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Glazes That Break

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I've seen and used some commercial glazes that break brown over edges. Is this because of the thickness of the glaze or something that has been added to the recipe, or both? 

What can I do to promote this?

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Iron encourages some glazes to break as does tin in a more limited fashion in my experience. 

Glaze thickness/viscosity will indeed play an important role as well.

Edited by C.Banks
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Thanks.

I had a feeling iron might be the one of the things that helps promote this.

I've also noticed this doesn't always happen with some of my glazes. What else besides titanium dioxide inhibits breaking?

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I presume the substitution of titanium for rutile only removed the iron from the mix as rutile is, as you might already know, titanium contaminated with iron.

Titanium is a interesting thing and very much subject to cooling rates. A glaze with rutile/titanium/iron cooled quickly with no holds will create a mostly shiny tenmoku breaking orange/light brown from dark brown. Put that same glaze in a larger kiln and forced to cool slowly will promote the development of surface crystals and a matte orange with no dark breaks unless thick and allowed to pool.

hope this helps a bit

Edited by C.Banks
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Surface tension of the glazes, viscosity and refractory oxides making up the glaze are going to determine how fluid the glaze is. Also, length of time/heat at top of the firing. Refractory oxides include magnesium, alumina, zirconium, calcium, tin, strontium, zinc and probably some others I'm forgetting. Lithium, sodium, potassium and boron will help make the glaze more fluid if used in high enough amounts. Simple way to make a glaze move more is to take out some of the alumina (kaolin would be a good candidate). If your recipe is high in any of the refractory oxides it's going to much more difficult to get them to break. Cobalt and copper are active fluxes and will increase glaze fluidity if used in high enough amounts.

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16 hours ago, C.Banks said:

I presume the substitution of titanium for rutile only removed the iron from the mix as rutile is, as you might already know, titanium contaminated with iron

If I'm correct in thinking ilmenite  is iron and titanium and has more titanium than rutile

 

15 hours ago, Min said:

If your recipe is high in any of the refractory oxides it's going to much more difficult to get them to break

I'm guessing silica is refractory too?

So the MC6G base may not be a good starting point

Edited by RonSa
Fixed Typo in MC6G

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29 minutes ago, RonSa said:

I'm guessing silica is refractory too?

If you take away some of the silica you would likely lower the melting point of glaze. Adding silica does the opposite plus it will increase the viscosity (less fluid). Problem with changing the silica is if you alter it too much you are going to change the look of the glaze insofar as changing the silica : alumina ratio (drastically), would change a semi or matte glaze to a gloss. Plus, the silica needs should be within "limits" for durable glazes. I would suggest trying to find a base with the less viscous fluxes, alumina not too high rather than fiddling with the silica. The silica you loose by removing some epk can, if necessary, simply be added by increasing the silica in the recipe to make up what is lost by epk reduction. Take out a little alumina and replace the lost silica (from removing some epk) by adding silica. make sense? Alumina still needs to be at a reasonable level to ensure a durable glaze but there is wiggle room. Have a look on the digitalfire website at some of the flow tests or even materials melt buttons, or better yet run some with your materials/glazes to see what they do. 

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Agree with Min about those variables, although I would say for breaking the surface tension is probably the most important of things to be looking at.

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Ron:

all kaolins average  37% alumina by weight. So by that number you get a grasp of how much alumina is lost by decreasing  kaolin.

to put the effect of alumina in terms of food: alumina does to glaze what corn starch does  to gravy: the more there is, the thicker it gets. Generalization I realize, but gives you the picture. There are minimums and maximums of alumina- formula limits. Being a clay guy I also have to add: the alumina content of the clay can add or subtract from glaze run. Some glazes will run more on some clay bodies more than others because of alumina content of the clay.

in cone 6 there is always the "cheats"...  Frit and lithium additions

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4 hours ago, RonSa said:

If I'm correct in thinking ilmenite  is iron and titanium and has more titanium than rutile

 

Actually, the opposite. Illmenite is a raw source that is refined into rutile, which in turn is further refined into titanium. The illmenite is roughly 50:50 iron and titanium oxides; rutile is roughly 10:90 iron and titanium.

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I think the idea with breaking is to get the glaze to run off of high points and corners and into crevices and depressions - without running off the pot.   Making a glaze runnier by reducing alumina may well promote breaking, but if it is too runny it just runs off the pot so you can only take this so far.  That said,  a little bit may work and you may get a satisfactory result.

