Isculpt Posted November 5, 2014 Report Share Posted November 5, 2014 I've read on CAD of potters firing their clay to mid-range maturity, and then applying a low-firing glaze and firing again. My raku clay has a firing range of cone 2-6, so I bisqued my sculpture to cone 05, then fired to cone 5 with Potter's Choice cone 5 glaze applied. Now I want to glaze the interior of the sculpture with another lower-firing glaze, but I've noticed that all the low-firing 05 glazes I have on hand have the following instructions: "Apply to matured cone 04 bisque". What do they mean by "matured 04 bisque"? Can I not apply them to a clay that has been fired higher than 04? I'm assuming that I don't want to use another cone 5 glaze, since I suppose it would affect the previously-applied glaze? I've made some vases and glazed their exteriors, but need to glaze interiors to water-proof them, so any advice on how to proceed would be appreciated. Jayne Link to comment Share on other sites More sharing options...
potterbeth Posted November 5, 2014 Report Share Posted November 5, 2014 Low fire pottery is usually bisque fired to a higher temperature than its glaze temperature (bisque to 04 and glaze to 05-06). Bisque firing to cone 04 ensures that organics in the clay have burned out so they don't cause problems during the glaze firing, and any clay shrinkage has already occurred before the glaze firing so the glaze does not have to shrink to fit the clay. Plus the clay is still porous and can absorb the glaze. The good news here is that you probably won't have any fit problems in applying and firing the glaze on your cone 5 fired piece. However, if your piece has become non-porous at cone 5, applying the 05 glaze will be more difficult. Commercially produced glaze purchased as wet mix usually contains enough gum to allow a decent coating, but it may not go on as thick as it would on bisque which could affect the color and texture of that glaze. Additionally, your cone 5 glaze may be affected by the re-fire at cone 05. I've seen both good and bad results. Until you finish one of these pieces, you won't know for sure what you will get. So, moving forward, some test tiles in your clay body using your glazes might be desirable. Otherwise, your work is the test. Finally, some low fire glazes do not respond well if they are fired in a kiln with work being bisque fired. Once again, I've seen both good and bad results. So your tests should take this into consideration. Have fun! Link to comment Share on other sites More sharing options...
Chris Campbell Posted November 5, 2014 Report Share Posted November 5, 2014 I agree with PotterBeth ... you won't know how the outer glaze is affected until you try it on pots or test tiles. Warm the pots in your oven before applying the liner glaze. My concern on this ... will the final result really be water tight? Make sure you test them. If you are doing decorative work, it is fine to fool around with surface and firings. Sometimes you need to fire high first in order to be able to use certain low fire glazes on other parts of the work. BUT ... with any functional work, I would prioritize the food surface and follow the directions on the glaze bottle. Link to comment Share on other sites More sharing options...
Isculpt Posted November 5, 2014 Author Report Share Posted November 5, 2014 Thank you both. Beth, now I understand the 'why' of the seemingly arbitrary "fire to 04 bisque". I was going to fire the 05 glaze along with a bisque load with no consideration of the effect of organic matter burning out, so your tip regarding that got my attention. Chris, next time I will fire inside and out at the same time! Can water-tightness be achieved simply by firing to vitrification or is a glaze required? Or does that vary according to clay used? Is there a rule of thumb that refiring glazed work to a lower temperature is less likely to have drastic effect on the existing glaze than firing a piece that was glazed at, say cone 5 all the way up to cone 5 again? Or are there no rules of thumb?! And finally, can you recommend a book about this subject? I will never, at this stage of my life and with limited need for glazes, create my own glaze. So I don't need a book that goes into that aspect of glazing..... Jayne Link to comment Share on other sites More sharing options...
Chris Campbell Posted November 5, 2014 Report Share Posted November 5, 2014 I am not a glaze expert so I will leave that part of the answer to others ... But as to water tight ... I never say my work is, I always advise customers to use caution. Yes, it probably is but I don't want to hear any stories about how my pot ruined great grandmothers favorite antique table! Link to comment Share on other sites More sharing options...
Isculpt Posted November 6, 2014 Author Report Share Posted November 6, 2014 Good point, Chris. You may have just saved me from having to pay for someone's priceless antique. Link to comment Share on other sites More sharing options...
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