Thursday, January 15, 2015

Raspberry, Biscuit or Bisque? High Fired Glazes Q and A.

A Taste of Summer!
The purpose of having summer is to be able to eat fresh raspberries from the garden! Sorry I cannot send any of them to potters who are shivering in the middle of winter in the Northern Hemisphere, but...

I glaze fired a kiln load of porcelain yesterday and put some of the earthenware jugs and pots that I have been making on top of the kiln to finish off their drying. I like to use the waste heat from a firing like that if I can. I started work today by selecting some of the dry jugs from the top of the kiln, and packing the other electric kiln with them. The other kiln is only about 3 cubic feet inside, but I managed to squeeze 7 of the jugs into the space. The tallest jug is about 17 inches high (43 cm), which is about as tall as I can go in this kiln. These will be having their first firing and it is quite OK for the pots to touch together for this one as no glaze is used in this firing.

Packing even a small kiln like this can take a lot of time as using this limited space to the full is a bit like solving a jigsaw puzzle. It is also horribly easy to damage a pot when loading the kiln, as the pot is just dry clay at this stage and is very brittle. It is best to never hold the pot by the handle or rim, but to try to cradle the pot with both hands when manoeuvring it into the kiln. Of course there are exceptions to this when fitting pots and hands into tight spaces. I make use of scraps of insulating fire brick to stand a pot on if I need to prop a pot up a bit higher in the kiln to make it fit.

I usually incorrectly call this firing a "bisque" firing. I think most potters I talk to in this part of the world speak of "bisque" firing pots. According to the Holy Book of potting, Frank Hamer's "The Potter's Dictionary of Materials and Techniques", the word "bisque" should be reserved for a firing of the pot where the pot is fired to maturity, and the subsequent glaze firing is done at a lower temperature. This is often the practice in industrially produced pots. The studio potter usually fires the glaze firing at high temperature, after the - so called - "bisque" firing at a lower temperature. In this case the "bisque" firing should really be called a biscuit firing.

Q. Do you say "bisque" or "biscuit" when you refer to such a firing?

What is the Purpose of a Biscuit firing?
It is always something of a nuisance to have to fire pots twice. It is costly in time and power having to load and fire kilns, but the biscuit firing can achieve two very useful things and most potters seem to find it a worthwhile part of the process of making  a pot.

Useful thing number one...
If the biscuit firing is taken slowly enough (it should not be rushed!), then carbon from plant material, and other contaminants, can be burnt out of the clay, it is a case of "better out than in"! For example, if carbon is left in the clay, then a problem called black core can result, and the pot become abnormally fragile. If the pot with black core is broken, there is a characteristic dark grey layer within the thickness of the clay wall of the pot that is glassy, and brittle. In extreme cases a molten magma will form in the wall of the pot when it is at high temperature and cause the wall of the pot to bubble and bloat!

Useful thing number two....
A biscuit fired pot, though weak and porous, is strong enough for a potter to glaze with confidence, safe in the knowledge that it will not crack or collapse when wet glaze is applied to it.

Some potters single fire pots, and avoid the biscuit firing altogether, but the clay, the glaze, and the way the potter makes and fires the pot have to be right for this method to work without having too many failures.

Generally most clays are biscuit fired in the range of about 800 - 1050 degrees Celsius (1472 - 1922 F), but this depends very much on the clay and the method the potter uses to apply the glaze to it.

Black Hole and High Fire Glazes!
This has nothing to do with bisque or biscuit firing, or with the dreaded black core problem that can occur in pots..., but I made a largish bowl late last year that made me think of black holes in space!

The bowl is stoneware, and probably measures something like 14 inches in diameter (35.5 cm). The glaze is Black Tenmoko, with a crystalline glaze put over it around the rim and allowed to waterfall towards the centre over the course of the firing.

There is a recipe for Black Tenmoko (BTM) on my High Fire Glazes page. I am in the process of updating the glaze page and have also added a High Fire Glazes Q and A page. The Q and A page is mostly made up of questions people have asked in the comments of the High Fire Glazes page, and my attempts at answering them. I have also added some extra material and photos to this.


Anonymous said...

Wooooo! Love the 'black hole' it reminded me more of a mandelbrot set.

Maybe I just disagree with Hamer (gasp!) but I suspect both the words bisque and biscuit come from the French/Latin for 'twice cooked', which is true which ever way round you do your temperatures.

Michèle Hastings said...

I say "bisque" as well. Biscuit makes me think of, well, biscuits... which would be very tasty with those beautiful berries in your post!

Peter said...

Hi Mike,
Mmmmm, fractals!:)
Lovely to hear from you, welcome to the blog. I am glad to get your input regarding bisque/biscuit... It would be a relief to be able to be correct with either word, particularly as "bisque" feels familiar and right to me. Maybe the Good Book of Potting is open to a more liberal interpretation after all!

Hi Michèle,
Yay, another vote for "bisque"! I never think that biscuit fired pots look particularly appetising, so it is just as well not to confuse them with biscuits! Mmmm, I could imagine a cheese cake with fresh raspberries and a biscuit base!

Linda Starr said...

I like that black hole bowl, bisque here too

Peter said...

Hi Linda,
Good to hear from you. Bisque appears to be winning unopposed, thank goodness! I think I will lose my biscuit and bisque onwards in the happy knowledge that I do so with good company! :)

Anna said...

great post. thanks for the explanation of the difference between biscuit and bisque... guess I'll stick with saying bisque :)

Anna said...

Hi Peter
is it OK if I share your blog site to an Australian Facebook page called Help for Australian Potters, Tips and Advice.
Are you on Facebook?

Peter said...

Hi Anna,
Lovely to hear from you twice in a row :)

Right now I am kiln watching early in the morning and I am trying not to eat a biscuit... it would be all too easy to just keep munching away whilst trying to stay awake! No risk at all of eating bisque..., maybe I should use the word "bisque" for the edible variety, to put me off overconsumption!

Regarding Facebook... You are most welcome to share my blog site on Facebook, and the page you mention sounds a really interesting one. It is nice to hear of people taking time to share tips and advice.

I am not currently on Facebook, although they still regularly send me stuff that goes into my spam... I briefly signed up years ago when a friend (a real one!!) encouraged me to give it a go, but it just seemed one thing too much to do as I am a bit slow putting together my blog posts, so I unsubscribed.

spondoolio692000 said...

Hi Peter what the crystalline glaze over the tenmoku on this bowl it's great :)

Peter said...

Hi Spondoolio (love the name by the way!),
I am not sure what crystalline glaze I used for that one (I seem to remember using up a small quantity of left over crystalline glaze for that bowl), however the base glaze was probably either:

4110 47 (4110 an alkaline glaze fritt that is the same as 3110)
Zinc oxide 27
silica 24
Titanium Dioxide 2
Bentonite 2

4110 51
Zinc Oxide 25
Silica 24
Titanium Dioxide 6
Bentonite 2

There was probably 2 percent of copper carbonate added to either of those too.

I am using the crystalline glaze in this case for its fluxing powers, rather than to grow crystals. This combination of crystalline glaze over a high iron glaze such as a tenmoko mostly works quite well. It is worth testing carefully first on something that doesn't matter too much as I have found that the glaze combination does leave annoying bubbles when over some clay bodies.

Also, just a small amount of crystalline glaze put over a glaze will make it run like crazy, so do take care to protect kiln shelves!