Surface tension and  viscosity are related but different.  Low surface tension glazes have an ability to "wet" the clay surface to achieve this effect.   Many oxides in our materials do both, that is lower viscosity and reduce surface tension, but have different effects in these two regards.   Magnesium in particular lowers viscosity as a flux, but has relatively high surface tension (compared to silica, which is kind of the benchmark).  Same goes for zinc (high surface tension).  See Hamer and Hamer for more on this.

If you have zinc, talc or other magnesium bearing materials in your glaze think about lowering them to promote breaking.

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8 hours ago, RonSa said:

Sigh, I feel like I'm starting from the beginning all over again :(

Thanks for the input

Why don't you just take an existing glaze you know breaks and change the colorants around? Unless it's a very refractory colorant it should work fine. If you need some glaze recipes I can post some.

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28 minutes ago, Joseph F said:

Why don't you just take an existing glaze you know breaks and change the colorants around? Unless it's a very refractory colorant it should work fine.

That's what I was hoping to do but the only base glaze that I've found that doesn't craze on Standard 630 is MC6G and it seems to be very stiff

 

32 minutes ago, Joseph F said:

 If you need some glaze recipes I can post some.

That would be nice, thank you.

 

I was just checking this base out on https://digitalfire.com/4sight/recipes/cone_6_transparent_base_4.html   since it has no talc and MC6G has 10% (Of course I'll need to test it)

Transparent Cone 6 G1214W
EPK Kaolin    25
Silica    25
Wollastonite    10
Ferro Frit 3134    25
F-4 FELDSPAR    15  (swap out for G200 HP since that is what I have on hand)

 

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"Cream breaking red is a classic". Here is a 20 year old post from Clay art. Tom Buck, the originator was an old friend. a former ceramic technician from Canada.

John Post on wed 21 may 97

Collen Raynor recently posted Tom Buck's Cream Breaking Red glaze. She 
tested it and found that it is "a predominately rust glaze with a cream 
colored speckel". This is the recipe she posted... 

Glaze name: Tom Buck's Cream Breaking Red 
Cone: 6-7 
Firing: Oxidation 

Recipe: 
41.00 G200 feldspar or equal 
22.00 Gerstley borate 
9.00 Whiting 
3.00 Strontium carbonate 
25.00 Flint 
100 Total 

Also add: 
13.00 Tin oxide 
6.00 Red iron oxide 


This glaze sounded interesting to me and I decided to test it on my white 
stoneware body. I found that it is rust colored where thin or where it 
breaks over an edge. It is cream colored where it is applied more thickly. 
Even though it is not red as the name implies, I think it is quite a nice 
looking cream and rust gloss glaze. 

I found that the glaze tended to settle at the bottom of bucket. In order 
to improve the working characteristics of the slurry I recalculated the 
glaze formula to include some clay. In this new version which I call CBR-3 
there is 8.4% EPK. I also replaced the gerstley borate with Frit 3134. 
Wollastonite replaces the whiting and some flint. The reasoning behind 
some of these ingredient substitutions is based on information I found in 
the article "Getting a Cone 6 Oxidation Glaze Right For You". (This 
article is located on the IMC webite. Thanks Tony!). 

After I fired CBR-3 on a white stoneware body at cone 6 & 7 I found that it 
has the same visual characteristics as Tom Buck's Cream Breaking Red. It 
is speckly rust where thin and more cream colored where thick. A very warm 
attractive glaze. The slurry of CBR-3 seems improved because it stays in 
suspension better. 

I tried using less tin oxide in order to cut the cost of the glaze, but the 
glaze was not opaque enough with 3% or 6% tin. At 9% the glaze was opaque, 
but didn't have the creamy brightness of the glaze with 13% tin oxide. I 
also tried 15% superpax instead of the tin, but that glaze test turned the 
rust color a dull brown and the left the creamy areas looking dingy. I'm 
going to stick to using the 13% tin because it gave the best results. 

The nicest looking results occured when I sprayed CBR-3 on the top 1/3 of 
some pots and Bob Kavanaugh's Berry Rust glaze on the bottom 2/3. I then 
lightly sprayed some Berryrust over the CBR-3 at the top. The pots came 
out of the kiln with a rich cream and speckled rust color at the the top 
and a wonderful deep rusty red on the bottom. The combination is 
wonderful. If you don't already have it, Bob's Berryrust glaze is listed 
below. It is a wonderful rich oxidation red. 


