Raise firing temperature of high thermal expansion glaze

[mr_glazy_man]

What material could I use to raise firing temperature of a crazing glaze? For the record, I like that it's crazing and want to preserve that. I am using copper as the colouring oxide to get a beautiful turquoise blue, and am noticing that it becomes opaque as we reach temperature (1040c)for this base glaze. I am presuming that the copper is therefore overfiring in the base glaze when it turns opaque. 

90% FZ-16

10% EPK

5% Copper Carbonate 

I have tried adding silica in incremental amounts of 1%, but this reduces thermal expansion and still yields me an opaque copper at that temperature. 

I was thinking Nepheline Syenite because of its high sodium content, but not sure if this is a misdirected effort. Next test will be a line blend of additional EPK at 1% increments to see if I can raise that firing temperature without reducing the crazing.

Note, this is NOT for functional work. 

 

[Bill Kielb]

Generally to raise firing cone one would increase alumina and also silica to try and maintain the surface texture.This recipe has few components so EPK is likely a place to start. I am unsure that will solve your colorant issue though. Changing fluxes could be a solution but also may cause more complication for sure so the neph sy idea might not be the direction you want to begin testing as neph Sy has some of the lowest silica and alumina content for natural materials. Things fire by cone, so best to figure out what cone it fires to, peak temperature is not generally effective or indicative enough for fluxed reactions.

Edited June 7, 2023 by Bill Kielb

[BobMagnuson]

Zinc can be an opacifier when the calcium is low.  This recipe has loads of zinc and very little calcium.   It's also shy on alumina.  In order to reach the zinc eutectic point and prevent it from opacifying, both CaO and Al2O3 will need to be increased quite a bit.  I don't have much personal experience with low temp glazes, but I would start by doubling the EPK and adding whiting in 10% increments.

[Min]

20 hours ago, mr_glazy_man said:

What material could I use to raise firing temperature of a crazing glaze?

How much do you want to raise the firing temp by?

Have a look at the melt fluidity tests done with Fusion FZ16 as it compares to other frits in this link. It softens and starts to become fluid at around 1500F and by 1650F has run right off the fluidity tester. If you are looking to raise the firing temp by a few cones I would swap out some of the Fusion FZ16 for Ferro 3110. Should be able to do a line blend of the two and find a balance. Ferro 3110 is more than likely to retain the crazing you want to keep.

Image below from the above link from Hansen.

[mr_glazy_man]

Thanks for the responses team. 

@BobMagnuson I didn't know that Zinc opacifies sans calcium. Definitely a trail for me to explore. 

Note below the line blends I did. To clarify:

First column: the recipe fired across the temp range. You can see that at 940C and 1000C, it fires absolutely magnificently (and this is with Copper Carb, which has given me nothing but problems blisters, orange peel surface in every other base glaze I've tested at the temperature)

Second column: Line Blend adding 1% Silica. Zinc begins to opacify and thermal expansion is reduced

Third column: Line Blend adding 1% EPK (on top of the 10% already present). Zinc begins to opacify almost immediately. This shocked me.

Next test would be to proceed as @BobMagnuson suggested (but with Wollastonite given the LOI issues with whiting) and @Min suggestion of 3110. 

Would it just be the lack of calcium causing this opacification?

It seems that the opacification is correlated with the copper as I'm getting opacification when put into other zinc free low fire high thermal expansion transparent bases. For example, see below triaxial test of 94% frit (3110, 3195 and 3134 in each corner), 6% EPK and 5% Copper Carb fired at 1040C

 

 

 

Edited June 8, 2023 by mr_glazy_man

[mr_glazy_man]

Also, shame I cannot upload larger than 1000kb per post – that first image is particularly small and doesnt show the sheer beauty of FZ-16 and Copper at 960C. Marvellous I tell ya.

[mr_glazy_man]

@Min just realised I also didn’t answer your question . I would like to raise the temperature by about 4 cones to 1040C

[Bill Kielb]

On 6/8/2023 at 4:16 AM, mr_glazy_man said:

I would like to raise the temperature by about 4 cones to 1040C

Do you fire with cones?

