What causes glaze color variation?

[MissyLee5]

Hello, 

I am new at glaze making and trying to learn all I can on the chemistry behind it and glaze predictability. For now I am making glaze from formulas found online/in books (almost ready for my first test tile firing!), but once I have enough knowledge & experience I'd like to make my own formulas and tweak existing ones to create new effects. My favorite glazes are ones that break a different color over texture, or have color variations within them. What is it that causes each of these? Is it an ingredient, or the glaze chemistry?

I know rutile and titanium dioxide can give variegation like rivulets and pooling, but what about when different colors are within the glaze? I see this much more in commercial glazes, for example Mayco's Seaweed, Amaco's Aurora Green or Blue Hydrangea (pictured below respectively). Some glazes that seem more speckled or have focal areas of different color I assume are using sintered glaze sprinkles (is that the same as crystals?) for this, but others seem to more bloom colored areas (AG and BH), or where the color breaks or runs its a different color (Seaweed and the coveted Honey Flux). I've looked at the MSDS on these glazes for more ideas or possible reverse engineering (a habit from cosmetic formulation), but not all of the ingredients are listed. The MSDS for Seaweed shows 5-10% copper carbonate, 1-5% rutile and 1-5% zinc for colorants. The first 5 ingredients are water, feldspar, frit, neph sy, and gillespie borate. It also shows 5-10% silica and 1-5% hydrated aluminum silicate. I found it interesting all the potter's choice glazes use gillespie borate since I believe it's no longer available.

So anyway, I'm curious what causes these effects and if it's possible to tweak existing formulas to bring it out. I've searched but can't seem to find an answer. The only thing I can think of with my newbie brain, is for the blooming type glazes to add varying sizes of sprinkles sintered from a runny glaze of a different color/colors, which would allow the secondary colors to spread out? I'd really like to make more of these types of glazes, but have only found a few on glazy that behave this way. Can anyone enlighten me? Am I looking for something within the chemistry, like higher boron, or is it an ingredient that achieves this effect, or both? I have an electric Kiln and will be firing in oxidation, so the lovely effects of reduction aren't possible for me. Although if I could choose again, I might have opted for reduction. 

[Hulk]

Hi Missy Lee,

I'm not familiar with commercial glazes at all, would like to offer a few thoughts though.

The clay/glaze combination can be a factor; I'm seeing the variegated green I use break much more over one white stoneware than the other white stoneware I use, for example. Also the red glaze I use breaks over the two light red clays I've used, much more so than other clays...

How much glaze "moves" can accentuate the breaking look? Some glazes droop and run more than others.

The translucent to transparent glazes might also highlight the breaking look, where opaque glazes mute it.

Mainly though, wanted to suggest looking for recipes from reputable sources, and especially recipes that come with useful descriptions, comments, test results, application notes, etc., and test your glaze fit, also durability.

Mr. Hansen's article might be a good start: Breaking Glaze (digitalfire.com)
see also Rutile Glaze (digitalfire.com)

 

[Hulk]

Reputable sources, hmm, many of the contributors here (this forum), certainly; aforementioned Digitalfire.com, where one may find several detailed articles, some of which describe reformulating/adjusting glazes; Hesselberth's FrogPond pottery website, the tested glazes page (looks like his website has been preserved here Home Page | Frog Pond Pottery (archive.org)); the book Hesselberth wrote with Ron Roy, Mastering Cone 6 Glazes; Bill van Gilder's book, Wheel Thrown Pottery (I use one or two of the recipes from that book*); see also Lakeside Pottery website, where several glaze recipes come with notes (notes! golden).

...and many more books, periodicals, etc. Glazes that apply well, fire predictably over a reasonable range of heat work, fit the clay, and hold up well, oh, heh, and look good - go!

*it has been pointed out here (this forum) that Bill works in high fire in his own studio. My favorite glaze is from his book! It's not everyone's favorite though - as one of the regular contributors to this forum had pointed out - which takes me back to first point, above, the clay can make a huge difference in how the glaze looks!

[Pyewackette]

@Hulk So it was YOU who posted those links about breaking glaze!  I've been working my way through that information and had already forgotten where it came from LOL!

All I can say is, its going to be fun when I finally get my new computer set up and I have to transfer all these links I've accumulated on my son's laptop to it. 

Can you share your "favorite glaze"?

[GEP]

My take on glazes that break nicely in edges:

Glazes opacified with Tin or Titanium are more likely to break, compared to glazes opacified with Zircopax.

Glazes that are slightly runny will break, stiff glazes will not. Boron is a good choice of flux if you’re trying to make slightly runny glazes. 

Thickness in application can make all the difference. 

I don’t have an answer regarding the speckles of different colors, but I like your thought about adding crushed up fired glazes. That sounds like it would be worth trying, 

[Min]

Glazes with high glaze fluidity are likely going to have low viscosity (of the glaze while firing) plus lowish surface tension. This will enable them to break and flow over texture or vary with glaze thickness. 

