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Theoretical Reduction Firing   John Tilton Letter   Porcelain Misconceptions   Potters Guild
Refiring   Crazing     Porcelain Evaluation

Porcelain Evaluation
I had to convey my appreciation of your latest incarnation of Turnerís Best Porcelain (Standard # 600). As soon as I started centering this porcelain I knew it was different from your previous formulations. It has a very tight feel that continues throughout the throwing process. I never felt like I was losing control of the form. I tend to throw with more water than the average potter and this porcelain took the excess while moving to wherever I directed. I was just running some preliminary tests and found that I didnít want to stop throwing this body. Thatís probably the highest compliment a potter can give to a porcelain and its creator. As you know, Iíve thrown pots for 26 years now and have used or tested upwards of 80 different stonewares and porcelains. I can say with confidence that this is the best throwing body that Iíve come across in some time and definitely the best porcelain period. I am in the process of studying its drying properties, how it takes handles, testing shrinkage at cone 6, and how my glazes react to this porcelain. I am interested in constantly improving my work so I begin with the best equipment, raw materials, and clay at my disposal with the end goal being consistency in quality. Having Standard Ceramic Supply mix and distribute #600 removes any concerns I would have regarding batch to batch quality. Iíll let you know the results of my further testing. I already have an educated guess that the cone 9 #600 will be plenty vitreous at cone 6 from testing its predecessors. Talk to you soon- Chris Powell, studio potter.

Here we have another border line issue-crazing. Is it a defect, or a decoration? Crazing is caused by the glaze shrinking more than the body during cooling of the glaze firing in the kiln. Crazing can be seen as a flaw, especially on low fired pottery where liquids will go through the craze, through the body, and onto furniture. Crazing is also called crackle, and has been developed to a high art by the Oriental potters. I have said before, "When does crazing become crackle, when you put ink in it". Crazing on a vitreous body such as porcelain, is not a flaw, or detriment. Some colors prefer the chemistry that causes crazing. Sometimes adjustments can be made to the glazes, or the bodies, but sometimes it just has to be lived with. My refires tend to craze more than on the first firing, but that's because they are getting more melt, creating much more vibrant color. There is also delayed crazing that happens months and years after the piece is fired. Here again, I would prefer not to have it, but I accept it for certain glazes.

I have gotten more and more into refiring pots to really max out color and interesting surfaces. When I refire, I run the risk of loosing the piece completely, or changing it into something I like more than before. Sometimes I don't loose the piece, but there can develop what I will call "ceramic acne". Pimples erupt from captured air in the walls that expand upon refiring into what we call bloats. There can be other surface blemishes as well. Normally we do not want any sort of "ceramic acne", but with the refires that come out better, I accept some of these anomalies. There can also be discoloration of the unglazed porcelain and many times the glaze crazes. The first firing melts chemicals into a glass or glaze; the second refire remelts a glass or glaze. With refiring, very different reactions take place and the results are different from the first firing. Sometimes I just refire to get more melt and sometimes I will heat the pot in the electric kiln to about 250 degrees F and spray another glaze over the first glaze and then refire. If I accept the piece as finished, I also accept the acne that comes along with the process. I have read where the Japanese prefer and treasure such anomalies. Those of you who know my work, know that my standards are extremely high, maybe too high, so I ask that you accept my acceptance of these "in the process" anomalies. No plunge: no pearl.

Since I was accepting these anomalies as part of the process of getting a greater pot, I was blind to them once accepted. A very good customer brought this to my attention and I will try to be more vigilant when I photograph them to show what I have accepted. I also understand that web purchases are from photographs and if you are not satisfied when you receive the piece, please contact me and I will accept a return, or exchange, minus the shipping costs.
Thank you for your support


Theoretical Reduction Firing

Each kiln is different, each load is different, the weather is different, and each potterís ideas are different. I believe candling a kiln overnight is a waste of time and fuel. Universities do it because they donít pay for the fuel and it makes the firing day shorter. I light a kiln and keep it under 250 degrees the first hour and then proceed on up.  Ten degrees per minute is 600 degrees per hour.

If you are firing bisqued ware, there is no reason after the first hour that you cannot go 300-600 degrees per hour up to reduction. I go slow the first half hour to remove any water from the glazes and bisque, cone pads, etc. Industry fires to cone 14, up and down in 90 MINUTES. You can be at cone 05-06 in 4 hours and start reduction. I start at cone 05 with the oxy-probe at .65.

Atmospheric burners-maintain clean, efficient combustion all the way to reduction.

Forced air burners-maintain clean efficient combustion without excess air.

I start reduction at cone 05, and only the lightest amount of reduction to get the results I want. NEVER is there carbon anywhere. Carbon does not reduce, carbon monoxide reduces. I start at .65 on my oxy-probe.

DO NOT REDUCE AIR INTO THE BURNERS! To start reduction, simply push the damper in until there is slight pressure out the bottom peephole. At first there may not be a flame out the bottom, but there will be pressure and if you smell it, it will take your breath away, (or burn your lungs if you are not careful).

