Techno File: Clay Veneers Ryan Coppage, PhD

Appears in the May 2024 issue of Ceramics Monthly.

Do your glazes pinhole, bubble, or blister? And have you noticed that this mostly happens on darker clay bodies far more than lighter ones? This article provides strategies to avoid these unfortunate kiln results, reduce costs, and save more of the work you fire. 

Define the Terms 

Bloating: The build-up of gaseous pockets under a clay wall that, instead of escaping, creates a bulge on a vitrified clay surface. This happens when a clay is vitrified before its off-gassing has completed. 

Bubbling/Blistering: A more extreme version of pinholing, large bubbles or blisters are present in a glaze after firing, making it sharp/dangerous to handle and also possessing pockets for bacteria to grow and contaminate food or drink as the vessel is used. 

Off-Gassing: The evolution (giving off) of gases through thermal decomposition of metal oxides, sulfur/nitrogen content, and even some getter reduction material in clay bodies. This happens slowly during a heat ramp and is not ever quite finished at one specific temperature. 

Pinholing: Small pockets or holes in a glaze from evolution of gases that can trap food or drink and ultimately let that organic matter rot, creating a scenario in which bacterial growth is cultured inside the holes. 

Firing Gases 

There are only a few truly profane words in pottery. Everyone knows them, sometimes we utter them in disgust, and there are often hours and hours spent trying to avoid them at all costs. These are: bloating, pinholing, bubbling, and blistering. There are a few others, but these four are all caused by the same phenomenon: off-gassing. More often than not, this is most prevalent with dark clay. And typically, the darker the clay, the worse it is. This is most often because those same metal oxide blends that make your clay dark also give off gases while they’re heating. Those gases join together at the surface, and then push their way out through the glaze, leaving their escape path behind as a pinhole. 

A more extreme version of pinholing is bubbling or blistering. This is when the quantity of gas escaping is so great (from the clay or the glaze), that the channels they escape from are huge bubbles—effectively boiling at the glaze surface during the firing. Finally, bloating, the worst of all the pottery gas profanes, is when the surface of a clay vitrifies or seals over before all thermal decomposition gases have escaped and it builds an air pocket under a matured clay surface, bubbling it out. This happens when fluxes are used to lower a clay’s vitrification temperature, but there are materials in the clay that require more thermal energy to decompose than is required to vitrify the clay body. This has become less of a problem lately, but was uttered commonly during the big transition from cone 10 to cone 6 about a decade to a decade and a half ago. 

Dark Clay Bodies 

There are commercial dark brown, dark gray/slate/black, medium brown, and every other color imaginable clay bodies available for purchase. These bodies are often finicky to work with—and a nightmare to experience when you open the kiln. The pigments in them are pretty homogeneously dispersed throughout the clay body. In order for a clay to be dark, you often need up to 20–25% pigment blend, which also fluxes the clay and lowers its vitrification temperature. At that point, you lose many of your workable clay-like properties that we assign to throwing/handbuilding and it starts to become more appropriate as a Wedgwood pouring slip. An easy thing to do is to assume that the darker a clay body is, the more pigment it has blended throughout its body, and the worse it will eventually pinhole and bloat/blister/bubble during a firing as the pigments undergo thermal decomposition and give off gases. 

A somewhat controversial opinion: in order for a clay body to be any particular color, those pigments only have to be at the surface of the clay. That means that you don’t have to wedge expensive stains into porcelain; you just have to make a stained slip and dip the work into or brush the slip on at the leather-hard stage. If you want a black clay, just make a black slip and dip your piece at leather hard. Let it dry slowly and bisque fire. If you want blue, make a blue slip. If you want a porcelaneous, beautifully vitrified surface, make a slip out of a bag of Laguna Midrange Frost and brush it on when leather hard. 

There are so many potters wasting money by mixing expensive (but unmatched in performance) stains into their clays, when they only need about 5% of the total amount to exhibit color at the surface. 

Firing Schedules 

After fighting with dark clays, stains, and a huge range of firing schedules to try to off-gas at bisque, I stumbled across a Robin Hopper slip recipe. It is formulated to be dippable, can take most any pigment, and is stable from cone 04–12. 

An additional trick to managing off-gassing is the “Stop-and-Drop” (by John Britt) or the “Drop-and-Soak” (by Tony Hansen of Digitalfire) firing schedule, listed below. 

Or a “Drop and Hold, Slow Cool” schedule (listed below), which is just one added step, slow cooling to 1400°F (760°C) before shutting off. This is best for matte glazes that notoriously bubble/blister/pinhole.

What this firing schedule does is creep up to just barely vitrify clay, then drop down to cone 3–4, in which glazes are still actually very fluid and allow bubbles/blisters/pinholes to seal from surface tension with a longer hold time—when the glaze isn’t trying to sprint off of the pot toward your kiln shelves. This schedule changes somewhat depending on what temperature your test cones melt at, but it is pretty close to what I use (I go to 2210°F (1210°C), with a longer hold time at 2110°F (1154°C) (45 minutes). 

If you have both a dark clay body and a black slip on it, you very likely need a longer hold for your “drop” or “soak,” contingent on whose firing schedule you are following. 

The glaze recipe for the mug below is above. Most importantly, it wasn’t made with dark clay! It is Standard 213 clay, a cone-6 porcelain, dipped in Robin Hopper Dark Clay Slip at leather hard and then bisque fired normally. This is infinitely easier and less problematic than using black or slate/dark clays—both in throwing and glaze troubleshooting. 

What It All Means 

Hopefully, this saves a lot of people the headache of troubleshooting bubbles, blisters, pinholes, and bloating. I was struggling with a dark clay body blistering horribly—and then putting a black slip over it and it being even worse; however, putting Robin Hopper Black Slip over the Standard 213 clay body was a life-saver for emulating certain black clays that are notorious troublemakers in many pottery studios. It also allows for gorgeous rutile/cobalt blues that are partially translucent, and make for aesthetically interesting bleed-through effects from the black-clay veneer underneath the glaze. On the other side of that spectrum, I have a bucket of Laguna Frost white slip veneer that provides the most amazing emerald greens from oribe glazes—also over Standard 213. 

Special thanks to Tony Hansen for the “Drop-and-Soak” firing schedule: digitalfire.com/schedule/plc6ds. 

the author Ryan Coppage is currently a chemistry faculty member at the University of Richmond. He fiddles with various glaze projects and makes a reasonable number of pots. To see more, visit www.RyanCoppage.com.