Jon Singer (![[info]](http://l-stat.livejournal.com/img/userinfo.gif){kind=small} jonsinger) wrote,@ 2009-06-29 20:25:00 |
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Glazes, mostly...
I continue to mess with glazes (in the broad sense), and I am currently pursuing at least 5 tracks:
China Paint
I have a couple friends who are china painters. Seems that china paint (which is essentially the lowest-melting ceramic glaze) was classically made with lots of lead oxide, which melts nicely at low temperatures. Also seems that lead has to go away, because everybody is kinda crazy on the subject. There are now some supposedly leadless china paints, but according to Paul Lewing, some lead shows up when those paints are tested. I could wonder about this; perhaps the tests are showing Bi as Pb? They shouldn’t, but that may not prevent them from doing so.
I am working up some recipes that are seriously unlikely to contain any lead, and we’ll see how far I get. I am not a china painter, btw, and my test objects are very different from real china painting. Here is a recent one, painted onto an ordinary wall-tile that has little speckles, so I can see whether the plain clear version really is transparent (it isn’t, and I will have to reformulate it to obtain more complete melting) —
Black Ding Ware
I am reformulating my version of the Black Ding glaze, bringing it more closely into line with the original, and having the expectable problem: the Song dynasty folks fired their wares for several days; I fire mine for a few hours. My recent versions melt in my kiln; but they don’t have enough time to smooth out properly, and they come out of the firing with dimpled surface texture. I am messing with the formulation in order to make the melt more fluid. There are several possible approaches to this, and I will post about progress from time to time if and as there is any.
The other side of this particular project is that I am also attempting to approximate or imitate the original ingredients. It appears likely that the Song potters made these glazes from dirt and ashes and maybe some rust. The dirt (which I think is described as “Glaze Earth” when it is used in ceramics) is decayed Loessic soil that seems to have blown in off the Taklamakan Desert a few millions of years ago, and blankets a huge area of northern China; in some places it is hundreds of meters deep. My first shot at imitating it is a fairly decent glaze all by itself, so I am feeling encouraged:
The broad stripe down the middle is a mixture of Rutile and Gerstley Borate, which tells me that this tile was fired to cone 10 in reduction, in my little gas kiln. The narrow stripe is iron oxide. I think my next step is to try adding small amounts of wood ash, to see what happens.
It is, I will admit, peevish-making that it takes me at least 10 ingredients to make a mediocre imitation of something the Chinese potters could (and still can) just dig up out of the ground, make into a slurry, and use. Sigh.
Aventurine
Aventurines (see the Lancastrian vase on this page if it is still there, or this piece by Lasse Östman) are crystal glazes, and they want the kind of firing cycle that gives the crystals a chance to grow. This involves (among other things) stalling the kiln for a while during cooling, at some appropriate temperature. Sometimes there are several such “holds”. For certain crystal types it gets even more complicated, but let’s not go there just now.
A crystal firing is very different from my usual cycle, which involves a fairly long hold at peak temperature and no pause at all during cooldown. It is no surprise that I do not obtain great results from aventurine tests. I can, though, get some sense of the character of an aventurine test, and eventually I will get something that looks really promising, at which point I may ask a friend to put a test tile through a proper crystal firing.
I fired two test aventurines last night. One of them did approximately what I expected it to, and would probably be fairly decent if I put it through the right kind of firing, but does not appear to be in any way exceptional.
The other is quite possibly the most astonishing failure I have experienced in 13 years of glaze development.
I was originally going to call this “DragonPox”, but have been gently guided in a different and more temperate direction; it currently stands as “Chocolate Toad”. (I suppose I can use whichever name is appropriate for a particular piece.) Either way it is definitely a keeper, and will probably end up on a few delicately distressing sculptural items.
Oilspots
Oilspot glazes have been around for quite a long time; they are high-iron formulations, generally fired in oxidation, so they are nicely adapted to electric kilns. The process by which the spots form is reasonably well understood, but the description is sufficiently geeky-tweaky and protracted that I would prefer not to go into it here; if you are interested, please send email.
There seems, nowadays, to be a fashion for large spots, which are associated with very thick applications of glaze. I don’t really like that look, so I am working on glazes that are applied in ordinary thickness and develop small-to-medium-size spots, similar to the ones I’ve seen on occasional Song dynasty pieces (for example, this one). My first test glaze fired out the wrong color but was otherwise pleasant, and went from version 0.1 to version 1.0 immediately when I took this test bowl out of the kiln and got a good look at it:
(I only mixed up 100 grams of the test glaze, so I had to paint it onto the bowl with a brush. The thickness, needless to say, was somewhat uneven.)
Because it is not what I was going for, the mere fact that it is a keeper did not stop me from continuing development. Version 0.2 did not fully melt, and I reformulated it; last night I fired tests of versions 0.3 (right side) and 0.4 (left side):
What I think I want is probably about 90% v0.4 and 10% v0.3, applied fairly thinly. I think I will go ahead and mix up a large enough batch to dip things in, as there is a good chance I can make minor corrections on the fly if it becomes necessary.
Once I get this type under control, the next step is to get spots that are more pronounced and are either silvery, like these, or perhaps iridescent if I can engineer that. Thinking about it...
The one orchid I own, which I think is Neofinetia falcata ‘Tama Kongo’, is once again putting out flower buds. (The flowers are fragrant, mostly in the evening.) It does this every year now, despite the fact that I have no clue how to take care of it and no idea if/when it should be repotted or divided or whatever you do with these things. This plant has survived multiple frosts, and is either remarkably hardy or astonishingly stubborn. Maybe both.
I took that with the camera in my phone; apologies for the image quality.
