Mark McCombs's Profile
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- 13-August 12
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Posted 11 Feb 2013I need to re-fire a ^6 ox load and get to the proper temp. Programming error on my part.
Do I use the same cones that have seen the same heat work as the ware, or do I use a new set of cones?
Posted 17 Nov 2012For the benefit of other beginning potters I would like tooutline a series of mistakes that has put me in a rather awkward and embarrassing situation.
I start potting in June of this year and was sampling various clay bodies to find one that I could move forward with. I have a friend who has done slab work for many years and she offered a bag of her clay for me to try. It turns out, though a bit stiff to get started on the wheel , it finishes well and is real pleasure to trim. This is a very white stone ware that she fires to ^5.5 and ^6 and was developed over several years with her supplier. It is a very good clay body to work with IMO. I split a 2000 pound order with her and started potting. The clay came from the supplier marked withher name and cone 4-6 on the box.
In spite of all that I have read on this board regarding vitrification and testing the clay body maturity, I never once consider this to be an issue with my friend’s clay. The box said 4-6 and she fired to ^6. That was good enough for me.
In October there was a thread about vitrification that I responed to. It was the first time I thought to myself that I should check even though I was sure there was not a problem.
And a week ago, Istarted glazing my 3rd run and added an unglazed cup to the firing( ^6, final temp at 2203F, 5 minute soak, ^7 just starting to move). Imagine my surprise when the cup started leaking water onto the paper in about 5 minutes. What the heck??? Then I thought about checking a cup with glaze on it. I found one that had a small crack in the bottom and “might” have a crack on the inside and it leaked badly. I tested another cup with tiny crack in the bottom and looked good on the inside and it to leaked within 10 minutes or so.
By the way, at this time I am planning my first sale at our town’s annual bazaar three days away. yeah….
I loaded the kiln for the next glaze firing and programed alonger ^6 firing with a final temp at 2232F with a 5 minute soak. I ended upwith a ^7 firing. Again I had placed an unglazed cup in the mix for testing.
After the kiln had cooled to ambient temps, I unloaded andtested the unglazed piece.
It leaked within a half hour. The gravity of the situation was just starting to hit me.
I decided to test a perfectly good, glazed cup from the previous run. It leaked. Right through the glaze, it leaked. So now I am on the phone to the person who developed the glaze and he said that that particular glaze is finicky with certainclay body types and will easily craze. He said to take a permanent marker and colour the bottom of the cup to see the crazing. Sure enough. Crazedlike crazy and I couldn’t see it.
I made a decision to water test all the ware with that glaze. Most all the pieces leaked.
The second glaze load from that run did not have any of thatglaze but another issue was rearing its ugly head. By hitting ^7, I over-fired some other glazes that were developed for ^5-6. Pots were pinging left and right. I thought to myself thatif they craze, they will leak. Sure enough I tested a few bowls and they leaked.
Knowing that none of my pieces were vitrified, I decided to water test my entire inventoryof cups, mugs , bowls and such. The results were extremely disappointing. Istopped counting, but there are more than 75 pieces that are not salable andwhat I was left with would not make a very good display at the bazaar so I hadto cancel my table for tomorrow.
I have learned a hard lesson or lessons:
· Specifically ask at what cone the particularclay body matures or vitrifies. Use this information as a guide only.
· Run multiple tests (thickness, glazes, cones andfinal temps)on the clay body glazed and unglazed
· Run absorbency and shrinkage test
· Check opaque glazes for crazing with a felt tipmarker
· Specifically ask about glaze compatibility with your clay body
I’m sure there are other lessons and I hope that some of you will point them out.
BTW. My friend who I got the clay from has a well-developed glaze that fits her clay body at^6. She does not make functional pottery but has made ichibanas that do not leak for several years.
The glazes I use to not fit that clay body and I have noidea at what cone it vitrifies at.
A new search begins.
Posted 5 Nov 2012New glazes I picked up:
Stellar Rust (I have used before)
These are glazes from Clay Art Centre in Tacoma, WA.
If anyone has experience with these and knows a (M50 cup) viscosity number start with please let me know.
The manufacturer will only tell me "medium cream" and "thin cream"
Posted 3 Nov 2012This is a test post.
Posted 16 Oct 2012Question to those in the Kalispell/Whitefish area:
As a child in the 60's and early 70's I went fishing in many lakes in NW Montana including a few just West of Whitefish called; Beaver, Little Beaver and Dollar lake.
On the shores on Beaver lake in a couple spots was a deposit of what I can only describe as a white clay that extended into the water. I spent many hours as a youngster playing with that clay, making various figure and forms.
I would like to ask if anyone has use a locally produced (white) clay from the area?
Coordinates to where we used to launch the boat and comp on the deposit.
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- September 24
- SW Washington
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