Pinholes And Craters

[Joe_L]

Hi Folks,

I have a couple of issues I'm trying to get a better understanding of. I'm using using commercial clays and glazes.

 

The first picture shows pinholes on the inside of a lidded jar. This is iron-rich stoneware bisqued to 1000C with 3 coats brushed Botz Satin Cream Matt. It's sold as a cone 8-9 glaze which I fired to 1240C with 20 mins soak (cone 8 fully down). Test tiles didn't come out as bad as this, though going back to them with a lens shows some evidence of tiny pinholes in the thicker areas I didn't notice initially.  I have a recollection that on stirring and brushing the glaze it seemed a bit "foamy" which I didn't pay too much attention to, it may be the cause but I guess its more likely there's a reaction to the clay as test tiles with a white clay don't do this.

 

The second picture shows a scattering of craters, again on the inside of a lidded jar lid. Each of them is well healed but has a distinctive peak in the centre like a lunar crater. This is Earthstone ES5 with a Spectrum Cone 5-6 glaze single fired at 1220C 20 mins soak. (Also cone 8 completely down - eek!!) I know this is firing higher than the manufacturer says but test tiles don't show cratering and the interior of the body of the jar has only 2 isolated craters, the rest is ok. Are these healed craters because of over-firing or caused by the particle that forms the central peak? Would that be a contaminant or an intentional ingredient that needs to be seived out? (BTW the glaze does have a slight speckle).

 

Joe

 

[Foxden]

I would suggest bisque firing to a higher cone. It seems as there is some out gassing. Perhaps 04 might do it. The pin holing might resolve with a longer soak or a higher temp. Trial and error will get you there.

[High Bridge Pottery]

Number one looks too thick, I have a similar glaze that does this. I would also fire it a little hotter if possible. Number two could just be a dust on pot issue. 

[Celia UK]

Number one looks too thick, I have a similar glaze that does this. I would also fire it a little hotter if possible. Number two could just be a dust on pot issue.

 

Ditto - even to my inexperienced eye!

[Joe_L]

Yes, time for more testing! But thanks everyone for your comments, it's so useful to know where to focus experiments so I don't waste time pursuing the wrong avenues.

Joe

[bciskepottery]

If you fire with your lids on, the inside temperature will not get as hot as the outside -- which can affect glaze melt.  In your first picture, the underfiring might be the result of the glaze not hitting mature temperature.  In the second, the cone 5/6 glaze likely only reached around cone 7 if fired with top on.  The isolated craters may be from outgassing of the clay body during your once-fire; bisquing might get those gases out of the way or slowing the rate of temperature climb during the last 200 degrees of the firing. 

[Joe_L]

Yes I fired with the lids on. Maybe that's why the test tiles don't show it as much. And more evidence for raising the temp a bit.

 

On a related point, when you fire with the lids on how do you make sure there's sufficient burn out because fresh oxygen can't get inside? I guess the inside effectively becomes a bit reducing as a closed pot is effectively an empty saggar. Does slowing the firing further manage to pull some carbon from inside through the wall (or at least make sure the outer surface of the wall is as clean as possible)? Can oxygen or CO diffuse though the wall? Are there any techniques that help in addition to slowing the firing?

 

Joe

[Babs]

Don't know but the inside of jars etc are often much more attractive, to me..