What happens is that the same amount of gas continues to go in, but the damper has restricted the amount of air going in, both primary and secondary air. More importantly, by reducing with the damper you create a pressure that provides a more even firing, and with CLEAN reduction.

Once I learn a kiln and the damper setting for the reduction I want, I set that and leave it alone throughout the firing. If you finish a kiln with the damper at 3 inches, then start the kiln with the damper at 4 inches. We were taught to grossly over-reduce because our teachers really didnít know any better.  In the 60ís everyone was learning. So they would over reduce and loose temperature, then oxidize to get temperature up again, then over reduce, and so on. I think this procedure also echoed earlier firings with wood where you stoke and get reduction, it burns neutral and oxidation, then you stoke again, etc. When potters over reduced they would always ďclean up the kilnĒ at the end; but my point is if you donít over reduce, there is no need to clean anything up. Now, oxidation at the end is not bad, just donít over reduce so that it is necessary.

Without an oxyprobe we judge reduction by how much flame comes out the peep holes and the character and color of that flame. IDEAL reduction is greenish purple that is clean and steady pressure of about 6 to 8 inches of flame out the top peep hole. I highly recommend an oxyprobe! They will pay for themselves in no time. You will save fuel, time, and firings will be consistent. The oxyprobes vary and we canít be specific on numbers. The directions call for milder reduction than we usually want, and without enough pressure, you will not get an even firing and even reduction. Many potters start reduction at .7 on the oxyprobe. If you have very clean reduction and sufficient pressure, I think you could start at maybe .6.  All of this is trial and error and once you get the gas and damper settings that work for you, you use those numbers as a guide for successive firings, realizing each firing will be different do to loading, weather, etc. Potterís using natural gas will have slightly different settings and results as compared to those using propane.  Point is, once you get YOUR settings, all you have to do is go there next time. The oxyprobe will show a different reduction reading if you move the damper ONLY the width of a pencil line. In my Ohio studio, if you opened the front door, reduction began to drop and you could see it on the probe. Likewise when you open a peephole, the numbers start to drop. The peep hole becomes more chimney, allowing more air into the kiln. Realize that the chimney pulls air from ANY opening, ANYWHERE in the kiln. It draws from the opening above the damper, it draws from the burner ports, and it draws from ANY crack ANYWHERE in the kiln or chimney. Holes in the chimney are passive dampers.

So back to firing- once the proper amount of reduction is set, I do not change it. As the kiln gets hotter, it will burn off some of the reduction and the numbers will lower and the flame get even cleaner. When my desired cone has fallen, I turn the kiln off and close the damper. End of firing and leave it overnight.

Hi Tom,
My friend Terry is firing your porcelain to cone 11 in a crystalline firing with no problems. He has built a very impressive electric reduction kiln which automatically reduces and a solenoid and oxy probe adjust the amount of reduction.
I have begun to institute a hold of 45 minutes at 800C/1472F and it appears to be very promising in stopping bloating -this is a new experiment and I have only done 2 firings but neither pot bloated and they were the ones that were bloating. This may be the key -- just some volatiles that need to come out of the clay.
John Tilton


April 26, 2006

Porcelain Misconceptions

I have been receiving phone calls and emails about problems with porcelain and it is time to discuss this so these errors can be eliminated. I have done high fired porcelain pottery for over 40 years and have experienced many of the problems people are having today. I must say that part of the problem today is that many people learn to make pots without any technical training to go along with the mechanical training of learning to center and throw. Then they buy someone elseís porcelain, misuse it, and then want to blame the porcelain. This is unfair to themselves and to those of us who provide porcelain for them to use.

Porcelain is not clay, it is porcelain. Porcelain is man made and formulated for the manufacturing process and firing temperature required to do the job. I have seen porcelains from cone 01 to cone 14 which were formulated for a specific use. Naturally a slip casting porcelain will be different from a throwing porcelain in the type of materials chosen for each body. I am a potter who loves to throw pots and my porcelain was formulated by me, for me, to throw as well as possible while staying very white and firing to total maturity, or vitrification. It throws incredibly well, is very white, and as a bonus is translucent. I can throw 5 pound pots totally translucent. Many commercially available porcelains have a leeway of several cones to satisfy many customers and to cover mistakes and over firing. As a result people have been led to believe that all manufactured bodies can be over fired 2 or 3 cones and should survive. This is not being realistic. My porcelain is sold as cone 9 because it is totally done at cone 9, as are my glazes. If you choose to fire over cone 9, you are on your own as I only recommend cone 9. I do know that Otto Heino in California is firing it to cone 12 in a wood kiln, but thatís because Otto understands the mechanics of porcelain. If porcelain pots are not thrown correctly, trimmed correctly, dried correctly, and fired correctly, you will have problems. If you want to side fire them with other pots on top, naturally they are going to warp. If you donít design, throw and trim properly, naturally they are going to warp, especially when you over fire them 2 or 3 cones.

