Earthenware Bisque Temperature

[PottaFella]

Is it best to bisque to a higher temp with earthenware?

 

The clear glossy earthenware glaze looks fine dipped and then fired at 1110 degrees c, 

 

But I have been bisque firing to only 1000 (as with stoneware clay) and I'm concerned the earthenware clay may not be fully matured.

 

Is it more difficult to dip-glaze if it has been bisqued higher, say to 1150 degrees C?

[Marcia Selsor]

I would think 1100 C would be better. That would be about ^03. I know this is better for majolica glaze but can't say about what it is you are using. ]

Marcia

[PottaFella]

I would think 1100 C would be better. That would be about ^03. I know this is better for majolica glaze but can't say about what it is you are using. ]

Marcia

 

Thanks Marcia

 

I'm finding the earthenware thing quite puzzling. Stoneware in theory you basically match clay and glaze and fire to a temperature that matures everything nicely.

 

Earthenware it seems the glaze/decoration won't take the heat so you have to bisque higher than the glaze firing or else the clay isn't matured - if I'm understanding it right?

[Sputty]

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[PottaFella]

Oly - if you're firing your glazed ware to 1110 deg C, then the clay is probably going to be matured anyway, if it's an earthenware clay. I would think under those circumstances you can biscuit to 960 deg C, or something similar. I'm doing exactly that - biscuit to 960 deg C (plus half an hour soak), and then glaze fire to cone 03 (1105 deg C). Everything is good.

 

With people firing to lower glaze temperatures (perhaps 1040 deg C, or lower), then biscuiting higher first is sometimes done to ensure body maturity. But in your case, given the final glaze temperature, I would've thought that unnecessary.

 

Further, if you biscuit fire too high, you run the risk of over-maturing the body, so that its subsequent lack of porosity makes dip glazing difficult.

 

Which clay are you using, and what do the manufacturers say about its maturing temperature?

 

Hi Sputty

 

Thanks I've been using Valentine's standard terracotta – no problems – nice results bisque-ing at 1000c and then the Bath Potters clear glaze fired at 1110C with 10min soak made watertight vessels and no noticeable defects (not to my untrained eyes anyway). I should say it's prob the glaze that is making it watertight, I think the terracotta is pretty porous.

 

However, I need to find a white clay to use, thinking of Valentine's PF580 (1080-1180c)  or they suggested low-fire white smooth VBP1 (1060-1160c). From what you say, it sounds like I could just treat it like the terracotta?

[Sputty]

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[Marcia Selsor]

It really depends on the glaze used. For majolica it is important to fire the clay hotter to get all the gases and impurities out before glazing. Otherwise, you can have pin holing.

Marcia

[PottaFella]

It really depends on the glaze used. For majolica it is important to fire the clay hotter to get all the gases and impurities out before glazing. Otherwise, you can have pin holing.

Marcia

Thank you Marcia, yes pin holing is exactly the sort of thing I am worried about.

[Sputty]

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[PottaFella]

Soak for 30 minutes at your top biscuit temperature.

The cheap, no-frills Spanish terracotta I use here in France is biscuited to 960 deg C, with a 30 minutes soak, and then glazed with a gorgeous, fat, stiff majolica glaze. Never a pinhole in sight.

In fact, I've never had a glaze pinhole after a bit of a soak at top biscuit temperature, especially using clays from Valentines, which I did a lot when in the UK. They're so processed, there's precious little in them to burn out, to be honest! Nice, fuss-free clays to use.

 

 

Actually I was just watching a video about using small electric kilns and default should be to soak for half an hour to even-out that top temperature across the shelves - even at biscuit firing. This is something of a revelation to me, so I'm going to try building it in to all my firings, bringing down the top temp a bit if need be – to keep the same cone.