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Glaze Behaviour Why?

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we have some new glazes at school and they are behaving differently when on a vertical surface compared to a horizontal surface. 


bowls had 'oil spot' looking spots on the floor and the sides had 'hare fur' looking results. this is ^5 (really ^51/2 as its 2100) electric and i use the terms so you kinda have an idea of what i am talking about since i dont have pictures. 


what is consistent is it does it on the floor of high wall bowls. i have seen quite a few bowls other students have made. and they are all consistent. no spots on the sides or 'fur' on the floor. 


i am just trying to understand why. its a mix of two glazes.


when i saw that i tried it on a vase form to see if my conjecture was right. and sure enough my vertical bottle showed no spots. i have not tried it on a bowl or a cup yet. 


what are the factors that could be causing this to happen?


heat inside is warmer than the heat outside?


maybe more glaze pooling on the floor?


i have yet to test to see if there is a difference between a shallow bowl and a cup (cylinder shape). does heat behave the same way inside a bowl as it does in a cup in the kiln or do the curved walls and straight walls make a difference

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It's just gravity. On vertical walls the glazes flow downward. On horizontal surfaces they pool. These two different actions result in different appearances. They mix and move differently, so they look different. The top half of a pot will often look different than the bottom half, as the glaze runs down from the top and gets thinner there, but gets thicker at the bottom where the runny glaze collects.

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Gravity is the correct answer; but how to deal with it?


Glazes leech alumina from the clay to help hinder glaze flow; coupled with alumina in the glaze itself. Between the two sources, generally there is enough alumina present to prevent glaze flow unless a piece is intentionally loaded up to cause running. Then it becomes a matter of heat and flux levels: excesses of both will cause running, which is even more pronounced when alumina sources are lower. So you are dealing with a new glaze with higher levels of flux, so it can be fired at lower temperatures. Then further complicated by a likely lack of alumina both in the glaze and in the clay: and probably fired higher than it is formulated for. This problem may even more pronounced on the clay body of choice. Stoneware bodies can have as much as 25% more alumina than porcelain bodies: so do not be surprised  to see more glaze run on porcelain rather than stoneware.


The simple fix would be firing a 1/2 of cone lower than presently fired; but that could also cause problems with the clay maturing fully. So I would recommend adding 1/2 to 1 percent alumina hydrate into the glaze before application. Test it on some damaged or practice cylinders or pieces to check glaze flow with these additions. Most of the time alumina additions are not required; but in some rare cases they are.



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Darn! I am so disappointed by this answer!!!! gravity?!! really!! as simple as that! here i thought i was going to expand the breadth of my education in glazes and learn some new theories. And its just about gravity. So 


Nerd I am grateful for your explanation.


but i am highly elated by all your answers too. Really?!! a falling apple can do all that? as simple as that. my mind is blown. Now I look at my school glazes with new eyes because i'd notice subtle differences between bowls and cups. It opens up a world of potentials. and a whole world of test bowls to make. 


So here are my questions.


Mark C I know you have worked on your glazes for years and it is your signature. but what do you base your choice of glazes on? by the chemistry/ingredients or test tiles? I have access to 15 glazes at school. is it possible to look at the ingredients and make an educated guess that they might work and choose just a few to test, test, test?  since i dont have access to a glaze class what can i read to up my knowledge. 


the test tiles at school only do two combos. i have worked with a few combos that make somewhat interesting surfaces. 


so Nerd what you are saying is if I add alumina to it, the glaze combo should make 'fur' on the floor too because the addition of Alumina hydrate will make the glaze a little more runny. so should i reduce the alumina if i want spots on the wall?


If it helps here the two glazes ^5. first dip Smokey Blue and then Rhodes White. the result is blue and white


Smokey Blue

EPK                 28

Neph. Sy.        18

Frit 3124         23

       3134         13.5

Flint                 13.5

Whiting              4.5

Zinc Oxide         4.5


Manganese Dioxide 2.5%

Cobalt Carbonate 0.75%

Epsom Salt 1%


Rhodes White

EPK                  2.5

Gerst. Bor       13.7

Minspar 200    45.5

Dolomite            6

Flint                 20

Whiting               8.3

Zinc Oxide          4


Bentonite 3%

Zircopax 12%

Epsom Salt 1%


Now here are two more glazes ^5 first dip Tan and next dip Black Shiny. with this combo there are always spots no matter what the surface - vertical or horizontal. Shades of blue. 



