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BornonSunsetCeramics

Crawling matte glaze. Any hope of saving these?

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Hi All,

I'm having a heck of a time with these Duncan true 5 matte glazes. They keep crawling! I'm guessing the glaze pooled in the edges of these low serving bowls, and that's why they crawled...? Is there any hope is saving these? Also, that crack in the dish on the left is in the white underglaze... and happened during the bisque firing. There's no crack in the dish it's self, what could have caused this to happen?

 

Thanks so much everyone!

crawling plates.jpg

Edited by BornonSunsetCeramics

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I fired to cone 5 with a 15 minute hold on the medium program in my Skutt. All were bisque fired to cone 04 on slow. They each have white underglaze on the interior (Amaco VU) which was applied at the green stage before the bisque. I applied the duncan glaze to the middle of each and then  speedball clear just around the walls of the dishes prior to the glaze fire, since I was planning to use them as serving vessels. The clay is SBF cone 5/6 Aardvark. 

I placed witness cones and they read as follows: ^4: flat and broken, ^5: touching the kiln shelf, ^6: slight bend to about 1 o'lock position.

Thank you so much!

Edited by BornonSunsetCeramics

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Looks like the matte shrunk away towards the center of mass, I've had a similar problem when glazing only one surface.  The glaze was fine if I glazed the inside and outside, but would pull and sometimes warp a rim if I only put it on the inside.  Something about the inside shrinking at the same rate as the outside.  Probably not what happened here but it could be.  I don't think there's any saving those though, you could try refiring but don't count on it

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I was careful and brushed the clear on the sides, so if there is overlap its very minimal. 

I used a white underglaze because I was hoping for a very rich opaque, yet light color matte interiors to contrast the warm buff clay body. I was afraid not using an UG would muddy the color of the glazes. 

Any clue why the white UG would crack in a circle that way? I guess the glazes may not fit my clay body very well... which is a shame because it's a pretty buff color and throws so nicely! 

I'm so bummed. 

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I would be surprised if that crack does not go through the clay pot also , it Will eventually.

When applying under glaze to one surface of the pot it  pays to run a dampened brush over entire pot then apply the under glaze . Differences in moisture can cause cracking like this as  pot dries at different rates according to underglazed or not 

Would appears glaze may be too thick 

The crack on base. How thick is the base conpared to the walls?

Edited by Babs

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Matt glazes for liner glazes are not a good solution for functionality . Its not as easy to clean them as they are not as smooth.

It looks like your clear and the Matt to not work well where they touch.

I do not think there is any hope for them-just start over

I would use a liner glaze on the inner walls and bottom thats all one glaze and is not matt.

Matts can put tension on pots as well especially on the inside.

Edited by Mark C.

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@BornonSunsetCeramics, going forward I would really suggest making up some test tiles or even quick crude test pots to try your underglazes and glazes out on before trying them out on actual pots. Try all the combinations of underglazes beneath glazes, clear over the matte and vice versa. If they look okay after the glaze firing try rubbing a fork or spoon over the glazes and see if they cutlery mark and if crazing is something you want to avoid then test your test pots for that (with a stress test). 

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Just an add or two here as much of what has been covered is reasonable. The white underglaze appears fairly thick and having applied it in the green stage I am guessing that you needed to do so for 100% coverage of the dark colored clay body. A note about underglazes: that they contain clay in their makeup and as such bisque firing only sinters the clay within the underglaze. Further the underglaze will change the way the overglaze is absorbed by the body so in cases of thick underglaze application in the green stage,  overglaze compatibility often suffers in perceived thickness of application and when finally fired the clay in the underglaze will vitrify with the clay in the overglaze. (Yikes, got that all in one sentence)

When these two clays play well (clay in the under and over glaze) often you will see a matte glaze go glossy in areas where the underglaze contributed a significant amount of silica during vitrification.

A true matte glaze is one that has the correct ratio of silica to alumina. A true matte will always be matte even when overfired and runny. A test for true matte is simply to add more silica and it will go glossy. A final add to the true matte stuff, true mattes have tested as durable or more durable than glossy glazes and their coefficient of expansion is chemically what it is so there are high, medium, and low coefficients with mattes just like glossy glazes. Glazing one side of a ware however often creates tension on the glazed side and if it exceed the strength of the clay failure of the body is often sudden, We see this with mugs glazed on the inside only. One day, hot coffee, cold mug and whamo!

If you were into glazes then I would suggest formulating a matte that fits well with the clay and adding 10% zircopax for the white bottom and brush or spray  apply the base clear to the sides. In this way overlap should be fairly compatible and easy to manage while the overglaze reacts with the clay body during your application so your sense of application thickness is normal.

And if you use a durable matte glaze, the pot will be durable for food and dishwasher service.