Oxides with high viscosity are alumina, zirconium, silica, chromium, tin and nickel. Oxides with the lowest viscosity are potassium, sodium, boron and lithia. (partial list)

Oxides that contribute to high surface tension are magnesium, alumina, zirconium, zinc and to a lesser extent calcium, tin and chromium. Oxides with a low surface tension are lithia, sodium, potassium and boron. (partial list)

You can see there are oxides that overlap both viscosity and surface tension insofar as both  being in the high vs low range are concerned. There is a balance though, too much fluidity can lead to problems with blistering etc. The simplest way to make a glaze run is to remove some of the alumina in the recipe, usually from kaolin, this can work if a slight change is only necessary since removing too much alumina will likely increase the glazes chances of crazing.

 

[Hulk]

Can you share your "favorite glaze"?

The van Gilder book "Rutile Green" - it's listed on MMM*

I'm seeing it come out flat pond scum - well, muted then - in some depictions, however, am getting nice bright lively fine-grained freckling on white stoneware, weathered breaky look on buff clay, rich darker undertones with red clay...

I'd forgotten that the Teal Blue and Variegated I use are from that book as well.
Favorite, well, that was last week, I really like the six glazes that have stayed in the lineup; I've been away from the studio for a few weeks now, missing that red. It's from a local college lab, same as in John Britt's book, "Chrome Red" - which seems to pick up fine blue pinpricks, I'm guessing from either fuming neighbors in the kiln, underglaze or glaze under the red, somewhat else, or come combination of factors, I don't know, I find it fascinating.

*Mighty Mud Mixer

 

This is the rutile green over white stonware, then buff stoneware

 

[MissyLee5]

@GEP and @MinGreat information, thank you! That is exactly what I was looking for, I will add this to my notes and keep that in mind when tweaking glazes.

@Hulk That is a lovely glaze, I really like how it emphasizes the chattering marks. Are those photos of the recipe you shared?

While we are on the topic of colorants, what is everyone's preference as far as carbonate vs oxide? Is there any reason to carry both? I went for oxides since when factoring in the price difference and strength, oxides ended up being cheaper. But I've noticed most recipes call for carbonate. From what I've seen in photos, as long as the substition guidelines are followed, the color comes out nearly the same. I'm debating on whether or not to add copper/cobalt carbonates to my next purchase order. I know it is said oxide can cause some speckling since it is coarser. I've got my first 15 100g glaze batches to test, where oxides were subbed anywhere carbonate was called for. I did sieve well, so hopefully I don't get any unwanted mottling or speckling. I'm waiting on a my dark brown clay to arrive before I begin my tests. 

[Hulk]

Hi Missy Lee,
Yes, both images are the BVG rutile green.

The chatter marks were filled with a contrasting color(almost sure it was blue glaze that time).
I'm using Speedball underglaze or just glaze; having sponged the area a bit, I use a brush to fill the marks. Using water helps to fill the marks without leaving a thick layer on the clay - especially where the pot was ribbed smooth and tight. Then I'm sponging away the excess, so the marks are filled and the rest is clean. After drying, glaze per usual. Even where I want to use just one color/product, I'll fill the marks against the infrequent but rather annoying dry chattermark, where the bottom of the mark didn't "wet" with glaze and hence a chattermark shaped crawl/bare spot emerges in the firing.

Colorant forms, I remember reading up some on safety/toxicity, relative cost, and recipe substitutions, but I don't remember why I chose carbonates. Your testing may reveal any speckling.

[GEP]

1 hour ago, MissyLee5 said:

what is everyone's preference as far as carbonate vs oxide?

At least when it comes to cobalt, the carbonate is already very concentrated pigment. Make a small mistake when measuring and the result will be off by a lot. The oxide is even less forgiving! I prefer cobalt carb for that reason. 

[MissyLee5]

@HulkAh I see, smart. That may have happened to me recently with a plate, I figured I must've somehow got a dot of wax near the chatter marks. I just made some underglaze over the weekend, I'm looking forward to using them to infill amongst other things.

 

@GEPThat's good to know. My small scale measures to 0.01 and I did my best to measure dead accurate with my first tests, but bigger batches won't be as accurate with a less sensitive scale. I bought such a small amount of the oxides and other colorants thinking they would last a while, but after about 20 test batches I'm realizing everything goes quicker than you think. 

[Babs]

19 hours ago, Hulk said:

Can you share your "favorite glaze"?

The van Gilder book "Rutile Green" - it's listed on MMM*

I'm seeing it come out flat pond scum - well, muted then - in some depictions, however, am getting nice bright lively fine-grained freckling on white stoneware, weathered breaky look on buff clay, rich darker undertones with red clay...

I'd forgotten that the Teal Blue and Variegated I use are from that book as well.
Favorite, well, that was last week, I really like the six glazes that have stayed in the lineup; I've been away from the studio for a few weeks now, missing that red. It's from a local college lab, same as in John Britt's book, "Chrome Red" - which seems to pick up fine blue pinpricks, I'm guessing from either fuming neighbors in the kiln, underglaze or glaze under the red, somewhat else, or come combination of factors, I don't know, I find it fascinating.

