion of fluxes in a glaze lowers its melting temperature. Adding the glass-former boron to a recipe, or substituting it for some of the silica in the glaze, has a similar effect. Both permit a glaze to remain molten longer during a firing, and that extra time can heal potential pinholes.
Adjusting a glaze formulation can help heal pinholes, but it’s better to prevent them from forming in the first place.
Studio glazing is typically done with glazes applied to work that has been fired to a sturdy but still porous state. The porous ware absorbs water from the glaze, assuring that a glaze coating of adequate thickness adheres to the body.
The process of producing porous ware in the first firing makes later glaze application convenient, but also guarantees there will be air under the glaze in the pore spaces in the body. If the glaze melts before all air is driven out of the body by densification, the air coming from the body is trapped as bubbles in the glaze and can produce pinholes. To compound the problem, air can also be trapped between the applied glaze and the body.
Industry fights the first of these problems by firing hotter in the first firing, which is typically to full body density. However, the dense clay cannot absorb moisture so glazing is typically done by spray application onto heated ware. The heated ware dries the water from the glaze rapidly before the glaze has a chance to run off.
Industry also commonly uses some kind of surfactant or low-foaming soap as a wetting agent in the glaze mix. This helps the glaze wet the body and minimizes air trapping between the glaze and body.
In the glaze mixing studio one can also use a surfactant in the glaze mix. If twice-firing, wetting the surface of the ware before glazing can also be helpful. This will permit the glaze to more completely wet the surface of the ware. It is most convenient to do this while washing the ware to remove dust which may have accumulated during or after the first firing.
When using a fuel-heated kiln and firing to cone 10, firing in reduction up to peak temperature will reduce iron and prevent high-temperature off-gassing of oxygen from iron oxide. This is essentially preventing oil-spotting (created by thermal decomposition of iron oxide beginning around cone 8) by reducing the iron before the glaze melts in a thick, high-iron glaze. The oxygen given off creates craters that heal so the surface is relatively smooth but has a characteristic look.
In the final analysis, slowing the glaze firing near peak temperature to allow time for pinholes to heal over can be helpful. If the last 15 to 30 minutes of the peak cone bending occurs at a constant temperature and pinholing still occurs, then a less viscous glaze may be required.
This article was excerpted from the June/July/August 2013 issue of Ceramics Monthly.