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knobs popping off

updated tue 31 dec 96


Robert Speirs, M.D. 766 X4450 on thu 5 dec 96

Hi Brad. I can relate. I have lost a lot of knobs in the past. I was
told that this is due to air having been trapped under the clay as I put
it on to trow it into a knob. To solve this I also use vinegar over my
scoring (I use a mid-range porcelain), and really almost throw the
little ball of clay against the scoring to be sure there is no air under
there. Then I throw the ball down and cone up a bit and get it good and
attached. Since I have done this, I haven't lost any knobs. (I have
lost a couple, but this is because I wasn't paying attention and heeding
my own advice!) Hope this helps you.

About the silver scratch marks on your white glaze - try "Zud" which you
can get in the grocery store next to the Ajax and Comet cleansers. It
is a stronger abrasive cleanser than those two, but I use it on some of
my plates which also have a white glaze and scratch marks and with a
little elbow grease they come off. It got rid of pot marks on my sinks
when nothing else would.

Laura in Oregon on fri 6 dec 96

I have never had any problem with knobs popping off. I like to trim my lids
quite thin in the center, so I have never liked to throw the knobs on the
lid. I simply throw a batch of knobs off the hump, which is extremely fast,
and then as I trim the lids, as a final stage I score the center of the lid,
apply plenty of THICK slurry, press the knob into place, start the wheel to
make sure the knob is centered, and with the wheel spinning smooth down the
joint. I often add a grove with a wooden modeling tool where the joint is.
In the last twenty years I have never once had a knob pop off if applied as
described. If the knob is freshly thrown, then I only score the surface of
the lid. If the knob has stiffened up a bit, I score the bottom of the knob
as well.

My students have had knobs pop off LOTS of time, and it is invariably the
result of trapping air in the joint. The secret is to score well and use
PLENTY of THICK slurry. When parts pop off in the bisque the cause is
almost invariably trapped air. We all know how things can explode into
thousands of fragments if heated too quickly in the bisque. The point here
is that even if the bisque-firing ramp is very gradual, trapped air spaces
will still cause parts to pop off.

Throwing knobs off the hump is a cinch. Raise up a tall thin collumn of
clay, and form the desired knob shape at the top of the collumn. With the
wheel spinning, and with plenty of water on the knob, support it with the
fingers of one hand beneath the knob, and with the other hand cut it off
with a needle tool so that there is a very slight flange at the base of the
knob-shaft. When it comes free, simply pick it up with the hand that is
supporting the knob, and set it on a wareboard. With a little practice you
can make knobs like this at a rate of three to five per minute.
- Vince

Vince Pitelka - vpitelka@Dekalb.Net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts, Smithville TN 37166