"Pay the Rent" Blue glaze with even coverage

[davidh4976]

I started working up some blue glazes by starting with a some good gloss clear glaze recipes I've used before and adding 1.5% cobalt carbonate. On the batches I've run, the fired glaze goes clear over edges. It looks great if that's what you want, but I'm looking for a blue glaze that stays blue even over edges/texture. I also want to avoid Gerstley Borate, Neph Sy, and Talc.  GB because of end-of-production. Neph Sy because it somewhat contributes to hard panning. Talc because of recent quality. For cone 6 stoneware. Any ideas or suggestions? 

[Kelly in AK]

I expect people more knowledgeable than me will check in, but for my initial reaction, I suggest you’ll want to look into various boron based frits. They will give you some modicum of certainty, that is  at least one of your glaze ingredients can be depended on to be exactly what the analysis says it is. You’ll get the boron content you need to flux things at cone6. I lean heavily on Ferro frits 3124 and 3134, but the are several others you can use to tune your glazes with specificity.

As to blue at cone 6 my thought was including an opacifier (zircopax, tin) would help even things out and reduce that “breaking” quality. That’s just a guess. I have used a glaze from John Britt called “John’s Noxema” that fired very evenly. I didn’t like it for exactly the reason you might seek it: it didn’t break enough on edges and texture. 

[Bill Kielb]

3 hours ago, davidh4976 said:

I started working up some blue glazes by starting with a some good gloss clear glaze recipes I've used before and adding 1.5% cobalt carbonate. On the batches I've run, the fired glaze goes clear over edges. It looks great if that's what you want, but I'm looking for a blue glaze that stays blue even over edges/texture. I also want to avoid Gerstley Borate, Neph Sy, and Talc.  GB because of end-of-production. Neph Sy because it somewhat contributes to hard panning. Talc because of recent quality. For cone 6 stoneware. Any ideas or suggestions? 

What comes to mind is as a general rule 0.15 boron ought to get you a cone 6 melt regardless of source. And maybe search Glazy.org defining the characteristics and color you desire. (Example below)

 

[davidh4976]

I'm using Frit 3195 and it's breaking.  I'll try John’s Noxema”.  Thanks for the tip! I saw that in Britt's book, but it's so hard to tell if it's breaking by just looking at the photos. Same challenge with the glazes on glazy.org.

[Hulk]

I'm using Lakeside Pottery Clear Blue; it goes on and fires well, does not move much, and is fairly consistent over edges.

The formula is on Lakeside Pottery's website; it's been copied to Glazy as well:
Clear Blue | Glazy

It does call for Talc.
I'm using the no longer widely available C-98 talc, and have enough to last several years, however, if/when the supply runs low, I'll be testing alternative sources of MgO, an important part of several glazes I use and really (really!) like...

There's some lightening at the edge of the knob, which may be difficult to avoid with any transparent blue over white clay; any road, it lays flat and doesn't move, excepting if put on very thickly.

[Min]

You want to look for a stiff glaze (little glaze mobility / fluidity) if you don't want it to break on edges. Think of majolica type glazes, they are high in alumina making them a stiff non fluid glaze so when the overglaze brushwork is put on the designs don't run or move. 

If you don't want to use talc then try your hand with a glaze calculator tool, like the free one on Glazy, and have a go at removing the talc and supplying the now lost magnesium with dolomite  instead. You will have to tinker with the amounts as it isn't a 1:1 swap then will need to rebalance the remaining oxides in the formula. 

I had a look at Tom's Lakeside Blue recipe above, couple thoughts about it. First thing that jumps out at me is the gum in it, I'm guessing it's there because the glaze might have a tendency to crawl without it due to the super high amount of epk. (Epk shrinks as it dries, excessive shrinkage = cracking which more often than not results in glaze crawling during the glaze fire). Bit of a band-aide solution to add gum when supplying part of the epk through calcined epk is simpler. Other thing is the colourants are included in the total, not as an addition. Makes it easier to adjust a recipe if the colourants are a separate addition from the main recipe. If possible I would reduce the cobalt, using less of a transition metal  is preferable if the colour is still okay.

I redid the original recipe first with just replacing the talc with dolomite to supply the magnesium. Then I took that recipe and replaced about half the epk with calcined epk. Glaze with the calcined epk is going to appear thinner in the bucket with the same amount of water. (measure specific gravity) I didn't include the gum in the recipe as it doesn't contribute to the fired chem. If you try this please just mix up a small test amount first as what looks good on paper doesn't always translate to a pot. 

[davidh4976]

Min, thanks for the info! It's good to see someone else's work.  Yes, I use the Insight tool and do 200mg test batches. I think I'll try your conversion with the calcined EPK in addition to John’s Noxema.  I don't mind testing more than one version at a time and then selecting the one I like best.  And, I always do tests that include crazing tests for our clays.

