Friday, September 9, 2011

Salt Glazing

Top of German kiln during firing. Kannofenbrand Film.
I recently had an e-mail inquiring as to what the process of salt glazing is. This is such an excellent question! So, I thought I would make it into a blog post in order to share some photos and go into a little more detail. I think all too often potters assume that people know the processes they use to make their pottery, and our audiences do not always ask! For me, I love questions, but I will admit that I do not always get the chance, or take the chance, to explain the process. If you know what salt glazing is, then this is an opportunity for you to see some really great images a friend of mine (thanks, Angelika!) captured from a video I have of a kiln firing in Germany. The video is called Kannofenbrand and is one of my favorite nerdy videos. The video shows a kiln firing from 2004 of a restored circa 1840 kiln in Hohr-Grenzhausen, Germany. The kiln is 42 cubic meters in size. Just think about that for a few moments.

I visited this monstrosity in 2007 and when I first saw the building above I said, "where's the kiln" and my host said, "That's it!"

Photograph of the same kiln from around 1950. From Alt-Hohr-Grenzhausen by Heribert Fries
I often start by telling people that I do not generally put any glaze on the exterior of my pieces. "But how are they so shiny?" The glaze is created by the salt. The shiny factor is determined by the amount of silica (component of glass) in the clay as well as the amount of salt put into the kiln. Silica basically attracts more salt. When the kiln is firing potters can tell the temperature of the kiln by using what we call "cones." These cones are made with different kinds of clay which melt at varying temperatures. By setting these cones up in the kiln we watch the temperature rise in the kiln as they fall.

Cones at the beginning of the kiln firing, all standing. Kannofenbrand Film.

Two cones down, one going down, the temperature is rising! Note the glow in the kiln as well, that is the heat building in the kiln interior.  Kannofenbrand Film.
When the kiln reaches roughly 2,200- 2,300 degrees Fahrenheit it is ready for salting. Some potters wait for a higher temperature, some actually salt at a lower temperature (salt will actually flux or break down at earthenware temperatures).  The temperature in the kiln must be hot enough to basically evaporate the salt once it enters the kiln, the clay must be hot enough, and the silica in the clay is fluxed, in order to grab the salt once it enters the kiln. Salting is done in many different ways. With the kiln in Germany, the salt ports are on top of the kiln, meaning that the salt is dropped directly on top of the stack. The salt is loaded into metal cans which are attached at the end of a metal rod.

Loading the salt into the cans. Kannofenbrand Film.
 Then the salt is poured into the ports on top of the kiln.
Salting the kiln. Kannofenbrand Film.
Notice in the two above photographs that the people are wearing cloth to cover their mouths, and look closely at the second photograph with the large plume of gray/white gas coming from the kiln. That is the salt. The salt basically turns into a gas, technically hydrochloric acid, and fumes inside the kiln. When the air pressure and moisture in the air is just right, it can get so foggy-looking around a kiln being salted! Now, you are probably thinking, hydrochloric acid, eh? Yes, it may not be good for you in large doses, and it definitely clears your sinuses.
An American potter who attended the 2004 German kiln firing told me that when the kiln was being salted, people started coming out of their homes and collecting around the kiln. He asked what they were doing and was told that traditionally people came to the salting for health purposes, that they thought the salt was beneficial to their health! In Europe now, most salt kilns are required to have a scrubber on their chimney in order to clear the fumes before entering the atmosphere. Here in the States we do not have regulations on salt kilns as much, and for the comparatively few and small salt kilns that there are, the amount of gases released is minimal. Historically, in parts of Europe there were regulations requiring potters to be moved and kilns to be put in other regions because people complained about the smoke and cloudiness.

This is a photograph of a kiln while salting from the 1920s in Hohr-Grenzhausen. From Alt-Hohr-Grenzhausen by Heribert Fries
Either way, the gas/cloud of salt created adheres to the clay body of the pottery in the kiln, creating a glassy surface. Depending on how much contact with the salt the pottery has, and how much salt is put into the kiln will determine how glassy or dry the pottery feels. When the kiln is done the result of the salting is a texture referred to as "orange peeling." You can make it out pretty well on the rim of the mug below. It basically creates small beads of glass created by the salt and feels like the surface of an orange.

Mug made by Liberty Stoneware. Good orange peeling on rim.
Synopsis of salt glazing:
1. Bring kiln to temperature (2,200-2,300 Fahrenheit)
2. Insert salt into kiln (various methods)
3. Salt turns into gas which adheres to the clay body of the pottery, making a glaze.

The German kiln firing in 2004 turned out pretty well! They had a mishap in the center of the kiln, where the floor collapsed partially, but all in all the results were good.
Results of the kiln firing. Kannofenbrand Film.


Michael Mahan said...

Is that last picture actually inside the kiln?

Liberty Stoneware said...

Yes! That last photo is of the interior of the kiln after it had cooled. Isn't that amazing?! I anticipate playing with similar stacking methods to this with my new kiln!

Anonymous said...

Hi Brenda.. I didn't expect an entire post just to answer to me (a completely unknown). You made a great and well done work. I don't know how to thank you. It's everything so clear. The only further question related to my status of ceramic hobbist is.. can I do some experiment with an electric kiln (3 kw)? Micaela

Liberty Stoneware said...

I am glad to hear I answered your questions! It was my pleasure. I would not recommend salting in an electric kiln. I have only known one potter to do this, and it was with a very old kiln and it was outdoors with very little salt. They basically lay salt on their pieces and it fumed during the firing. The problem is that you risk ruining the entire interior of your kiln because salt will break down the metal parts as well as the brick.

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