Gerst. Bor     10

EPK                9

Magnes. Carb 1

Neph. Sy.       26

Whiting           16

Spodumene      1

Silica                37


Rutile   5%

Bentonite 1%


Black Shiny

EPK               5

Custer Feld.  79

Gerst. Bor      11

Whiting            5


Copper Carb 4%

Mang. Diox    4%

Cobalt Oxide  4%

Bentonite        1%


sadly enough i am not a big fan of blue. but these are the only two combos that yield spots.

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Back in college I tested all the class glazes and the combos on stoneware and porcelain clay bodies-that is the basis of of my start with glazes-since then I have added so many glazes and dropped so many glazes-I maybe have only 4-5 original glazes from that time (1971-1976) that I use today in my current 15 that I use each week.I try new glazes every year.

All glaze work for me involves testing at the start with test tiles.

What works in the market place determines weather I keep using them. That is if they sell well. Many glazes I like personally like do not sell well. I'm in business to sell pottery not make what I like -this is a key point.It works the same with forms and glazes -what sells well I make weather I like it or not. Its about selling and making a living.

For the hobbyists its about playing  with clay and making and glazing what one likes. I know nothing about this anymore. Its my business and its work-yes its fun but its work. People pay me to work in the studio but that means I have to work in the studio not play.

Testing glazing and how they react to one another in the firing  atmosphere you choose to work in is key to learning about glazes. It takes time and discipline . Make the tiles from forms like cutting a small pie plate (cut up like pizza )so each sample has a horizontal and vertical surface-this you want to see how it looks when gravity is pulling on it (the wall) and a flat surface. These are the best tiles you can extrude them as well 

. Put a hole into  the  to hang them on the wall.

test all those school recipes-keep a notebook .

That was how I started.

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Pretta: "since i dont have access to a glaze class what can i read to up my knowledge." 


somewhere to start https://digitalfire.com/4sight/education/index.html

(the level of colourants in the shiny black glaze does not look healthy for functional pots, sorry if I sound like a nag but you never know who will take posted recipes and use them without knowledge of glaze materials / testing)

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Gravity i think was the answer to the rabbit hair running from the wall to oil spot in the base.

Here is the most simple easy to understand the Fluxes RO, Intermediates R2O3, Glass Formers RO2.

Fluxes are the flux the melts the recipe, R2O3 includes the alumina,clays like Kaolin as stiffeners, RO2

are the glass makers.

The ratio between the RO and the RO2 determine the melting point. The ratio between the R2O3 and the Ro2 determine the the surface texture.




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You first posted your question, I made the assumption that you were working with premix glazes. Glad to hear your school is teaching you to custom mix glazes. That said, adding alumina to a premix that is overly runny will work just fine. However, if custom mixing; following formula limits and unity formulation will prevent that problem right out of the gate. Marcia gave probably one of the easier to understand explanations of unity formulations. I would also recommend working your formulas om Insight live or buying a glaze calculating program like GlazeMaster once you have the general principles down.



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Sadly Nerd the school is not teaching us anything about glazes except application. they are supplying the glazes. 


i am at a community college. the number of proffs hired is low and he is overworked with the number of classes he is teaching. so i beg and ask the lab tech who has given the recipes to those interested and answered some of my questions. the ones who are interested in glazes have moved on to a 4 year college. those at school are not really interested. i cant ask the proff as  he is so overworked. but in a semester or so he will let me volunteer so then i can experiment with the glazes. i know all the advanced students and volunteers and no one is actually playing with recipes. 


thanks Min - my glaze knowledge comes from reading digital fire, Hamer and hamer and old copies of CM (amazing what a wealth of knowledge and tips there are in the magazine) and books here and there. min i am going to look into your nagging. black shiny is an old glaze recipe here. i know all the glazes are lab tested as well as tested in school so i am surprised. 


Mark C - i just made some test tiles out of my favourite red claybody but have yet to actually glaze them. i used john britts method of a partial bottom pie plate and even used a stamp. i am going to listen to your wise words and use more blue in our student holiday art show and see how things sell. you've also got me thinking about making pots for whom. btw i love sketchbooks and have detailed notebooks (both for throwing and glaze) from the very first semester. 


neil i hope to be mixing glazes if not next semester then by fall for sure. 


marcia i've been experimenting with top 1/4 thick glazing to see what it does to cups. 

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you said and asked:

"I have access to 15 glazes at school. is it possible to look at the ingredients and make an educated guess that they might work and choose just a few to test, test, test?  since i don't have access to a glaze class what can i read to up my knowledge."


Glaze is just a potter's name for "molten rocks" that have been cooled fast enough to become a glassy coating on a clay based ceramic object. 

