Wow - I wanted to introduce semi-porcelain into my repertoire, so I started with a series of tests back in October to test it's plasticity over a wide range of forms (wide and low serving dish shape; bottle form, tall cylinder, small and large plate, etc...). Lots of warpage and hairline fractures in the gloss firing.
So, first go, not too great.
I decided to see what the experts had to say on potters.org about clay body additions and molochite seemed to be a popular choice so I went ahead and tried two grade additions in two two different quantities. After some testing, I decided 30s grade too bulky (see below) and 80s grade much better. Went with 5% addition in the end, as 10% seemed to really open the body too much and cause way too much tooth when turning.
Since I had settled on the molochite addition, I needed to see if I could get the clay up to 12 inches on the wheel, quite an achievement for porcelain in what would ordinarily be quite straightforward with stoneware. Tuesday was a painful day for me. All morning Iwas trying to throw tall cylinders. I would get them around 10 inches and then they would just start warping on me. When I tried to work out the twist, they would shrink down in size by almost 2 inches.
I'm gonna have to post some photos of this weird phenomena next time it happens! (well, I'm hoping it won't happen again and in that case, you are out of luck seeing photo documentation).
So, I did what any decent potter worth her wait would do. I walked out of the pottery and proceeded to cry for an hour, managing to question my whole pottery existance asking all the while "what the heck am I doing trying to switch to a more difficult clay after all these years of getting stoneware where I want it?". I redeemed myself by then researching all the tips and pitfalls for throwing with porcelain and this is what I found out:
- limit water usage to almost nil. use slip instead
- go very slow when knuckling up to avoid warpage (this seems to be a big problem)
- use a blowtorch to stiffen things as you work
Well, two hours of crying/researching seemed to do the trick, because when I went back out to the pottery for a second "go", it seemed to go well!! I managed to do 10 inches, but beggers can't be chosers!
Yesterday I went at the clay again. This time with smaller items in mind. I decided to test the clay with molochite and without. To prepare myself, I had a bowl of slip, a bowl of water, blowtorch and batts ready to go:
Although I feel great about the day's throwing achievements, feeling like I may just figured this darned clay out, I'm finding the whole process tedious!!! With stoneware I can just be fluid, quick and in control without much effort at all anymore. With semi-porcelain, the whole process is more disjointed and lengthy:
1. prep batt on wheel. batt essential as I find clay very unforgiving once thrown
2. get clay wet with slurry NOT water!
3. throw clay slowly over 4-5 pulls. more than the ideal 3, but hey, what can I say? tenderness is key. fight the slump! everytime I shape inward, clay seems to get shorter. fighting this results in that 4th to 5th pull! this really slows me down!
4. clean all slurry from pot to ease in drying
5. blowtorch 30 seconds for small piece, 45 seconds for medium, etc...
6. list off gently with batt in tow
As a result of following these steps, I managed to throw the following:
Unfortunately, following these steps tends to take the immediacy and excitement out of throwing that I grew accustomed to with stoneware!? Wow - whiteness does come at a price!
They say that this semi-porcelain is actually much better to work with that normal porcelian. Damn, I wouldn't want to experience that nightmare.
I do have to say that to get this stuff working how I want it to, I really think my experience with throwing is the only thing getting me through this stage. I cannot even imagine what the poor souls who use this stuff as their first experience on the wheel must think about their abilities with clay? I hope they don't blame themselves.
I will fight on darn-it. I have invested too many hours with clay testing, slip and glaze development to throw in the towel now. In fact, the ironic thing about this whole experience is how much I now appreciate my stoneware clay. The whole reason I went into this process was to replace stoneware with a more exciting fresh look. Now I feel it is the stoneware that is more exciting to work with and the porcelain more exciting to look at.
What is a girl to do?