Lowell Hill Pottery

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Glazes Part 2 – Many (Overwhelming) Choices

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Chalices

Potters face a bewildering set of choices when creating and using glazes.

There are many different surface treatments for pottery and ceramic sculpture, but glazes are by far the most popular and common. Glazes can be shiny, matte, stony, cratered, crackled, glassy smooth, crystalline, and textured in a number of ways. They can be colored in almost any color that can be perceived by the human eye. These variations can be created, effected and affected by formula, method of application (brushing, dipping, spraying, splattering), thickness of coating, the firing schedule and type of fuel used for the kiln.

If all of this leaves you overwhelmed, add in the multitude of choices when it comes to raw materials for creating glazes and it’s easy to see why so many potters choose to use commercially produced products. These tend to be very stable formulations that behave predictably when used as suggested. Many potters use only a couple of base glaze recipes and alter them with different colorants or by changing the ratio of silica to alumina to adjust the gloss or matte of the glaze. I follow that philosophy when creating new glazes. But even that method has its limits. Here is a tale of my own experience.

I needed a new green glaze that was unrelated to the Rowantrees reproduction Evergreen that I already sell. This new glaze needed to have an “antique” character to it, with the green more muted and not as bright. I already had a good, stable recipe for a clear glaze that I had used to make a similarly antique white, blue and yellow. So I decided to try that recipe to make green. Reasonable, no?

Well, no, it wasn’t. Green is usually made by using either copper or chrome. Copper creates a reliable green only in lead-based glazes and in some high-fired glazes. Because all of my glazes are fluxed with boron – and I fire in the medium temperature range – copper created turquoise to blue. So copper was out for making green. That left two choices for me. Either I could use a green stain (more on that later) or chrome. Chrome was the less expensive choice and I already use it for the Evergreen, so I had a supply of it. I only need about 1% chrome to make a strong green color, which is very little as well.

Here’s the thing; every time I made a test batch of my base glaze using chrome to make green, the best I could manage was olive drab. Mind you, it was a pretty glaze that I intend to explore (particularly in combination with other colors), but it wasn’t what I was trying to achieve. Nor was it what my customer was waiting for.

What was going on here? Well, a little research told the story. The base glaze contains both lithium and zinc. Lithium is an alkaline metal and exists at the “reactive” end of the periodic table. In fact, lithium is possibly the most reactive after hydrogen. Zinc and chrome are transitional metals, with chrome being more reactive. When chrome is added to a glaze containing either lithium or zinc the result will tend to brown. So there you have it. I needed a totally new glaze recipe.

Nobody likes coming to that conclusion, but there you have it. I chose to adapt the turquoise recipe, which contains neither of the troublesome fluxes and got a satisfactory green. Sometimes the answer is right in plain sight.

Next time: common glaze defects to watch out for.

Answering Your Questions – Glazes Part 1

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I often get questions about pottery glazes: exactly what are they, what are they made of, are they safe, how long should they last, and quite a few others. In this and several coming articles, I will be discussing these questions. The more you know about pottery and glazes, the better informed your purchasing decision will be and the happier you will be as a result.

So to the first question: What exactly are glazes?

In the simplest terms, glazes are suspensions of minerals and metallic oxides that are used to coat ceramic forms. When applied to those forms, the water soaks into the clay form leaving an even coating of the minerals and other materials on the surface. When fired to the correct temperature, the glaze materials melt to form a glass coating on the form.

Sounds pretty simple, right? Tomes have been written on glazes. New recipes are created every day. The vast majority of these are discarded as failures. Only a few of those that succeed will see regular use. Even fewer will become favorites destined to be used for many, many years. A fraction of a percent of them will become heirloom glazes recognized throughout history and in many cultures. The art and subtle science of glaze making are complicated, and as time goes on, the preference for mere beauty is equaled by a need to safety and permanence.

But enough marketing talk, what are they made of?

At its simplest, a glaze contains three things; a flux to promote melting, a glass maker to form glass and an intermediary to harden and stabilize the glaze. The most common fluxes are sodium, potassium, boron, calcium, magnesium, lithium, zinc, barium, strontium and lead (pretty much in that order). Each of these has its own specific set of properties and behaviors; advantages and disadvantages that need to be kept in mind. Usually a glaze will contain more than one primarily because they do not exist in pure form naturally. They must be “sourced” from various minerals. The exact chemical composition of the various minerals used in ceramic glazes helps to determine which material to use.

The most common glass maker is silica. Silica is also found in virtually all minerals including clay. It can also be found in a relatively pure form by powdering quartz. Boron is a secondary glass maker in addition to being a flux.

The most common intermediary material is alumina, and while it can be had in pure form, it is most often obtained from clay – the pure form being extremely expensive. The intermediary hardens and strengthens a glaze. The ratio of alumina to silica in a glaze will decide how shiny or matte the surface of the glaze will be. The more silica, the shinier the glaze will tend to be – as long as there is sufficient flux to maintain the melt.

Ideally, a glaze recipe will have as few ingredients as possible. The more materials in a glaze, the more issues that will tend to arise as various chemicals in the composition react with each other. Keeping things simple helps to avoid trouble.