One note: when zircopax is added you may need to increase your boron levels slightly as the zircopax is a refractory.

Well that's a little about glaze solutions, maybe too much. If interested drop me a note, there is a simple glaze published on Glazy that would likely fulfill your need. 

Just my suggestion towards a possible solution

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18 hours ago, Bill Kielb said:

A true matte glaze is one that has the correct ratio of silica to alumina. A true matte will always be matte even when overfired and runny. A test for true matte is simply to add more silica and it will go glossy. A final add to the true matte stuff, true mattes have tested as durable or more durable than glossy glazes and their coefficient of expansion is chemically what it is so there are high, medium, and low coefficients with mattes just like glossy glazes.

Don't forget about cooling rates. Many matte glazes will only achieve their matteness if the cooling is slow enough. I've seen many a matte that is glossy without a controlled cooling. There has to be time for the crystals to grow. Steven Hill has some great examples of this on his Instagram account. It doesn't necessarily have to be a really slow cycle, but slower than the natural cooling rate.

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Yes controlled cooling will provide more time for growth, but glazes that are true matte are matte regardless so nothing special about cooling usually for a true matte. Interestingly enough, fast cooling can freeze glossy surfaces but are rarely discussed. I know folks who intentionally gas fire for high gloss and take their time closing their kiln down  just to promote the highest gloss they can get.

 

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57 minutes ago, Bill Kielb said:

Yes controlled cooling will provide more time for growth, but glazes that are true matte are matte regardless so nothing special about cooling usually for a true matte. Interestingly enough, fast cooling can freeze glossy surfaces but are rarely discussed. I know folks who intentionally gas fire for high gloss and take their time closing their kiln down  just to promote the highest gloss they can get.

 

Considering that every kiln cools at a different rate, how can you define a glaze as a 'true matte' regardless of cooling rates? I've seen glazes that are glossy in my 4 cubic foot kiln but matte in my 21 cubic foot kiln. Which is correct? If I only had my large kiln you would say it's a true matte because I didn't add a cooling cycle. In my small kiln you'd say it wasn't. As with most things in ceramics, you can't define it that precisely. You can also alter the degree of matteness with the cooling rate. I've also got glazes that are glossier with a slow cooling. The main point here is that you achieve matteness with a glaze that is properly melted, not by under-firing a glossy glaze.

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True mattes are defined as such because chemically they have an appropriate silica to alumina ratio and crystalize accordingly. Over fired True matte glazes are runny overfired matte glazes as opposed to obtaining a matte by under firing a glossy glaze where a tight crystal structure has not been established.  Its matte, but not fully melted.

A good test for a true matte is to simply add silica and it will become glossy. By chemical definition they are true mattes. They occur in Stull at about 5:1 or less. 

This may seem as a silly distinction but for folks who understand it as defined, it helps them understand that the matte they like so much but is severely under fired likely is  for non functional stuff. Not everything crystalizes well even in slow cool, so assuming cooling rates are the decider is a bit misleading as well. Better to just understand the actual chemistry and then freely experiment from there. The poster purchased a commercial true matte glaze so I expect that the manufacture provided one that does not need a special cooling cycle to perform. It is what he paid for. The definition is not mine, it is defined chemically as such, so I believe you are really saying the same thing. For clarity True mattes have a silica to alumina ratio of 5:1 or less and fire as such else something is likely wrong. 

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I never said that cooling rates are the only decider, or that every glaze responds well to it. I didn't say that the si:al ratio isn't important. I didn't say anything contradictory about under-fired glossy glazes.I'm simply said that cooling rates can affect matteness and every kiln cools at a different rate so it should be taken into consideration.

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Micro-crystals formed during the cooling of a high calcium or magnesium matte glaze will cause mattness, with a slow cool.  Silica:alumina ratios can and often are higher than 5:1. I've found high alumina mattes often cutlery mark and prefer mattes from higher levels of the former mentioned fluxes. Silica:alumina ratio is an indicator of the degree of gloss but it's not the only factor.

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3 minutes ago, Min said:

Micro-crystals formed during the cooling of a high calcium or magnesium matte glaze will cause mattness, with a slow cool.  Silica:alumina ratios can and often are higher than 5:1. I've found high alumina mattes often cutlery mark and prefer mattes from higher levels of the former mentioned fluxes. Silica:alumina ratio is an indicator of the degree of gloss but it's not the only factor.

I have a great magnesium matte glaze that has a si:al ratio of 6.1:1, is durable, slightly fluid, and quite matte. When crystal growth is contributing to matteness, the si:al ratio is less of an issue in determining matteness, and the increased silica can make for better glass formation.

