Confused Newbie. Cone 6 base matte not firing as expected

[cwells]

Hi all,

 

I started potting as a hobby about a year ago. A couple weeks ago, I got into glaze making and decided to try to make my own matte base as an exercise, after consuming all of the free information I could. It seemed to have worked well, as I got the desired effect of a cone 6 matte black. I followed all of the information I could find, and made sure it was 0.3:0.7 alkali:alkaline metals, 0.16 boron, and it charted on the stull map as a matte at 0.74 alumina and 2.54 silica. At that time, I did not fire a test tile without the colorants. I replaced some of the EPK with calcined EPK as I was having some crawling issues with application, and made two more test tiles.

 

Where I'm confused, i tried to fire the base without the added colorants, and fired the two test tiles together. One with 8% yellow iron oxide, one with no iron. The one with the iron fired nicely and seems durable to the eye (added cobalt didnt seem to dissolve when soaked in acids,  glass didn't seem to erode in bases, and doesn't scratch or chip). The one without the iron is a dry brittle glaze that I can flake off with a pin tool, so I assume it's underfired.

 

From what I understand, in oxidation fe2o3 should have a higher melting point than cone 6, and doesn't give off it's oxygen until cone 7-8 to become feo. I'm not sure where my misunderstanding lays, does iron oxide act as a flux in cone 6 oxidation, or am I missing something else entirely?

 

Thanks!

Edited March 9, 2023 by cwells
grammar

[High Bridge Pottery]

FeO is certainly a much more powerful flux but Fe2O3 can help with melting. I don't think you are missing anything entirely, there's quite a fine line/small area of chemistry that will make a good matte glaze as you have found out by removing the iron oxide.

 

All the B2O3 Fe2O3 Al2O3 oxides are a bit weird and can act as fluxes and stabilisers. Can be called intermediate oxides or amphoteric oxides.

[Min]

2 hours ago, cwells said:

added cobalt

Cobalt acts as a flux. Was there cobalt in both samples?

Welcome to the forum!

Edited March 9, 2023 by Min

[cwells]

7 minutes ago, High Bridge Pottery said:

Fe2O3 can help with melting.

I'm so confused by that with its melting point being so high. Guess that comes with being new, can't understand everything at once. Thanks!

 

5 minutes ago, Min said:

Cobalt acts as a flux. Was there cobalt in both samples?

Thanks No, neither of these two tiles did not have cobalt. I did a test earlier with the same base of 8% iron and 1% cobalt to get a dark black.

[Bill Kielb]

6 hours ago, cwells said:

0.74 alumina

This is fairly high for cone six. Usually in the range of 0.58 alumina things are pretty stiff at cone 6. I am curious why you picked 0.74 and 2.54 respectively and how you obtained 0.74? Also just curious about why a flux ratio of 0.3:0.7 and 0.16 boron. There may be no reasons here so no worries ……  but if there are I am curious what they are.

Edited March 10, 2023 by Bill Kielb

[cwells]

24 minutes ago, Bill Kielb said:

Also just curious about why a flux ratio of 0.3:0.7 and 0.16 boron.

My reasonings on the ratios are that the stull chart shows true mattes seem to sit below a 1:5 alumina to silica ratio. ~0.15 boron and a 0.30:0.70 alkali:alkaline ratio look to make the most durable glass when fired at cone 6 as per an NCECA talk about glaze durability I watched, which included a chart of measured durability of a glaze with varying boron levels and alkali:alkaline ratios at cone 6. Every other tile melted just fine at cone 6, just this one without iron. Maybe it just didn't get mixed well enough. Perhaps it's a mistake I made somewhere when making the batch since all of the other tiles with colorants matured.

 

I was looking at glazy for recipes with similar ratios and found https://glazy.org/recipes/147183 which is very similar to what I came up with and it melts at cones 4-7 in the semi matte region of the stull chart.

[Bill Kielb]

Nice! 0.3:0.7 is often considered an ideal range for likely durability.  When the ratio departs significantly especially for excess alkali >0.3 it may not be durable. Some reading here:  https://www.ceramicmaterialsworkshop.com/uploads/5/9/1/2/59124729/nceca_2012-  @High Bridge Pottery (the glass former / flux debate emerges again)

So in and around that ideal range …..generally a decent ratio.  0.16 boron at cone 6 also good as shown by Katz and Dr Bill Carty, pretty linear findings below:

https://www.ceramicmaterialsworkshop.com/uploads/5/9/1/2/59124729/katz_tf_boroninglazes_0912.pdf

The Gloss line

Or matte line,  in Stull which is relative - is a Si:Al of ratio of around 5:1 or less and is regional within Stull, trending toward gloss for his glaze at 7:1 and for him highest gloss.

so pretty good picks thus far, my question and caution was very high alumina levels and how did you get there. Things melt by composition though so as the alumina increases so likely does the cone it melts at. I could only find two similar mattes at 0.7 alumina so yours is interestingly high, higher than these and may not melt effectively at cone 6. Old forge below, Si:Al in the 4:1 range and 0.62 alumina.

so nice work, the worry would be its just under melted and testing it to cone 7 or 8 would yield a better melt. Still wondering how you got to 0.74 though.

Old Forge

Only two maybe three similar at 0.7 Al @ cone 6

Edited March 10, 2023 by Bill Kielb

[cwells]

2 hours ago, Bill Kielb said:

Still wondering how you got to 0.74 though

Thanks I appreciate the long post. That definitely reinforced that I'm getting on the right track to start making proper glazes. There's been so much information i've tried to absorb from the internet over the last couple of weeks.

