[Clayart] Thank you for your guidance

Paul Gerhold gerholdclay at gmail.com
Sat Mar 3 07:23:25 EST 2018


Snail,
A good summary. Agree with everything except your statement that absorption or bisque temp. Is not relevant when applying glaze by brushing. In fact having a relatively non porous bisque is critical to getting long brush strokes and a streak free even glaze coat. A high bisque temp. and  multiple applications are critical to getting an even glaze application by brushing. Adding glycerin will also help.

Paul

> On Mar 1, 2018, at 11:40 AM, Snail Scott <claywork at flying-snail.com> wrote:
> 
> 
>> On Mar 1, 2018, at 5:06 AM, Rebecca Evans <cariadcraftswales at yahoo.com> wrote:
>> Thank you for your reply, I  bisque to 1040, I thought I’d have to increase it but you are the pro! 
> 
> 
> Rebecca-
> 
> Bisque firing is never essential (many people single-fire their work), but it does serve several useful purposes, and reviewing them can help make better choices when picking a bisque temperature. They are: 1) to make the clay permanent, so that overwetting during glazing does not slake it to a lump of mud; 2) to toughen it so that objects can be safely (or even roughly) handled; 3) to optimize absorption for glazing, and 4) to burn out any impurities that might cause glaze flaws or reduced clay strength later.
> 
> For the first factor (making it into permanent ceramic), anything above 1000F (about 550C) will suffice for that, since that’s above the point when the chemically-bonded water is driven off of the clay molecule, never to return.  For the second, exact level of toughness is likely unimportant unless you are doing extremely delicate stuff (or shipping the bisqueware), and a higher/harder bisque becomes necessary to prevent breakage during handling.  The third factor, absorption, is relevant if you are dipping your work to glaze it, since lower-bisqued clay is more absorbent, and will soak up a thicker layer of glaze than higher-fired bisqueware. Typically, this merely requires adjusting the length of the dip, and if you are applying glaze by brush or spray, it may be nearly irrelevant.
> 
> The final factor is usually the most relevant when choosing a particular bisque temperature.  Some clay bodies are very ‘clean’ and have few of the impurities that might cause problems later, so a low bisque will suffice, but others have lots of carbon, sulfur, etc, and need a higher and/or longer bisque firing to burn it all away. Ironically, earthenware clays are more likely to have lots of these, but many highly plastic stoneware clays do, too. How to tell? Do your glazes look good on work that was bisqued to a low temp? Then no problem. If you have blisters, or the clay seems punky and over-fragile even after vitrification to the right temperature, though, then try bisque-firing hotter or longer and see if it helps.  (For example, my usual sculpture body is fairly ‘clean’ and also very coarse, and does fine with a bisque at around ^010 (1500F, around 800C), and I often glaze it as greenware and single-fire it with no problems. In contrast, the clay I use for throwing is a bit ‘dirty’ and tight-grained, and has occasional glaze flaw issues unless bisque-fired to around ^04 (1900F, 1040C).)  Few clays need to be bisque-fired any higher than that - that’s plenty high enough to burn out all the offending impurities, as long as the firing is nice and slow - long enough for proper burnout - or at least has a bit of a soak at the end. Thick objects may also need more time, just as pizza cooks faster than a casserole. (The importance of time is why we use cones to determine firing, since they measure heat-work and not temperature alone.)  
> 
> Though I tend to shy away from absolute statements, I will say this: There’s essentially ZERO chance that you will ever need to bisque hotter than you are already. (Cooler, maybe, but not hotter.) If you are getting good results from your earthenware at this bisque temp, then it will definitely be sufficient for your stoneware.
> 
> Many earthenware glazes are sensitive to sulfur impurities (cadmium-based colors like red, especially), and earthenware clays often contain a lot of carbon which can weaken the clay if they are still present when fired to maturity, so earthenware is nearly always fired to your current bisque temperature. Stoneware glazes seldom contain cadmium or other sensitive pigments (except as encapsulated stains), and stoneware clay gets fired hotter anyway, which can burn away many of the impurities that would interfere with vitrification. Because of this, many stoneware bodies can be bisque-fired lower, but not all. It depends on the specific clay body composition, AND on the glaze composition!  If you want to experiment, though, it’s possible your stoneware can tolerate a lower bisque, saving you a bit of time and money. Testing is the only way to find out for sure. If you prefer to keep things simple and stick with your present bisque temp, that will be just fine. Go right ahead! 
> 
> 
> Snail Scott
> claywork at flying-snail.com
> www.snailscott.com
> 


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