Potters & Sculptors - Making Rock from Mud
I am using a cone 6 stoneware clay from Pottery Supply House in Toronto, Canada.
I have fired my work many times using SH glazes and schedule with no problem (definitely no pinholes). Doesn't he suggest to slow down the cooling to 50F/hr?
Pinholes most often result from the points you're addressing. If we have a heavily loaded kiln, or thick pieces, I hold the bisque firing for at least 20 minutes or more at the peak temperature. Always a minimum of 10 minutes regardless.
Chemical reactions like oxidizing out sulfates, nitrates, chlorides and carbon take sufficient oxygen, sufficient temperature and sufficient time. If these are nor fired out in the bisque they off-gas during the glaze firing causing either pinholing or bloating.
When our kiln reaches that temperature or higher the lid fits loosely so we can clearly see the orange glow between the kiln and the lid. Combined with one peephole open that's plenty of oxygen.
Some glazes do not combine well with some clays. More specifically the glaze reacts chemically with the clay body and discovering those unfortunate combinations is just trial and error.
Laguna Clay WC-382 Half and Half which is half porcelain and half stoneware reacts very badly with fluorine containing frits like Ferro Frit 5301 and 3269. The combination bubbles and bubbles leaving pinholes in the glaze. This combination pinholes when fired at lower temperatures and higher temperatures - when fired quickly or slowly and without a hold and with a long hold.
If I spent enough time and money investigating the combination, I'm confident I would eventually find out what chemical reaction is creating the off-gassing. But it's just easier avoiding that particular combination.
It would be helpful to others to know what cone 6 clay you're using with which Steven Hill glazes so we can all avoid that unfortunate combination as well.
For the record:
The clay body I am using is LAGUNA 607 Cone 6 Buff Stoneware. It is one of their North Eastern clay bodies.
The SH glazes I use and am experiencing trouble with are:
SCM Warm, Red Orange, Jen's Juicy Fruit w/ iron
SCM, Carmen's Torquoise, Jen's Juicy Fruit, Watercolor Blue, Watercolor Green, 2D Blue
I was never able to get Jen's Juicy Fruit to flow until I replaced the Lithium Carbonate with Lithium Fluoride.
I think the mesh of our original Lithium Carbonate was too large.
You might try sending an email to firstname.lastname@example.org with the glaze recipes.
Your email will be routed to their Tech people. They know what's in their clay, so they may be able to easily tell you what the problem is.
talked to my kiln distributor today. He asked if the Envirovent on my Skutt was working, I said yes. He then asked if I was plugging all the spy-holes on the kiln when firing. I answered that I usually keep the top plug open. He told me that with the top plug (or any plug open) the envirovent does not function properly.
The test is to turn on the envirovent fan and hold a match or lit lighter over one of the holes in the kiln lid. If the flame is being pulled (even slightly) into or towards the hole, then YES the vent system is working.
I did this test and INDEED when the spy-hole plug is open the flame does not get pulled towards the vent hole. When the plug is in place the flame gets pulled towards the hole.
SO I WAS NOT getting enough oxygen flow through the kiln during bisque, thus leaving those organics not completely burned out.
Whether this will solve my problem with Laguna 607 Cone 6 Stoneware remains to be seen. I will do some tests.
This same distributor told me that they use Laguna 609 Cone 6 Stoneware (a whiter and smoother clay body) when they host the Stephen Hill workshops and do not have any pin holing problems.
So I will pick up some 609 and throw similar pieces with both clay bodies, fire them together with the spy-holes plugged up and then see what happens.
I was intending on switching to the 609 sometime next semester because it is a whiter surface than 607 when fired to Cone 6.
Tom, the cone 6 clay from Pottery Supply House I use is their own mix, it is stoneware 519. Their website is http://www.pshcanada.com, if you contact them I am certain they will be able to answer your questions.
As for the oxigen requirement for a good firing, my kiln is fitted with an Orton Vent Master which pulls the air through the kiln and vents it outside. I fortunately do not have to worry about this issue.
I have experienced SMC cool & warm, watercolour green, apricot, 2D blue and fake ash with success.
I, however, have a problem with Black Frost - I thought SH used this glaze as a liner - I get a flat greyish color very unappeeling. If anyone has tried it, could you let me know what kind of results I should expect?