Since we are talking about a body that is vitreous and soft like taffy in the firing, we must learn to design properly. Feet MUST be directly below the walls to support the form while it is soft in the kiln, or you will have warping. There cannot be thin spots from trimming too far or you will have warping or collapse. I have always said that I believed porcelain was as soft in the firing as it is on the wheel. You cannot cantilever forms out beyond the foot as easily as you might with an under-fired body. If you want to do these things then I suggest under fired porcelain, or donít use porcelain at all. An under fired porcelain wonít be vitreous or ring like a bell, which for me is part of the aesthetics of porcelain, the sounds it makes. Lids that are ground in place have a sensuous soft sound; and a tight vitreous porcelain rings like a bell when struck with a knuckle or small object. For me, that is all part of why I work with porcelain.

Now because this vitreous porcelain is soft like taffy in the kiln, naturally lids are going to want to stick to lips and feet are going to want to stick to unwashed shelves, or poorly wash shelves. Think of porcelain as molten glass, which it is in a way, and these events make sense. The easy solution is to put alumina hydrate in your wax resist at about a teaspoon of hydrate to a cup of wax. I use Alcoa c-33 which I guess to be about 150 mesh and coarser than what you would put into a glaze. I also use this in my kiln wash in a mixture of 6 parts alumina hydrate to 1 part kaolin and I spray this on my kiln shelves. The hi alumina wash resists the fusing action of the melting porcelain and the coarse particles of alumina act as little ball bearings to allow the foot to shrink and move during the firing. This wash adheres to the shelf until you take it off with a c-cup wire brush attached to a 90 degree body grinder. I clean my shelves in less than 60 seconds. A simple wire brush would do the same thing, just take longer.

There have been issues with potters using my porcelain with soluble zinc crystalline glazes and I would like to address that too. My friend John Tilton has found that he has to use a porcelain that is under fired at cone 9 or 10 to eliminate bloating. He basically took my porcelain and had it mixed to fire higher and itís working for him. He feels this bloating is from the solubles in the glaze entering the bisque surface and then reacting with the vitreous porcelain in the high fire. Frits today are soluble to some extent. John also found that he had to vent his electric kiln during bisque firing, or the bloating was worse. We think our new electric kilns are tighter than ever before and unless vented, actually fire in a neutral atmosphere and can in fact reduce. The vent systems that draw air through the lid and exhaust out the bottom seemed to work well.

Let me talk more about bisque firing porcelain. When we read books we must put everything into perspective, or we mislead ourselves. Books talk about long and high bisques and that applies to clays that have organic material that needs to burn out in the bisque firing. There are practically no organics in porcelain that need burning out, but I do recommend a good vent drawing air through your kiln to ensure a clean bisque as well as to lengthen the life of your elements. I bisque my porcelain in 6 hours and never have bloating problems. We bisque fire to eliminate chemical water, provide some of the shrinkage, and to strengthen the pot so we can physically handle the piece for glazing. We still want the piece to be absorbent enough for the glaze to adhere, but not vitreous enough that the glaze takes a long time to dry. Long drying times promotes crawling. Therefore you need only bisque high enough to satisfy these needs and if you can do that at a lower temperature than previously experienced-why not? My porcelain starts to sinter earlier than other bodies and I found that cone 010 to 012 is plenty of heat to do all those things just mentioned. This is especially necessary if you throw very thin and there is not a lot of wall thickness to absorb the glaze. I actually glaze the interiors and let them dry overnight before glazing the exteriors so the walls donít get over saturated with water, delay drying, and promote crawling. Stop and think for a minute. A shorter firing to a lower temperature is saving you TIME and MONEY; so what is the problem? If you want to bisque at cone 08 or cone 06 and you like the result, then you have no problem. When books or people suggest bisqueing at cone 02, they are referring to cone 14 porcelain. Once again, find out what works for you and simply do it.

Another issue is the cost of a body as compared to other available bodies. If a porcelain throws better for you, is whiter, is translucent, and eliminates some of the problems associated with porcelain, isnít that worth a few pennies? Kent McLaughlin of Penland recently told me that he has reduced his losses so much by using my porcelain that the extra cost isnít even a factor. I ask people how much they get for a one pound mug and what does .16 cents more per pound mean. (I might add that I am seeing $30. to $45. mugs). Even if you get $18. for a mug, what does .16 a pound mean, nothing in my book. The least expense of being a fulltime potter is the cost of our materials. The reason my porcelain costs more is because the materials cost more. The kaolin I use is four steps better than Grolleg. It is cleaner, smoother and more plastic. I do not make a penny by providing my porcelain to others, and I am sure Standard Ceramics makes no more percentage wise than on any other porcelain. The difference in cost is the difference in the cost of materials used. So for now I would like to suggest that you realize that porcelain is a very different animal than clay is and that to be successful I suggest you baby it a little and learn to work with it, NOT AGAINST IT. Learn itís characteristics and then design within the limitations of the material and processes you have chosen to use. Porcelain has always been the most challenging body to use and thatís why it has throughout history been very special. When it was first made in Europe it was more valuable than gold and was used as money. A great read is the book, "The Arcanum: The Extraordinary True Story", by Janet Gleason,  which documents the search and development of porcelain in Europe. Donít blame the porcelain, blame yourself, or better yet, formulate your own porcelain.

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