Need help with Pure Brilliance Glaze on Terracotta, PINHOLING problems

[Joanna Griffin]

I have had good experiences with this glaze in the past, and now seem to be encountering difficulty with pinholing, most especially on large plates and platters - almost a given.  Other forms seem to be fine.

The viscosity I use is approximately 21. Currently I am doing a mix of brushing application and pouring liners and flatware to achieve a uniform surface.  This has been a standard method for me that has worked in the past.  I am firing to a hot 04 bisque and glazing at 05.  I have never found the need to soak, however I do close the top spyhole immediately when kiln shuts off.  I do a slow firing approximately 10 hours and give ample time for cooling. I have an older model Skutt kiln model 231 that fires consistently even in temperature – manual controls.

In the past I have had success even with the platters, but now am troubleshooting.  Would love some help as my dinnerware orders are stalled in production, and pottery is my living with summer fair season already here. 

Thank you for any advice from a desperate Montana potter. 

[Joanna Griffin]

Thanks, Brian.  remembering my early days of stoneware/gas kiln firing, i would see that adding a flux to the mix would help it move slightly.   OK. I use a commercial glaze that arrives in gallon format.  How would i go about figuring 1 percent of the total, adding a dry ingredient into the liquid?   The total gallon is about 128 oz.  As an artist, this is beyond me, but as a cook, i'd say throw in a pinch!   However, i do want to be a bit more accurate.  Ideas?    I actually happen to have gerstley on hand.   I just finished up a firing to a high 04 and will use the same method when firing my 05 glaze so it also goes a bit higher.  Just adjusted the claw on the kiln sitter to catch a bit more of the bar and thus go a bit higher.    A question on soaking.  If i immediately turn the kiln back on once it shuts down, to say MED. for 30 mins or so, I assume the top spy hole should stay open until i shut it off?  Or should i close it down during the medium soak ?    thanks for your thoughts.  I do not have an additional venting system.       Joanna  

[Joanna Griffin]

thanks so much!  That was the general picture that was in my head re the mixing in of the powder... thanks for making it lucid.  The reason i asked about the venting system is that i visited a studio yesterday to drop off a platter to see how their usage of the same glaze might turn out... they use the same glaze effectively on commercially bisqued molds.  Their newer computerized kiln has an "Envirovent system" pulling fumes from the bottom of the kiln and thus they always keep ALL the spy holes closed through the whole glaze firing.  I have also come across info in my research that talks about effective burnout of organic materials, especially in terracotta, that is aided by this venting downdraft system.  Also aiding in elimination of pinholes.   One suggestion was to keep all spyholes open all through the firing up until red heat begins, to provide adequate oxygen for complete burnout.   What are your thoughts on this?  haven't tested this method.     I really appreciate having someone to bounce  questions off of :)  !