Glazed bisque mugs by Sarah Hall via Flickr
Today in the Intro to Wheelthrowing class at Mudflat Studio, Lisa taught us the basics about glazing. This is a huge topic and there is a lot to know and remember, but here goes. (Note: the following is not meant to be instructional — I am writing this post for my own reference. Please consult reputable instructors, books, magazines or online videos for instruction.)
Preparing a pot for glazing
After you create a pot, you will either be finishing it on the wheel or letting it dry to the leather hard stage and then trimming it using a tool on the wheel. Generally, you want to ensure that any marks are smoothed out during this part of the process. This way, when you are ready to glaze the pot after it has been fired the first time, you won’t have to deal with many rough edges or nicks.
(The pots I glazed today in class were from the initial intro workshop class that I took in January; we didn’t learn to finish/trim during that class. As a result, these pieces had lots of issues. I did a little smoothing with a damp sponge, but I think I was really supposed to be using auto-grade sand paper (the type that can get wet) or emory paper. Lisa said that the glaze itself would smooth out some of those imperfections. I am not too concerned with perfection at the moment — I am just trying to learn the basics.
Wax Resist
There are (at least) two types of wax used in glazing, white and green.
The white wax is quicker drying and is generally used to wax the bottom of your piece before you glaze it. Putting wax on the bottom helps you easily wipe off glaze before putting the pot in for firing. If there is any glaze on the bottom of a piece where it touches the kiln floor, your pot will stick, possibly damage the kiln surface, surely tick off the studio technicians, and the part that was stuck to the kiln may break off of your pot. Sounds like bad news. Use the wax.
The green wax is generally used during the decorating process (should you decide to integrate it). This wax takes longer to dry.
Selecting Glazes
There are many different types of glazes. There are also runny glazes, which should be used strategically, especially by beginners as they can run and make a pot stick to the bottom of the kiln, and thicker glazes which shouldn’t run. (There are also many other methods of decorating pots, i.e. using stains and acrylics.)
Oxidation glaze, Lisa explained, is better for porcelain pots while reduction glazes are more often used for stoneware. In oxidation firing, which is done with an electric kiln, there is ample oxygen during the firing and the colors stay true and even. In reduction firing, done in a fuel-fired kiln, the oxygen is reduced which has a sometimes dramatic effect the color and finish of the pot.
:: Further information about glazes and firing techniques:
- http://www.herwheel.com/pottery/explanations.html
- http://www.brothers-handmade.com/pottery-glazes.html
“Shino first or the pot is cursed.”
At Mudflat, there is an excellent glaze tile chart showing the effect of layering different glazes. (I will take a photo of this chart and revise this post.) The effect can be very different depending on which one you use first.
As I mentioned in the last post there are a lot of mnemonics in ceramics, and one of them refers to the use of a popular glaze called Shino. “Shino first or the pot is cursed,” or “Shino first or you can expect the worst,” both refer to the fact that if you use shino as the second glaze layer, you will likely have a finished product that is bubbly, grey and “nasty.” I can see how some people might like this effect, but in general, it is probably not the desired result. When it is used as the first glaze layer, it provides a smooth, rust colored base color.
Heaven’s Gate, Celadon, Shaner White, Kelly Green
I had three pieces to glaze today. I used Heaven’s Gate, a sort of pale heathery purple, Celadon, a pale, organic green, Shaner White, a very white and shiny glaze, and Kelly Green, which fires to more of a creamy blue green and not medium, shamrock green.
I started by using a yogurt cup to pour the Heaven’s Gate, Celadon and Kelly Green glazes in the interiors of the vessels and then swirl the glaze around. Next, I dipped the exteriors in the shaner white. Finally, I dipped the rims of the vessels in the Kelly Green. I started with a plan to try to keep it simple with color on the inside and white on the outside. However, I wasn’t really able to pour neatly or evenly, so I improvised. It will be interesting to see what the end results are.
Glazing tools
When you dip a pot in the glaze, you can use these tongs to hold the piece while you dip it. There may be marks left by this tool, which you can then smooth out with your finger once the glaze has dried. If you are dipping the exterior of a mug, you can also hold it by putting your hand inside it and lowering the piece down almost to the top.
You also want a bucket of water, sponges and a paintbrush handy so that you can wipe the glaze off of the bottom of your pieces and to dab on glaze where you may have missed a spot.
Eat, Drink and Wash
In order to eat or drink from a vessel that you make, the piece needs to be covered in glaze. Glazed pieces are also generally dishwasher and oven safe. I don’t think that they are all microwave safe as some glazes contain metals. I will need to check on that one. You also want to keep the interiors of these vessels as smooth as possible and not cause reactions like the shino “second” issue described above.