I have lots of texts related to glazes, but I recommend two sources as a starting point for students wanting to learn about glazes: 

  1. Susan Peterson's textbook, "The Craft and Art of Clay", any edition.  She presents a usable approach to glaze making and testing.  Her glaze testing grid of A over B testing is a very powerful tool, especially if you take the time to carefully examine the glaze that is actually on the test tile and forget the propaganda on the glaze bucket describing the glaze.  Also keep your testing simple.   [There are other ceramic texts, such as "Clay: A Studio Handbook" by Vince Pitelka, that have good glaze sections.   The Pitelka book is available from CAD].
  2. John Hesselberth's "Mastering Cone 6 Glazes: Improving Durability, Fit and Aesthetics" http://www.masteringglazes.com/, http://www.ronroy.net/ .  The book is focused on cone 6 glazes, but they discuss the principles that are behind the art of glaze making and application; these principles are independent of any target cone #. 


I suggest that you follow the hand methods discussed in Peterson's textbook using a spreadsheet (Excel perhaps) to do the calculations of converting from the "recipe" to the "oxide" to the "UMF" point of view of glaze compositions. 

When you reach a level of understanding of the principles and of the differences and strengths of each of the viewpoints on a glaze, then find a computer program such as Insight (http://digitalfire.com/insight/index.php   ), GlazeMaster (http://glazemaster.com/   ), HyperGlaze (http://hyperglaze.com/  ), or Matrix 2000 (http://www.matrix2000.co.nz/  ).  By first learning the principles thoroughly you will be better prepared to use the software with confidence.  Each of these software packages accomplish the same purpose of presenting a glaze from all the different view points, and their own operational start-up curves independent of glazes. 

Tony Hanson's "Digital-Fire" website is also a good source for a wide variety of clay body, glaze, and firing general information. 

If you want to go deeper into the chemical principles behind glazes I recommend you read the literature of Glass Technology and Igneous Petrology/Geochemistry.  You should find the books I suggest in a college library. :

  • Shelby, J. E., "Introduction to Glass Science and Technology, 2 ed., RS-C. 2005 .
  • Doremus, Robert H., "Glass Science," 2 ed., Wiley. 1994.
  • Mysen, B. O., P. Richet, "Silicate Glasses and Melts Properties and Structures", Developments in Geochemistry 10, Elsevier, 2005.

If you are more interested in glaze recipes than the science behind the general subject, then there are quite a few books on the CAD bookstore site that are excellent starting points and will at least introduce some of the generalizations that have been proposed for characterizing glazes. 

The  "Ceramics Monthly Guide to Materials and Glazes" is a good starting point, but it's coverage is not as strong  as either the Peterson or Pitelka textbooks.  Mimi Obstler's book "Out of the earth Into the Fire" approaches glaze from the point of view of how natural rocks and clay raw materials are converted in to glazes, but I found the book more oriented to glaze recipes than to glaze chemistry.  It may be out of print by now.  



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I ran your Shiny Black recipe, just to illustrate formula limits and issues:  ( formula limits by Hesselberth & Roy)


Knao (sodium & potassium)  0.585   (formula limits  0.30 max) ...almost double the maximum limit.


Ca (calcium)   0.413   (formula limits 0.60 max)  doing okay here.


Al2O3 (alumina) 0.723  (formula limits 0.50 max)  50% more than required, but still runs because Knao is double the limits.


SiO2 (silica)  4.490   (formula limits 4.00 max)  now we know why it is called "shiny" black.  excess silica also translates to "runny" and sheen.


another big problem COE  8.10 est.    Most stoneware and porcelains run between 5.25 up to 6.25 (average)  which means this glaze is going to have a "crackle glaze" effect because of COE differentials.  One thing that might help limit that is high alumina content.



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much gratitude LT. that kind of info is EXACTLY what i was looking for. i am excited. i now have much reading to do while on break. i have already put 3 of the books on hold. so excited!!!


thanks nerd for checking. first i am going to go to school and see if i wrote anything wrong down. i really appreciate your efforts. once i understand the principals and ready to experiment with glazes i will join insight. 

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You can get Mastering Cone Six Glazes, updated in 2013 directly from John's website for $25. I have the original book and it is highly recommended for all kiln temps, not just cone six. I fire at cone 10 and this book has been an invaluable help in my understanding of glazes and what you need to do in order to have food safe and durable glazes. http://www.thebookpatch.com/BookStore/mastering-cone-6-glazes/d2bea83c-2c34-4ed0-8a00-a6f12113515d

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