*Mighty Mud Mixer

 

This is the rutile green over white stonware, then buff stoneware

 

Where's the green in that?

[Hulk]

Hi Babs,

It's the copper carbonate.
...or does the colour not look green to you? It's the body glaze, the interiors are liner glaze with a light white.

Both these clays seem to eat away the coppery green more than the other white stoneware I use, where it looks more green. I'll look for an image...
It is variegated, eh? Medium brown rutile, the kind.

 

this jar and mug trio are over the other white stoneware, more uniform color

added, next day: in natural light, the blue green is more green, really!

[Pyewackette]

@Hulk The tops of the mugs look green but the first two pictures you posted and the covered dish look blue to me.  I did see what you meant about the "pond scum green" on the Mighty Mud Mixers website.  That glaze, and the other Van Gilder glazes I saw there, appear to be very strongly affected by the underlying clay.  On white stoneware (in those pics) it was pond-scum green but the more iron in the clay, the better looking the glaze got - same for the other two as well. Made me wonder if he works mostly with high iron clays?

[Min]

18 hours ago, MissyLee5 said:

I know it is said oxide can cause some speckling since it is coarser. I've

Cobalt and iron oxides  can also cause speckling from agglomeration in the glaze slurry.

[MissyLee5]

I thought they looked blue as well. The mugs are more green though. 

[MissyLee5]

1 hour ago, Min said:

Cobalt and iron oxides  can also cause speckling from agglomeration in the glaze slurry.

Ah OK, so just be sure to mix well? I'll be using an attachment on a drill to mix regular batches, and my mini mixer or homogenizer for the small batches.

I should have my brown bear clay this week, then can begin testing between it and my white stoneware. I'm really looking forward to the brown bear clay. Hopefully a decent amount of glazes will work on it, it's such a beautiful clay body. 

[MissyLee5]

Just thought of something- Can colorants can be switched out in a breaking glaze, to get a different color where it breaks? For example Folk Art Guild White (cream breaking red), can RIO be switched out for say copper carb & get a cream breaking green? The answer is probably try it & see 

 

FAGW

Dolomite 18.00

Gerstley Borate 18.00

Kaolin 18.00

Silica 18.00

Soda Feldspar 18.00

Spodumene 10.00

 Tin Oxide +13.00

 Bentonite +2.00

 Red Iron Oxide +2.00

 

Link to recipe

https://glazy.org/recipes/67304

[High Bridge Pottery]

On 4/16/2022 at 4:36 PM, MissyLee5 said:

So anyway, I'm curious what causes these effects and if it's possible to tweak existing formulas to bring it out. I've searched but can't seem to find an answer. The only thing I can think of with my newbie brain, is for the blooming type glazes to add varying sizes of sprinkles sintered from a runny glaze of a different color/colors, which would allow the secondary colors to spread out? I'd really like to make more of these types of glazes, but have only found a few on glazy that behave this way. Can anyone enlighten me? Am I looking for something within the chemistry, like higher boron, or is it an ingredient that achieves this effect, or both? I have an electric Kiln and will be firing in oxidation, so the lovely effects of reduction aren't possible for me. Although if I could choose again, I might have opted for reduction.

I think you are looking at phase separation in glazes where different glaze chemistry and temperatures allow silica rich phases and flux rich phases to separate out. These different phases can then dissolve and crystalize stuff at varying rates giving the different colours etc. Boron is particularly good because it is a kind of intermediate oxides that can be a  network former and network modifier.

 

Glass networks form this sort of silica rich and flux rich areas naturally and can form different kinds of stable networks at different temperatures so depending on your glaze chemistry, firing cycle and glaze thickness a lot can change visually due to what networks are formed and what might crystalize or separate out during the firing profile.

[Harold Roberts]

Just to chime in , all opacifiers, ie tin, titanium, rutile, and zircopax  have different effects on colors as do other materials. For example in both photos are the same glazes but the salmon color is opacified with titanium and the white to pink is opacified with rutile. Chrome oxide is the colorant. I imagine that tin would be pinker but less variegated. Also if the glaze was higher in kaolin it might be pinker because chrome pinks love high alumina glazes. It is also interesting how the copper glaze applied to the leaf motif is also affected.

[MissyLee5]

@High Bridge Pottery I believe you are correct, that's the effect I like. Phase separation, I couldn't remember what it was called. I wasn't sure if there was a surefire way to encourage this. It seems though as you said, it's not just the chemistry, but the application & firing too. 

 

@Harold Roberts Wow that is very interesting! I love the result with the rutile. That's what i was hoping for, a way to go from your first result with the tin to your second result with the rutile. Once I have a few glazes I like I'll do some tests, swapping out opacifiers & colorants to see what happens with different glazes. Thanks for sharing.