Glazes have to be carefully formulated if they are to be used for dinnerware and everyday use. Formulas that are not properly balanced and hardened sufficiently may be prone to leaching; the gradual loss of glaze material into the food or liquid in contact with the pot. The leaching of lead – once the most popular flux in ceramics manufacturing – came to the fore a few decades ago when people were poisoned by food that had been stored in lead glazed pottery. That pottery was not manufactured in the USA (there has never been a case of lead poisoning from domestically produced pottery to the best of my knowledge), but the fact that lead is toxic and accumulates over time in the body has made its use in pottery manufacturing impossible. It is not even possible to obtain lead from ceramic suppliers any longer.

More on glazes in my next article!

Black and Seagull

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I have had several recent inquiries regarding the Black and Seagull glaze pattern. Rowantrees was once asked by the Maine Republican Party to produce a set of dinnerware in this patter for Dwight and Mamie Eisenhower when they made a trip to Maine in the 1950s. A set of what became known as the Eisenhower pattern now sits in the Smithsonian Institute’s Presidential China collection. I don’t believe it’s on permanent display, but it’s there.

And I made much of it!

Sheila Varnum resurrected Black and Seagull while I was working at Rowantrees and it quickly became a popular choice. Indeed, it is one of my favor

So it should not be a surprise that I did make an attempt to bring this pattern back. The initial results were excellent as the picture shows. But Black & Seagull Mugboth glazes had one minor problem. They did not apply at all easily to the pot. Because I apply the glazes using a spray gun (as Rowantrees always did), the glazes need to dry almost on impact to assure a smooth and consistent surface. These glazes did not dry at all quickly and would begin running down the side of the pot if I did not apply them in very thin coats. That meant that I had to stand there for a great deal of time to get a decent coating built up on the pot.

Spending a lot of time on one pot isn’t good when you’re trying to glaze several dozen of them at a stretch. So I began the process of finding a better glaze for the purpose. I found one that looked very promising and that applied very well (and wanted to be applied thinly anyway).

Then I got the word that I was going to have to move out of the building. Oh well.

So yes, Black and Seagull will return. Once I have a new workspace, the process of developing a new replacement for this wonderful pattern will begin anew.

What Disaster Can Tell You

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Broken PotIt makes no difference how much experience I garner, every now and then I will do something monumentally stupid. Several weeks ago, I had a ware board full of ramekins. I placed the board on a shelf with about half of the board extended out over the edge of the shelf. Then I set to work unloading the board.

Starting at the wrong end.

Almost immediately, the ware board with all those beautiful ramekins began tipping like an insane seesaw causing several ramekins to “introduce” them selves to the floor. Sancho Panza put it best when he said, “…whether the stone hits the pitcher or the pitcher hits the stone, it’s going to be bad for the pitcher.”

I lost four ramekins to that little act of idiocy, but it’s interesting what such a disaster can tell you. Not so much about my working methods, but about the product I make.

Many years ago (actually, it was decades ago), I was working in the pottery studio of a private school. I actually worked in the kitchen of that school, but they allowed me to putter about in the pottery studio when I was not at work. It kept body and soul together for the few months I was there. The students were aghast at my habit of cutting pots I had just thrown in half so that I could see how the wall of the pot looked. I can remember the teacher telling them, “You guys should be doing that more often.”

I rarely cut pots in half these days. I don’t really need to as the lesson of an even wall has been well learned. Still, it’s always interesting to look at the profile of a pot when it meets an untimely end. And truth be told, I have actually broken pots on purpose to get the sort of information the little beauty in the picture above revealed.

So what can I tell from what I see? Three things.

First, I can see that the wall is nice and even. No surprise there. You may also notice the slightly thicker rim. That makes the pot less prone to warping during manufacture and chipping during use.

Second, I see that the pot only broke into about four pieces – most of them quite large. That means that the pot is extremely strong. A weaker vessel would shatter into a lot of small pieces.

Third, I see that the glaze perfectly follows the same break pattern as the clay. That means the glaze fit is exceptionally good. I knew that, given the tests I put the glazes through. But it’s good to see it up close.

Most people don’t realize it, but a glaze has to fit the clay it’s applied to perfectly. The critical measurement is what happens when the pots cool in the kiln. Everything expands while heating and contracts while cooling. If the glaze contracts more than the clay does, it will be under a lot of tension. In a case like that, something has to give, and the glaze will form a fine network of stress cracks. Potters call this crazing. Crazing weakens a pot and will inevitably shorten its life. Some glazes are specially formulated to produce the same crackle pattern seen in crazing without causing problems for the pot, but crazing as a gaze flaw is something to be avoided.

If the clay shrinks more than the glaze, then the glaze can pop off, resulting in tiny, razor sharp pieces of glass that can end up in the food or beverage the pot was holding. This problem is called shivering and it is one issue that can keep a potter up at night.

Ideally, a glaze should shrink slightly less than the clay, but not enough to cause shivering. That assures that the pot will be strong and last a good long time in normal use.

And the best way to prove the strength of a pot is to break it on purpose – or, in this case, by accident – and see how many or few pieces it breaks into. The fewer the pieces, the stronger the pot. If the glaze margins align perfectly with the edges of the broken clay, the glaze fit is perfect.

I make high quality pottery. The picture proves it.