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bare bottoms, leaky vases

updated mon 1 mar 99


David Hendley on sun 28 feb 99

Dear Leslie, Dave, and Phyllis,
If you make cone 5-6 pottery, you are making stoneware,
which means that the clay should have very little absorbancy.
About 1 to 2% is a good figure to aim for.
If your pottery is wet on the bottom when filled with water,
the absorbancy of your clay is too high.
Even with no glaze, inside or outside, the bottom should not be wet.

Here are three ways to fix your problem:
1. Fire to a higher temperature (as was already suggested), or
if you do a very fast firing, soak it at top temperature.
This "melts" the clay more, to make it more vitrious.

2. Use a new claybody. Buy several clays and see if any of
them work for you.

3. Adjust the claybody you are currently using. The most common
way to do this is to add feldspar to the claybody mixture.
Feldspar acts as a flux. Talc, in small amounts, can also be used.

Look in a ceramics textbook for instructions on how to test for
absorbancy. Or,....
Ron Roy has sent absorbancy test instructions to Clayart several
times over the last few years, so you might be able to search
Clayart for his instructions.
Ask him, (he's usually on-line) and he'll probably sent it in again.

Best wishes,
David Hendley
Maydelle, Texas

At 10:59 AM 2/27/99 EST, you wrote:
>----------------------------Original message----------------------------
> I am posting this reply to the list, rather than Phyllis directly,
>because I would like some feedback on this also. Does anyone have
>suggestions on what I should do to improve on the process below, considering
>the concern Phyllis voices? TIA for considering this!
> I am firing Laguna Calico to Cone 5, frequently making flower pots with
>attached trays. Therefor, in use the pots are always damp to wet. IF I do
>not glaze the inside, the pot will weep moisture through the bottom.(not
>condensation underneath, which is a separate problem).
> I deal with this by now glazing the insides of these pots. I add 3
>buttons of cork or felt on the unglazed bottom to get enough airspace
>underneath to control condensation/allow evaporation of the condensed
> I have felt this is enough to deal with the problem you are concerned
>about, Phyllis, and have done this same thing with vases. Pitchers also,
>but without the buttons.
> I have also trimmed bottoms on vases, which would permit glazing all but
>the foot rim on the bottom, but don't go to the trouble. I do usually on
>mugs, because I feel it makes them easier to wash, and they, I hope, get
>used a lot and washed frequently!
> Dave in wet windy winter-spring Idaho
>-----Original Message-----
>From: Phyllis E. Tilton
>Date: Friday, February 26, 1999 1:42 PM
>Subject: Re: bare bottoms
>>----------------------------Original message----------------------------
>>Hi: A concern of mine, in making slab pieces with bare bottoms, is the
>>porosity of the bottoms. They can retain water when washed that could
>damage a
>>table top or whatever. Have had no complaints but wonder if it does happen.
>>Most of these are for snacks--tortilla,chips, cookies, etc. so are probably
>>not used often enough that they have the opportunity to dry out. These are
>>symetrical and putting feet on them takes away something. These are fired
>>^5 or 6. I throw bowls to go with them.
>>Should I try to stilt or just forget it?
>>Phyllis Tilton