Pinholes and craters

[Tom Waggle]

I sent an email out to all group members. I should have just started this thread.

I am using Laguna 607 cone 6 stoneware.
I am getting pinholes and craters on about 1/3 to 1/2 of my pieces.
I contacted Stephen Hill via email to ask him about this. He suggested that I just switch to porcelain as it is the gasses from the impure elements in stoneware body.
I have adjusted my bisque schedule to slow down to 100'/hr between 1100' and 1700' ( the temp range where those organic gasses burn off). I am still getting the pinholes and craters.
Pinholes and craters supposedly come from
1. Gassing out during glaze firing.
2. Not enough ventilation and oxygen during bisque firing.
3. Possible glaze boiling during glaze firing.

Is anyone else experiencing this horrible problem?
Has anyone found a solution?
Is anyone using a cone 6 Stoneware and SH glazes and NOT getting pinholes and craters?
It is suggested to remove all spy hole plugs until pots become glowing red to increase oxygen flow. I have an environment on my Skutt and assumed that it would provide sufficient ventilation and fresh oxygen.
Thanks for your input.

[Tom Waggle]

If anyone is using cone 6 stoneware successfully without pinholes and craters could you share your brand of clay, bisque schedule and glaze schedule. I am using the Stephen Hill recipes from his DVD.

[Tom Waggle]

I cannot explain how or why some of my pieces come out with no defects and others do. I mix my own clay using Laguna 607 in a Soldner mixer. I do not pug my clay. I mix reclaimed Laguna Cone 6 into the mix. I am considering learning to use porcelain as it would solve this problem. I teach ceramics and the Laguna 607 is our studio clay. I have considered trying Laguna 609, which is whiter and may contain fewer impurities.

[Denise Rainville]

Hello Tom,

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?

[Tom Waggle]

Thanks for your reply. I slow down my BISQUE firing on its way UP, the range when the organics are burning off (1100'-1700' F).
I follow Stephen Hill's Glaze schedule, though I use the schedule he suggests for Bisqued ware vs. the single-firing schedule. The DVD provided two schedules.
Yes the down firing rate is 50'/hr.
I probably cannot get the same Cone 6 stoneware you have in Canada, but if you could provide the manufacturer and clay body name or number I would appreciate it.
I may remove all spy hole plugs and place a fan near the kiln to ensure adequate oxygen during bisque. The culprit is certainly the organic gasses not gassing out completely due during bisque.
They are then released during glaze firing and create the popped bubbles and pinholes in the finished surface.
I would love to solve this problem with Stoneware, though I know Porcelain would show the colors of these SH glazes even better.

[Norm Stuart]

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.

[Tom Waggle]

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:

Warm Palette:

SCM Warm, Red Orange, Jen's Juicy Fruit w/ iron

Cool Palette:

SCM, Carmen's Torquoise, Jen's Juicy Fruit, Watercolor Blue, Watercolor Green, 2D Blue

[Norm Stuart]

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 [e-mail] 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.

[Tom Waggle]

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.

[Denise Rainville]

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?

[Tom Waggle]

I asked Stephen Hill which of his glazes are suitable for liner glaze and he told me the white and Red Orange are the only one's that are technically "food safe". I did not ask him to elaborate or to give further explanation.
I use the Red Orange by itself as the liner for all my pieces right now, it is very reliable and a gorgeous deep red.
I have not fired the Satin Black Frost by itself. I have used it sparingly as a final touch of dark value over the other layered glazes.
Coyote Glazes make very reliable Cone 6 glazes, I know they have a good food safe black which could be used as a liner. All their glazes are food safe except their Mattes (as most mattes are not food safe).
I personally really like SCM Warm with Red Orange then Jen's Juicy w/iron, makes a gorgeous red-orange-orange-yellow-orange range of hues.

[Tom Waggle]

I still need to make a set of test tiles, actually throw some test cylinders so I can test out all the layer combinations of the SH arsenal, so I know what and why things are happening. I also have several non-SH glazes I want to test over the two SCM glazes.

[Tom Waggle]

If nobody has tried the SH Red Orange by itself as a liner it is gorgeous and has a nice solid gloss. I just pour it in and pour it out, do a really clean wipe off of the outside surface with a small sponge, then they are ready to spray. Makes a very nice contrast to the varied matte surface on the outside.
Sometimes a little overspray gets inside on top of the Red Orange, it blends in and actually makes the inside a little more interesting.
I have had good success with this little $13 spray gun from Harbor Freight. http://m.harborfreight.com/adjustable-detail-spray-gun-92126.html?u...
I bought this air regulator to go with it http://m.harborfreight.com/125-psi-air-flow-regulator-with-gauge-68...