 

How I got to it recipe wise, or how I decided on that level? Recipe wise, it has a fair bit of EPK/calcined EPK (it got real gummy without calcined kaolin especially with all the iron). For reasoning, to be honest, I wanted the glaze to be firmly into the matte region. I was also trying to not get close to the craze region as I've learned the craze region can move up the y axis depending on the glaze, so I was hoping to have it firmly there. I use a shared kiln that fires twice a week at cone 6, so not much room for playing around, and won't be able to test the glaze at a higher cone. I still find it odd that the same base seemed to have matured with the fe2o3 but not without. I'll try to remake the base and fire it once more incase I made a mistake somewhere. If it's still clearly underfired I'll lower the al2o3 in one test and increase the boron in another and try again.

 

The glaze is kinda pretty with the colorants and seems to be fairly resistant. Planning on making some pots that look cast iron. I'll at least keep using it as is, just quite confused about iron. Red iron oxide seems to have so much conflicting information about how it behaves in a glaze.

 

If you can see the images below, the left one has been dug at by keys and pin tools and left in detergent and acids overnight and seems fine. The one on the right is the base glaze, that chips off easily

 

[Bill Kielb]

All good but my thought would be why such high alumina. The recipe likely has silica that is adjustable. It may be easier to stay within alumina norms using conventional raw materials and adjust the silica accordingly to get your 4:1 - 5:1 ratio. As a benefit, It ought to result in a more optimal use of raw materials as well.  Adding more boron might work, but again more materials to potentially compensate. Creating a durable base glaze that performs well with a variety of oxides is handy. Certainly might be worth the testing. In my experience true matte glazes become reasonably easy to formulate, test and dial in the look. The more one departs from reasonable limits, the more difficult and random it can become though.

from the above NCECA publishing:  “ There is a clearly defined upper limit to silica and alumina in a glaze which will melt if cone 10 is more or less the peak firing temperature. Beginning after the 0.5 alumina level, glazes fired at cone 10 begin to become under-fired. Seger acknowledged that these upper limit glazes will melt with an increase of firing temperature, …..”

Edited March 10, 2023 by Bill Kielb

[cwells]

Is that assuming no boron in the glaze? We looked earlier at glazes with 0.62 alumina that fired to cones 4-7.

[High Bridge Pottery]

9 hours ago, cwells said:

I'm so confused by that with its melting point being so high. Guess that comes with being new, can't understand everything at once. Thanks!

It's the same way Silica doesn't melt until 1700c by itself but by having a mixture of oxides you end up with a lower melting point.

A good example is a mix of Silica and Alumina. "The melting point of silica is 1710c and that of alumina is 2050c. At the eutectic point, when 10% alumina and 90% silica are mixed, the melting point is only 1545c.  All other ratios of silica and alumina have higher melting points."

https://lindabloomfield.co.uk/wp-content/uploads/2017/04/Bloomfield_May17CM.pdf

[Bill Kielb]

2 hours ago, cwells said:

 

Is that assuming no boron in the glaze? We looked earlier at glazes with 0.62 alumina that fired to cones 4-7.

 

Yes it is. The point being .74 alumina is probably fairly high even at cone 10.  The highest I saw earlier was two or three similar boron recipes at 0.70.  The ratio is the thing that establishes the matte line, alumina is on the Y axis. Just curious, why is 0.74 important in your recipe and how do you achieve it?

Edited March 10, 2023 by Bill Kielb

[Callie Beller Diesel]

+1 for Highbridge’s thoughts on eutectics.

My personal favourite thing about Rose and Matt Katz’s research is that they’re incorporating colourants into UMF calculations through sheer empirical testing. While it was standard to assume that colourants had no effect on the base glaze when I was learning, it seems illogical that adding almost 10% of an ingredient would leave the melt unchanged. Not all of them are added in yet, but they’re working on it. 

When you’re looking at your recipes in glazy with and without the iron, are you looking at just the Stull map, or are you checking out the extended UMF calculations? How is the iron changing your flux balance?

[Bill Kielb]

On 3/9/2023 at 7:44 PM, cwells said:

My reasonings on the ratios are that the stull chart shows true mattes seem to sit below a 1:5 alumina to silica ratio.

I have seen matte questions a bit here so ……
Just noticed this came out! So posting it here in the event you are still working on these. Sue does a great job of explaining and providing answers to the question: How do I make a matte glaze / more glossy (Stull matte anyway) ? You might find it worth the read https://suemcleodceramics.com/how-to-turn-a-matte-glaze-glossy-with-one-ingredient/

She does mention other mattes and also provides good insight on under fired mattes etc…. IMO

Edited March 22, 2023 by Bill Kielb

[Roberta12]

3 hours ago, Bill Kielb said:

I have seen matte questions a bit here so ……
Just noticed this came out! So posting it here in the event you are still working on these. Sue does a great job of explaining and providing answers to the question: How do I make a matte glaze / more glossy (Stull matte anyway) ? You might find it worth the read https://suemcleodceramics.com/how-to-turn-a-matte-glaze-glossy-with-one-ingredient/

She does mention other mattes and also provides good insight on under fired mattes etc…. IMO

She is offering a free online workshop on Saturday the 24th.  Might be worth checking out.