Cheers —
jon
I continue to mess with glazes (in the broad sense), and I am currently pursuing at least 5 tracks:
- Low-melting glass (more about this in a later posting).
- Lead-free china paint.
- Two paths toward the Black Ding glaze, improving on my earlier copy.
- Aventurine.
- Oilspots.
China Paint
I have a couple friends who are china painters. Seems that china paint (which is essentially the lowest-melting ceramic glaze) was classically made with lots of lead oxide, which melts nicely at low temperatures. Also seems that lead has to go away, because everybody is kinda crazy on the subject. There are now some supposedly leadless china paints, but according to Paul Lewing, some lead shows up when those paints are tested. I could wonder about this; perhaps the tests are showing Bi as Pb? They shouldn’t, but that may not prevent them from doing so.
I am working up some recipes that are seriously unlikely to contain any lead, and we’ll see how far I get. I am not a china painter, btw, and my test objects are very different from real china painting. Here is a recent one, painted onto an ordinary wall-tile that has little speckles, so I can see whether the plain clear version really is transparent (it isn’t, and I will have to reformulate it to obtain more complete melting) —
Black Ding Ware
I am reformulating my version of the Black Ding glaze, bringing it more closely into line with the original, and having the expectable problem: the Song dynasty folks fired their wares for several days; I fire mine for a few hours. My recent versions melt in my kiln; but they don’t have enough time to smooth out properly, and they come out of the firing with dimpled surface texture. I am messing with the formulation in order to make the melt more fluid. There are several possible approaches to this, and I will post about progress from time to time if and as there is any.
The other side of this particular project is that I am also attempting to approximate or imitate the original ingredients. It appears likely that the Song potters made these glazes from dirt and ashes and maybe some rust. The dirt (which I think is described as “Glaze Earth” when it is used in ceramics) is decayed Loessic soil that seems to have blown in off the Taklamakan Desert a few millions of years ago, and blankets a huge area of northern China; in some places it is hundreds of meters deep. My first shot at imitating it is a fairly decent glaze all by itself, so I am feeling encouraged:
The broad stripe down the middle is a mixture of Rutile and Gerstley Borate, which tells me that this tile was fired to cone 10 in reduction, in my little gas kiln. The narrow stripe is iron oxide. I think my next step is to try adding small amounts of wood ash, to see what happens.
It is, I will admit, peevish-making that it takes me at least 10 ingredients to make a mediocre imitation of something the Chinese potters could (and still can) just dig up out of the ground, make into a slurry, and use. Sigh.
Aventurine
Aventurines (see the Lancastrian vase on this page if it is still there, or this piece by Lasse Östman) are crystal glazes, and they want the kind of firing cycle that gives the crystals a chance to grow. This involves (among other things) stalling the kiln for a while during cooling, at some appropriate temperature. Sometimes there are several such “holds”. For certain crystal types it gets even more complicated, but let’s not go there just now.
A crystal firing is very different from my usual cycle, which involves a fairly long hold at peak temperature and no pause at all during cooldown. It is no surprise that I do not obtain great results from aventurine tests. I can, though, get some sense of the character of an aventurine test, and eventually I will get something that looks really promising, at which point I may ask a friend to put a test tile through a proper crystal firing.
I fired two test aventurines last night. One of them did approximately what I expected it to, and would probably be fairly decent if I put it through the right kind of firing, but does not appear to be in any way exceptional.
The other is quite possibly the most astonishing failure I have experienced in 13 years of glaze development.
I was originally going to call this “DragonPox”, but have been gently guided in a different and more temperate direction; it currently stands as “Chocolate Toad”. (I suppose I can use whichever name is appropriate for a particular piece.) Either way it is definitely a keeper, and will probably end up on a few delicately distressing sculptural items.
Oilspots
Oilspot glazes have been around for quite a long time; they are high-iron formulations, generally fired in oxidation, so they are nicely adapted to electric kilns. The process by which the spots form is reasonably well understood, but the description is sufficiently geeky-tweaky and protracted that I would prefer not to go into it here; if you are interested, please send email.
There seems, nowadays, to be a fashion for large spots, which are associated with very thick applications of glaze. I don’t really like that look, so I am working on glazes that are applied in ordinary thickness and develop small-to-medium-size spots, similar to the ones I’ve seen on occasional Song dynasty pieces (for example, this one). My first test glaze fired out the wrong color but was otherwise pleasant, and went from version 0.1 to version 1.0 immediately when I took this test bowl out of the kiln and got a good look at it:
(I only mixed up 100 grams of the test glaze, so I had to paint it onto the bowl with a brush. The thickness, needless to say, was somewhat uneven.)
Because it is not what I was going for, the mere fact that it is a keeper did not stop me from continuing development. Version 0.2 did not fully melt, and I reformulated it; last night I fired tests of versions 0.3 (right side) and 0.4 (left side):
What I think I want is probably about 90% v0.4 and 10% v0.3, applied fairly thinly. I think I will go ahead and mix up a large enough batch to dip things in, as there is a good chance I can make minor corrections on the fly if it becomes necessary.
Once I get this type under control, the next step is to get spots that are more pronounced and are either silvery, like these, or perhaps iridescent if I can engineer that. Thinking about it...
The one orchid I own, which I think is Neofinetia falcata ‘Tama Kongo’, is once again putting out flower buds. (The flowers are fragrant, mostly in the evening.) It does this every year now, despite the fact that I have no clue how to take care of it and no idea if/when it should be repotted or divided or whatever you do with these things. This plant has survived multiple frosts, and is either remarkably hardy or astonishingly stubborn. Maybe both.
I took that with the camera in my phone; apologies for the image quality.
Cheers —
jon