[mr_glazy_man]

@Bill Kielb for production, no. I only use cones for calibration and to roughly validate temperature against what the controller reads.

 

[Babs]

43 minutes ago, Bill Kielb said:

Do you fire with cones?

What cone are you firing to at present?

[mr_glazy_man]

@BabsCone 05-04 is our standard range at the moment. All of our product gets fired between these cones. The FZ-16 base I have is melting beautifully at 09-08 (extraordinary really, and with no lead)!
 

Ideally, I want everything firing at the same temperature range, which is why I’m interested in increasing this base’s temperature. Silica and EPK have not helped me in this respect.

[Bill Kielb]

3 hours ago, mr_glazy_man said:

@Bill Kielb for production, no. I only use cones for calibration and to roughly validate temperature against what the controller reads.

 

So ……. When you validate  cone 04 peak temp you fire at a particular rate then verify peak when the cone bends? Is there a standard firing rate? Just curious.

[mr_glazy_man]

@Bill Kielb I fire the kiln at the ramp cycle recommendations set out in the Orton Self Supporting Cones chart to verify correct heat work and temperature. If I get the corresponding 90Degree bend, then my temperatures are correct

Do you suspect an incorrect temperature that might be causing the copper’s opacification?

[Bill Kielb]

48 minutes ago, mr_glazy_man said:

Do you suspect an incorrect temperature that might be causing the copper’s opacification?

No just curious to all the reference to peak temp without a rate. So looking at the samples, not really sure what cone they were fired to. The rate on the Orton chart is for the last 100c, but not knowing top temp, it’s hard to pick the last 100c. Complicating things further is many kilns struggle at rate in the last 100c so monitoring it is probably a necessity to know what it actually was.

The three rates in Orton are 15, 60, 150 with less kilns being able to achieve 150c per hour in the last 100c. Finally if anyone has been firing as elements wear, firing cone 04 at 15c per hour usually has an effect on the glaze versus say 60c per hour (the center column)

So all just very curious how to control this and relate meaningfully to peak temperature. Which brings to mind, the potential variations in fired appearance at a temperature without cones. Without rate, we may not really know what cone things were fired to. And in the case of really slow rates, many glazes change appearance even though their equivalent cone can be found in Orton. As things slow down too much, many glazes just don’t like it. Color and texture often become unacceptable.

Just a final thought, I have seen low silica and alumina (Bristol or zinc based) glazes melt very early but also have wild color shifts from blue / violet to golden brown and semigloss to flat matte with 1/2 cone firing difference.

Edited June 10, 2023 by Bill Kielb

[mr_glazy_man]

Broadly speaking, the higher I go in temperature, the more likely the opacification (depending on the base glaze I’m using). I would have thought that firing cycle is, in this instance, a negligible variable. I could be wrong on this of course.

[Bill Kielb]

3 hours ago, mr_glazy_man said:

Broadly speaking, the higher I go in temperature, the more likely the opacification (depending on the base glaze I’m using). I would have thought that firing cycle is, in this instance, a negligible variable. I could be wrong on this of course.

It may work for you. I have a hard enough time when using cones for each firing though so you might be at a disadvantage.. Hopefully your trend holds up.

[mr_glazy_man]

Unfortunately I’m at an odd’s end as to why the copper is opacifying at all - I haven’t been able to find anything on Digital Fire or other adjacent sites that suggest anything about copper opacification.
 

@BobMagnuson provided a nice trail to follow with the suggestion that it might be the zinc opacifying, but it doesn’t explain the opacification in other hosts - which leads me to suspect that this is specifically related to the copper. 
 

[Kelly in AK]

Firing times/rates have a large influence on crystal growth. Faster makes a less crystalline glaze, a more transparent glass. Not a negligible variable if you’re going for transparent. Your tiles are beautiful, by the way. Even at 1000kb, I see plenty. Thank you for sharing your work. 

