pinholes

[rebs]

hello all you helpful people, really grateful for all the help i’ve gotten in here i’m trying my luck again!

 

i’ve had some (new) challenges with ‘regular’ pinholes too. and i can’t seem to figure it out. 

 

i’ve made some marbled pieces with a blue and a speckled clay. they are slow bisque fired at cone 05 and slow glaze fired at cone 6. the ones fired in my relatively small kiln just had a few pinholes, and the ones i last fired in my new (old) larger kiln had a lot more pinholes in them. i have some theories about what went wrong after a lot of internet investigation and helpful people and posts in here..:

 

-this was the first time i closed all three peepholes at the side during a glaze fire (and more air flow makes pinholes less likely?)

-i’ve read that flat pieces is more likely to get pinholes, as to curvy stuff as for example bowls? 

-larger kiln vs smaller kiln and different cooling can give different results? (but in my case then it should be the other way around..?)

 

on top of this i heard a podcast episode of: ‘for flux sake’ (which i found a tip about in here), where they concluded with bisque firing at a LOWER temperature rather than a HIGHER was better to prevent pinholes. to have higher absorption of the clay and pack the glaze particles tighter against the surface and have less ‘open space’ between the glaze particles (air that eventually will become pinholes..). 

this makes sense i think, but is also very confusing as it seems that the most common advice is to increase the bisque temperature to prevent the pinholes.. the only thing i was certain about was that i would try bisque firing higher, but now i’m not sure anymore. and if any of you bisque fire lower (08 or lower) as a rule, do you hold as well, or is a ‘slow program’ sufficient? 

if i bisque to cone 08 (as suggested in the podcast) and then test glaze fire a plate with no difference in the result; how much do i re-bisque the rest of the bisqued ware on (cone) to get them to as if they were bisqued at cone 04 to begin with? (if that made sense..)

also, the pinholes appeared mostly in the speckled clay and not the blue. 

i was really sad about all these pinholes and desperately tried refiring them, again at cone 6; this time with all peepholes out. this resulted in even more pinholes and the speckles in the clay way more ‘melted’ than i like them. 

 

 

sorry for the loong post..any input?  in advance, thank you. 

[Jeff Longtin]

To clarify: in this photo we see "blistering". In your previous posting we see "pinholing". 

Blistering is when gas bubbles rise through the glaze, when the glaze is molten, and pop open. (And then the glaze cools before the blister can smooth over. )

While pinholes leave little holes, that are visually unappealing, blisters leaves sharp edges, that scratch the skin.

Unfortunately it requires several tests to solve both problems. (As suggested in the previous post.)

 

[High Bridge Pottery]

Looks like the glaze is full of tiny bubbles and firing a little hotter has let them join together and start rising to the surface.  Firing again just let more of them join together so you can see them, I would guess the ones fired in the small kiln are still full of tiny bubbles but they are just hard to make out and look more like a texture.

A lot of gas can be chemically dissolved into the glaze as it melts. As you keep increasing the temperature the chemical solubility of the gas decreases along with more silica arriving into the melt also changing how soluble those gasses are. 

I feel whatever you try with different bisque temperatures the glaze will still look the same and produce thousands of tiny bubbles that can't join together and escape because of high viscosity and surface tension

 

 

[Rae Reich]

A cone 06 bisque is so standard in pottery that few consider making this a variable. You can concentrate on the proper melting of your glazes by focusing on glaze composition and firing temperature/atmosphere/heat work. 

[Jeff Longtin]

To pick up on Rae's point: changing the bisque temp can sometimes greatly affect the absorption of your bisque pots. This, in turn, will then affect how your glaze is absorbed and, in turn , affect how your glazes turn out. 

If your problem glaze is just one or two, of many, then its best to go after recipe and not temp. If your problems are across the board, affecting all glazes in other words, then such changes may be warranted.

[Babs]

From your initial post, are you aware of cone labelling..08 is lower than cone05, like negative numbering . Just saying. I am reading that you thInk differently.

[glazenerd]

Rebs:

Pinholes come from off-gassing feldspars: blisters come from off-gassing inorganics- primarily from iron disulfide. (sulfides) You either slow the firing down between 1250-to-1800F, or slow the kiln down from 2050F up to peak (cone5/6). The brown splotches of color in the tan area of your glaze is a goodindicator of iron bearing clay: in this case iron disulfide. When a crater forms around the blister, and barren clay can be seen in the center of the crater: almost always inorganic materials burning off. -- Tom