Throwing tall
A technical question -- would love any pointers.
I'm trying to pull taller narrow cylinders, say with 2.5kg / 5.5lbs, to 30 inches or so with 4-5" opening. I can generally pull a nice, uniform cylinder to 16 inches with slightly coned walls (centripetally-sloping at 80 degrees). Beyond 16 inches, my inner and outer hand can no longer touch, so are unbraced and less stable and the pulls less steady, and the cylinder starts to flare from the bottom with each subsequent pull.. (This is also the point where my forearm within the pot widens the collared rim, which seems to contribute to the flaring cycle.) I collar it in again with some success, but each pull leads to more flaring and finally a short bowl with wide base...and tired clay after all my efforts to recover the cylinder. Beyond 16 inches or so, the bottom seems to have a round quality forcing the walls out.
I thought perhaps my first pull introduced a round departure point (from the base) instead of pulling at a right angle, but I've ensured over and over that I'm pulling vertically. Now I think perhaps my left pulling hand (inside) exerts more outward force than my right (outside) pulling hand, forcing the walls to flare. It doesn't feel this way, but seems like a reasonable explanation. Then I've also considered the wheel speed. As we pull taller, do we need to continuously slow the wheel (ie are taller pieces subject to more centrifugal forces and flaring?) -- even quite slow?
As for throwing tall, is there a general rule or guideline relating starting clay mass and height? For example, with 5 pounds of low-grog clay and a 5 inch opening, how tall a cylinder we can expect to pull (given desirably uniform, thin walls)?
Many thanks