[Kelly in AK]

I doubt it’s the copper, I suspect it has more to do with crystallization of other components. You’re shooting for glass phase and trying to avoid crystalline phase, in physics and materials science speak. That said, copper can certainly can opacify. Copper red glaze is opaque. The same glaze in oxidation is completely transparent.

I confess I can’t tell a great deal about opacity from the photos you posted, mostly color variation and different amounts of crazing are evident. The Ferro frits strike me by the color range and those awful blisters. Emerald green to robin’s egg blue. Those frits vary enough in their chemistry it’s hard to say copper is the deciding factor in opacity. As for the fz-16 tiles, I immediately was caught by the kaolin column and how the top tile and the fourth tile shared a “bluer” color, compared to the rest. Why would that be? It’s just where my eyes went. 

I can’t help wondering what 3134 and fz-16 would do with varying amounts of copper. Or leave out the high zinc frit altogether and play with blends of 3134 and 3124. I do a lot of work in the 03-04 range and that combo works well for me. 

 I suggest you do all your tests at the target cone range, eliminate that variable. Monitor the rate of firing and cooling, eliminate that variable.

Finally, I’ve made a lot of pots and made a lot of glazes, but I’m no chemist or glaze wizard, so take it all with a grain of salt. Add what I said to the pile and use what helps you move forward. Inspiring work, I’m sure you’ll get there and hope to see how it all plays out. 

[mr_glazy_man]

3134 and 3124 is another interesting avenue, for sure. I feel like that one, in addition to the magnesium based 3249, are both candidates to do line blends with (though the 3249 will have the opposite effect to crazing). 

The reason I test across the temperature range is to observe how a material behaves under different intensities of heat. It’s laborious, but enables me to visually distinguish symptoms of under and over firing. 

I’ll be playing with a number of other permutations of materials this week and I’ll post the update. Thank you for the compliments!
 

 

[Min]

If you already have a base that works at your chosen firing temp/cone then a simple solution would be to use a stain instead of copper. Vanadium blue stains will give you that turquoisey colour. Vanadium is combined with zirconium in stains so the colour might not be quite as crystal clear, depending on how much you use, this might not be a factor.

To keep using copper and get a blue rather than green colour the glaze needs to be very high in alkaline earths with little clay in it. Like you said magnesium will decrease the crazing so that leaves calcium, strontium, barium and then zinc. You already have boatloads of zinc in that frit (I'm thinking that must be one expensive frit) so look to increase the calcium at the expense of some of the zinc.  Calcium in the form of a frit would be best for lowfire. 

Are you using glaze calc?

Edited June 10, 2023 by Min

[mr_glazy_man]

@Min Ah, but it’s the beautiful oxide transparency that I want. Stains just don’t look the same

i am using glaze calc, but without having limit guides for earthenware (every target formula seems to be for cone 6) it just feels like I’m shooting in the dark. 

For calcium, I was initially thinking Wollastonite, but you’re right, frit form would be better. 3134 has close to 20% calcium, so I could line blend with that. Would you have any other suggestions?

[Min]

7 hours ago, mr_glazy_man said:

i am using glaze calc, but without having limit guides for earthenware (every target formula seems to be for cone 6) it just feels like I’m shooting in the dark. 

Limit charts aka Target Formula might not be that relevant given your recipe is bound to be way off them. If you want a couple for lowfire / mid and high fire

here you go:

7 hours ago, mr_glazy_man said:

frit form would be better. 3134 has close to 20% calcium, so I could line blend with that.

Yup, I agree, this would be a good choice to start with. It might raise the melting temperature a bit but I have a hunch it won't be enough. I would do the test Bob suggested at the same time with calcium carb plus a little more epk.