Glaze name: CBR-3 (John Post's Revision of Tom Buck's Cream Breaking Red) 
Cone: 6-7 
Firing: Oxidation 

Recipe: 
30.6 Frit 3134 
26.6 Custer feldspar 
10.6 Wollastonite 
2.3 Talc 
3.3 Strontium carbonate 
8.4 EPK 
18.2 Flint 
100 Total 

Also add: 
6.0 Red iron oxide 
13.0 Tin oxide 

Comments: 
A speckly rust glaze where thin, turning into a creamy beige glaze if it is 
applied more thickly. Great in combination with berryrust. Works at cone 
6 & 7. The one time I tried it in reduction at cone 7 it turned an 
unattractive speckled grey. 

Glaze name: Berryrust 
Cone: 7-10 
Firing: Oxidation 

Recipe: 
27.28 Flint 
18.18 EPK 
18.18 Nepheline syenite 
9.09 Dolomite 
9.09 Gerstley borate 
9.09 Talc 
9.09 Bone ash 
100 Total 

Also add: 
9.09 Red iron oxide 

Comments: 
Works best if applied thin. It turns greenish where it is thick. Seems to 
actually yield redder fired results as the glaze ages in the bucket. 

Thanks Colleen, Tony and Bob. 


John Post 
johnpost@c3net.net
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Here are two of my favorite glazes to use:

image.png.ca7c06c02f5a2c07aa315534e11ccf42.png

Folk Fireborn White is a modified version of a cream glaze that was in john britts book. It was originally called Folk Art White. I changed it pretty substantially, but left the folk part to give credit to the original glaze creator. This glaze is a glossy white glaze that if slow cooled turns into almost a glossy satin. If it is put on thin, it will show reddish colors where it is thin. Note you don't have to use 13 tin, it makes the glaze absurdly expensive, you can try different amounts to see what works best for you. I like it at 13 though.

DB-11090.jpg

If you want it more cream increase the RIO and you will get something that looks like this:

04c5786624e77a8e0b5fd516f05a47d1.jpg

The area on the right corner is a titanium glazed sprayed over it to add variation to the base. Obviously it was a bit thick = )

----------------------------------

The second glaze, is called Charles Titanium, I am not sure what the glaze is actually for. I like glazes with titanium in them so I tried it and it produced a clearish runny looking glaze. However when you slow cool it, it changes drastically. I ran a grid of it to find better versions which I use above.

gallery_63346_1349_120633.jpg

This is the glaze with a gunmetal stain added to it. Notice it breaks nicely on the edges. Also it reacts well to seashell flashing. Same glaze just fired on shells.

gallery_63346_1349_338278.jpg

Another version of the glaze. Again this has to have a slow cool or it will be a clearish looking glaze.

da44c182a4d6c98e16b27feca00b3885.jpg

Tile on the far right is the same glaze above. Except this time it has 2% yellow orche. So as you can see it is very adaptable to colorants.

I don't know what your firing schedule is, but give both of them a try and see what you get. Might be terrible, might be completely different, might be exactly the same. I usually dont post recipes because I find they don't travel well, but who knows you could find something you like.

EDIT: found a picture of it on stoneware that shows the clearish properties:

IMG_20171119_181824.jpg.78ca08d1ef0b2a7d0aea1f528342513e.jpg

EDIT FOR CLARITY(thanks @Marcia Selsor for reminding me.)

The above glazes are all cone 6, the clay bodies are red rock, and black ice. They were all single fired. Materials sourced from US Pigment.

Single Firing Schedule as follows:

1. 200F/h - 220F - 30min hold
2. 300F/h - 500F - 15min hold
3. 400F/h - 1550F - 30min hold
4. 400F/h - 2032F - 0
5. 108F/h - 2232F(other cone 6) - 5 min hold
6. 175F/h - 1800F - 30min hold
7. 50F/h - 1600F - 45min hold
8. 50F/h- 1500F - 30min hold

This is basically my current schedule for singlefiring. I found the holds at 500 and 1550 improved the surfaces on my single fired red rock. It is about a 10 hour slow cool to 1500F. 

Edited by Joseph F
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1 minute ago, RonSa said:

My kiln has a sitter and I don't know a way to slow cool so for the most part I let nature take its own course.

Well it is worth a shot firing them both, who knows. Glazes are soo different between kilns usually unless your firing same clay, same schedule, and mostly same ingredient sources. 

It doesn't cost much to mix up 100g of each and dip a few tiles. Please post your results, I am very interested in seeing how they traveled.

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GW1214 is a good glaze, but I found it to be a little stiff and slightly milky when I tested in my kilns. I tweaked it a bit to clear it up, and made some materials substitutions, and here is what we now use in my studio as the class Clear Glaze:

Frit 3134    26.08

Neph Sye   11.07

Whiting   7.98

EPK   25.11

Flint   29.75

It seems to fit well on all of the clay bodies we use.

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