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"Matte-ness" is an optical characteristic of a surface, not necessarily a chemical one.  That said, the elemental composition and thermal history strongly effect the optical properties of the glassy melt that we potters call 'glaze'.  (The article on to Ceramics Art Network Daily "What Makes a Matte Glaze Matte?" has good images illustrating the optical difference between glossy and matte surfaces).  


This discussion has been informative - at least to me - because:
1. The topic is currently relevant to my current assignment to develop a reliable cone 10R matte glaze using a constrained array of potential ingredients; 
2.  The discussion highlights the confusing usage of words in the realm of ceramic art/pottery to explain specific aspects of physical chemistry and mechanical physics where words have specific meanings in the disciplines. Too often a word commonly tossed around within one disciple will mean something significantly different within another discipline or the word is used to convey multiple meanings.  I have encountered the situation numerous times since I entered the ceramic art discipline. example:  a discussion about "thickness" of glazes:  is the discussion about the solids/liquid ratio of the slurry, the fluidity of the slurry, the amount of dried unfired material per square meter of pot surface, or the thickness of the fired glassy surfaced formed on the pot?  the solution of a problem will be different depending on which meaning you assume for "thickness").        

Stull's empirical analysis of the silica-alumina molecular ratio in common alkali/lime glasses is an important data set (Trans. ACerS vol. 14, 1912), so is the discussion of matte/gloss glaze in Parmelee and Harman (Ceramics Glazes, 3ed.,1973).  After reading Stull and Parmelee, and examining a few geochemistry phase diagrams, I am convinced that the mattness of the Stull glazes is most likely due to the precipitation of calcium aluminosilicate (aka calcium feldspar) from the melt created buy the ratios of the ingredients in the glaze and is not likely to be significantly different on fast or slow cooling  as used today.  
  
There are post firing treatments that can convert a mature glossy glaze surface into a mature matte glaze surface - sand blasting and chemical etching are two processes that quickly come to mind.  Also a matte surface can be polished into a glossy surface with rouge and diamond dust. 

I converted a glossy clear glaze to a white matte glaze by adding 20+ %w Zircopax to the batch (which was definitely an underfired glaze since Zircopax at that level is not soluble in the melt at cone 10).  The white matte glaze is as stable as the clear glaze, no more, no less. 
  
An observation from both my own studies and my understanding of the discussion so far:   The mattness depends on the surface having a preponderance of crystalline (Parmelee's term) species as part of the surface. This implies that the surface does not consist of the same species distribution as the bulk melt from which the crystals were precipitated. This leads to the conclusion that mattness may produce significant changes in the leaching of elements from the final glaze surface.  
Some glazes recipes can be forced into forming crystals on the surface by slowing the cooling rate, and some are likely to not form crystals within a reasonable time frame.  Stull's chart is a reasonable starting point for alkaline/lime melts (without colorants) for a cone 11 firing.    

 

Now, back to the original question of the "crawling" aspect of original post.

Is it the consensus that the crawling observed is an application issue (and/or the solids/water ratio in the slurry) and therefore is not specifically related of the glaze used on the pot being a matte glaze? 


Respectively, 
LT

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I think and therefore sort of ……. but his question contained the use and crack of the underglaze as well so I offered a potential reason for his reaction and a potential reason that his applied glaze did not appear to him to be excessive.   Since his surface desire was matte, white on the bottom and clear on the sides, I gave him a potential known glaze solution that was known true matte as a substitute for his purchased true matte clear and suggested adding zircopax for the bottom glaze and the unaltered base glaze for the sides for his intended look and a simpler application. In my view a potentially reasonable solution with a likely durable outcome.

I am sticking with Stull for true matte and the outliers can be as such and very possible. They just tend to add no clarity to the subject and serve to confuse many. In general Stull is correct from a trend standpoint and very relevant in my view with respect to glaze chemistry.

Mechanical means of conversion are what they are and remove material so generally not a practice to enhance durability. They are surface alterations, not necessarily glaze chemistry.  Not sure they add clarity to the definition of a matte glaze but they describe a matte look as measured by a gloss meter.  The same can obviously said for polishing as the surface will become higher gloss.

As far as the stability of the underfired 20% zircopax glaze, maybe ……  but being underfired why would it be assumed stable. Has this been tested in some way. Certainly it indicates to me it is underfired and therefore likely not as stable as the original glaze. It is not something I would suggest to someone for durable finish.

True mattes that have been tested have performed as good or better with respect to durability and leaching so the implication and conclusion cited are not necessarily correlating. Maybe a bunch more study will find a common indicator.

And finally -  yes crawling was likely due to a thick application, (relative to the glaze used)  so I am still sticking with my first suggestion for him to try and experiment with as a practical one and reasonably likely to create a good outcome. I am guessing at this point he was hoping to just glaze his pot and get the look he wanted. LOL

Thanks - fun stuff!

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