About Glaze

So your piece has been bisqued and you are ready to glaze. Nervous?

If your first exposure to glaze was anything like mine you are faced with a selection of ten to twelve 5 gallon buckets of grayish slurry with water on top. Maybe some old test tiles on the wall showing how some of those glazes look on a claybody that looks nothing like what you are using.

I am expected to dip my precious creations in these grey shades of muck and trust the kiln to do its thing.

Perhaps your first glaze experience isn't like mine at all and you have little pint bottles full of glaze all labled as to color and surface. You have a brush in hand and ready to give glazing a whirl.

What ever your first experience with glaze is like I think with some certainty I can say its kinda scarey in a fun exploritory way.

Don't worry once you get to know your glazes you will find your successes far out number your failures and eventually you'll actually start trying to get an unexpected result, for a result to call your own.

When clay workers talk about clays and glazes they often will refer to a particular clay or glaze with a cone or cone range designation. What does this cone thing mean?

Generally when one talks about a glaze being cone 10 or cone 6 or cone 04 glaze one is talking about the temperature the glaze will be fire to. If you visit the Pyrometric Cone link in the previous section you will see cone #'s and their temperature equivalents. So if I say I am firing to cone 10 what I am saying is that I am firing to approx. 2381 degrees fahrenheit.

The chart you see goes from cone 022 to cone 13 with cone 022 being the lowest temp and cone 13 being the highest on this chart. Somewhere in the middle of this chart cone # change from being in the format of having a 0 in front of a number to having no zero in front of the number so pay special attention to that zero placement, a cone 010 is not equal to a cone 10. Also note a Cone 010 is a lower temp than cone 06 but a cone 10 temp is higher than a cone 6 temp.

Often the word cone is replaced with a triangular symbol when one is writing out a recipe or writing down firing directions. You also might see cone temperature typed as ^10 instead of cone 10.

It is important to know the cone or temperature range of a particular glaze so you know what temperature to fire your ware. A cone 6 glaze fired to cone 10 can be a disaster to kiln shelves. A cone 6 glaze fired higher than is suggested could potentially flow and adhere your ware to your kiln shelf(this would not be a good thing for your shelves or your ware). Your cone 6 glaze fired to cone 10 might not flow so badly but it could so testing is needed to know what a glaze does when fired beyond the suggested range. Typically matte glazes fired higher goes glossy but fired too high even a matte glaze can ruin your shelves.

Glaze can make or break a peice...really I mean it...glaze can really break your work. Its called glaze fit. If the glaze you choose does not have a good fit(meaning it has similar shinkage) with your claybody your glaze may crackle, shiver off, crack your work, or break it into many peices. I have a story...

As I was preparing for my BFA exit show I was firing the last of the bowls I intended to put in it. These were very thinly thrown bowls using a heavily grogged claybody. These bowls were the thinest I had thrown to date and felt amazingly light in the hand when picked up. I was enjoying the heavily grogged claybody and so only glazed the interior so as to showcase the grog texture on the outside of the bowl.

I fired them in the downdraft kiln to cone 10 reduction as usual for my work at the time. The kiln cooled, I unbricked the door. The work turned out great and I started unloading. I unloaded the work from the bottom first and worked my way up to the upper shelves where my work sat.

I was moving work of others to the other side of the room when I heard a ping...a slightly too loud ping for the usual small pings of cooling pots. I turned and saw one of my bowls laying in 5 peices on the shelf. I picked up the the shards and examined them, nice smooth breaks running down the side of the pot. I was puzzled but the ways of the kiln are sometimes a mystery.

I finished unloading and brought all my work out and placed it on the desk to pack up later. Then I heard another ping...another self breaking bowl...then another...and another...that evening while I worked in the studio I listened and watched 8 bowls break in such a manner.

After a talk with my professor, we concluded a variety of factors contributed to the bowl carnage. The thinness of the bowls...I had reached the the point where there was likely more grog in the walls than clay, you might call it the clay body's breaking point. Reduction perhaps made the very thin walls more brittle and likely to give way to stresses. Glaze on only one side of the vessel walls stressing the bowl further. And lastly perhaps a bit of glaze fit problems while not enough to cause a problem normally but with me pushing the clays limits and glazing only one side the stresses were too much and the glaze pulled the bowl apart.

The bowls still were a part of my exit show...What a little thing like kiln chaos ruin my show? I framed the shards and hung them on the wall. They were well received.

Sometimes reglazing a piece will fix a problem or a disappointing result you encountered during the first glaze firing. Here are some tips to get more glaze to stick to an already glazed and fired piece:

Reglaze it when it is warm(I generally place it on a lid of a firing kiln to get warm), a warm surface will evaporate the water in the glaze quicker so the glaze will stay where you put it.

Also take a small amount of the glaze(about just the amout you need to reglaze) you want to use to reglaze and add a dab of karo syrup to it. The syrup will make it adhere to the surface better and thicker. Wash your brushes and utensil after using the karo syrup/glaze combo and throw away any glaze that you've added syrup to as in a couple of days it will start to ferment and become nasty...do not add the glaze/karo mixer back to your normal glaze containers or you will have a gross slimey yucky waste of glaze.

Place the work back on the hot kiln lid after reglazing to warm up again to dry faster.

When you are testing a new glaze or testing a old glaze to higher than suggested temps you need to protect your expensive kiln shelves from glaze flowing onto the shelves and adhering your work to the shelf.

When glaze comes into contact with your shelf the glaze does not notice the difference between the work and the kiln furniture. Glaze can and will coat your shelves if you do not protect them. Once glaze has made contact with your kiln shelf, it will begin to eat into that shelf every time that shelf is fired. An unprotected kiln shelf is an accident waiting to happen.

Kiln wash is the first step to protecting your shelves. Kiln wash is painted onto your kiln shelves to allow for easier removal of glaze that might accidentally flow or drop onto your shelves. Kiln wash is available commercially or you can make it yourself using just a couple of ingredients that you can order from most pottery/clay supply houses.

Another way I protect my shelves is to dry foot my ware. Dry footing means I do not glaze the bottoms of my work. Some people prefer to glaze the bottoms and then stilt their work but glaze can flow down stilts. Stilts seem to me rather a waste of time when loading a kiln. Dry footing a pot makes for easy loading and no stilts to keep track of.

My last method I use when I know either for certain a glaze will run(but I like the results) or I am testing something(either a new glaze or new firing method for a glaze). I use what I call glazing plates. I throw some plates of various diameters with very short walls and a very flat surface. I then bisque my plates. Before I load a kiln full of glaze tests I kiln wash my bisqued glazing plates on one side and allow to dry. I place my work I am testing on the kiln washed surface and place work and plate in the kiln. If the glaze runs the plate will catch the mess and I am only out a replacable plate instead of an expensive shelf. The plates can be used over and over again if they are not ruined by glaze.