If you do go ahead and try this I would suggest doing a double variable line blend with "A" being your recipe above with 90 FZ16, 10 EPK, 5 copper carb and "B" being something along the lines of 80 3134 (we know that will be too high in 3134 but it should work for a blend test such as this), 10 FZ16 10 EPK, 5 copper carb. (could use 90 frit 3134 but there probably isn't any point).

edit: looking at your test tiles again, looks like you could bump the epk a bit? Might look at the numbers with "B" and consider adding a bit more EPK to that. Given it will be diluted with "A" could give it a go with 20 EPK and see what happens. It would be great if you could get something to work, it's a very pretty colour. Is the glaze really slippery like Hansen found when using this frit?

Instead of doing the ratios like in the chart below I would divide it down further given in all likelihood you will need mostly "A" with little "B". Something along the lines of #2 being 9 parts A with 1 part B, #3 being 8 parts A with 2 parts B and so forth until you get to around a 1:1 ratio. (do the test volumetrically)

My guess about your opacity with the copper is there is far more melt fluidity with the zinc frit than the other more opacified looking ones which allows the clean gassing off of the copper carb. Do you have a microscope you can have a look at the more opaque tiles with to see if there are micro bubbles within the glaze?

 

Edited June 11, 2023 by Min
added a thought

[mr_glazy_man]

12 hours ago, Min said:

here you go:

Ugh, amazing. I love this community.  Thank you - that’s incredibly helpful. If you don’t mind me asking - is there a resource you pulled this from? I’d love to get my hands on it!
 

In terms of the double variable test - this would be a biaxial, correct? In such a case, I will test with:

A:

FZ-16 90%
EPK 10%
Copper Carb - 5%

 

B:

3134 - 80%
FZ-16 - 10%
EPK 10%
Copper Carb - 5%
 

Im hesitant to go 20% EPK again because of how thick it was last time with the FZ-16. I ended up using Tetrasodium Pyrophosphate and CMC to deflocculate, slow drying a little just to make it workable. I’m weary of adding too much extra water, because it’ll dilute my S.G and aiming for gram weights on the ware will mean a lot of glaze I don’t want.

I might also try another test with my initial base recipe and 2% increments of wollastonite. However, the unwanted added silica in the wollastonite has me thinking that whiting - as bob suggested - might be preferable. Besides, the zinc and it’s late melt should give time for the calcium carb to off gas properly. Time will tell I suppose. Maybe I’ll do both. 

12 hours ago, Min said:

12 hours ago, Min said:

Is the glaze really slippery like Hansen found when using this frit?

I can’t say, because I haven’t tested on any curved geometry - only test tiles. But the glossiness and ability to heal over defects absolutely rivals lead bisilicate, just as he said. This is why I’m so keen to get the temp up - if I can get it to 1040, I’m grinning.

 

12 hours ago, Min said:

. Do you have a microscope you can have a look at the more opaque tiles with to see if there are micro bubbles within the glaze?

 

Sadly not, but this has me thinking that I might want to invest in one now.

 

 

Edited June 11, 2023 by mr_glazy_man

[PeterH]

Three very minor comments on your last post.

> Im hesitant to go 20% EPK again because of how thick it was last time with the FZ-16

If people are recommending EPK for its chemistry, and you don't like its physical properties why not calcine some of it.

>I might also try another test with my initial base recipe and 2% increments of wollastonite. However, the unwanted added silica in the wollastonite has me thinking that whiting - as bob suggested - might be preferable. Besides, the zinc and it’s late melt should give time for the calcium carb to off gas properly. Time will tell I suppose. Maybe I’ll do both. 

Digitalfire's LOI versus temperature chart might help make this decision.
https://digitalfire.com/picture/huctibegac
This chart compares the decompositional gassing (Loss on Ignition) behavior of six materials as they are heated through the range 500-1700F. These materials are common in ceramic glazes, it is amazing that some can lose 40%, or even 50%, of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect your glazes (causing bubbles, blisters, pinholes, crawling).

Re microscopes, a short article by Derek Au (of glazy?) on using a macro lens on a phone
https://www.derekau.net/blog/2018/07/12/smartphone-macro-photography