[Min]

Is this the Noxema Blue recipe below? If it is the expansion is going to be super high from all that custer spar in it. It's unusual to see that much custer in a ^6 glaze. What might control the crazing from such a high COE glaze is the high amount of cobalt carb in it, which is another point, if it can be reduced without causing other issues that would be good.

https://glazy.org/recipes/2475

[Kelly in AK]

I dug up “John’s Noxema,” it’s from John Britt’s book The Complete Guide to Mid Range Glazes. Some similarities, some differences. Lots of feldspar, but soda not potash. No wollastonite or Gerstley borate, rather strontium and 3134. An abundance of cobalt, for sure. 

Kona F-4 spar  50

Frit 3134  24

EPK 11

Strontium carb  11

Silica  4

Add:

Cobalt carb  3

Bentonite  2

[davidh4976]

1 hour ago, Min said:

Is this the Noxema Blue recipe below? If it is the expansion is going to be super high from all that custer spar in it. It's unusual to see that much custer in a ^6 glaze. What might control the crazing from such a high COE glaze is the high amount of cobalt carb in it, which is another point, if it can be reduced without causing other issues that would be good.

https://glazy.org/recipes/2475

No, that's different from John's Noxema (in John Britt's "The Complete Guide to Midrange Glazes).
But, now that I put John's Noxema into Insight, it too has a high expansion and I would expect crazing on stoneware.

Edited September 14, 2023 by davidh4976

[Min]

Thanks Kelly and David.  Same principal applies though, really high spar.  Sodium will be higher in the Noxema from Britt's book than the Custer one from Glazy but the COE is still going to be super high, would probably craze on most bodies without the cobalt carb. and might still craze even with the high 3.0 amount.

[davidh4976]

[I edited out my mistaken reply; don't know how to simply delete it.]

Edited September 14, 2023 by davidh4976

[Min]

If you have a good blue already and just want to make it less fluid so it doesn't go clear on breaks try adding increments epk and silica in the ratio by weight of 1:1.25 epk to silica. ( to keep the silica:alumina ratio the same). There will be a point when the glaze is still melting well but won't be so runny. 200 base (include the cobalt) then add 2 epk + 2.5 silica dip a tile and repeat 2 or 3 times. Not super accurate and the cobalt will be diluted a bit but it will get you in the ballpark.

[Callie Beller Diesel]

Another avenue could be to add some zircopax to the recipe of your choice. Usually you don’t want it in a cobalt glaze because cobalt is pretty opaque by itself, but if you have a base glaze that works in all other respects, that could serve to get rid of any pooling. 

[davidh4976]

I think I finally got a good blue that does not go thin on edges.  I'm at the point of having also tested it for crazing on 3 of our 6 studio cone 6 clays.  It looks pretty good.  Here is what I ended up with. Now that Custer Feldspar is no longer being mined, I'll have to retest with Mahavir Feldspar.  It looks like I'll have to add some more silica to get to the same chemistry.

whiting     5.9
dolomite     5.2
custer feldspar     28.8
ferro frit 3134     14.2
epk     17.3
silica     26.4
cobalt carbonate     1.9
rutile     0.3

[Babs]

On 9/13/2023 at 11:40 PM, Hulk said:

I'm using Lakeside Pottery Clear Blue; it goes on and fires well, does not move much, and is fairly consistent over edges.

The formula is on Lakeside Pottery's website; it's been copied to Glazy as well:
Clear Blue | Glazy

It does call for Talc.
I'm using the no longer widely available C-98 talc, and have enough to last several years, however, if/when the supply runs low, I'll be testing alternative sources of MgO, an important part of several glazes I use and really (really!) like...

There's some lightening at the edge of the knob, which may be difficult to avoid with any transparent blue over white clay; any road, it lays flat and doesn't move, excepting if put on very thickly.

Have you tried it without the CMC? Why is it included? Glaze application?

[Hulk]

Hi Babs,

No haven't tried Lakeside Pottery Clear Blue w/o CMC.
Not sure why it's included; the notes don't mention anything about it.
Next time mixing some up, I'll pour off some and withhold its CMC to do a side by side. My guess is the CMC is for keeping it stuck on.

This blue is one of the recipes I'd selected when first making up a list of materials for glazes - was driving the truck to the big city, so.

Any road, this blue and the red I use are somewhat translucent; I really like them.

[davidh4976]

After a bit of testing, I found a Cone 6 recipe that gives a good, deep blue color that stays blue even over edges/texture. I avoided Gerstley Borate, Neph Sy, and Talc.  GB because of end-of-production. Neph Sy because it somewhat contributes to hard panning. Talc because of recent quality. This has been tested and found to be craze free (boiling water/ice water test) on the following clay bodies from our local supplier, New Mexico Clay: Marilyn's BOD, Ochre, E-Speckled, Cashmere, and Chocolate (these are all of the Cone 6 clay bodies that we now support in our pottery).  On Chocolate, the glaze surface does not look as smooth as on the other four which look great. Here are the two recipe versions, one using Mahavir Feldspar and the other using G-200 Feldspar (because we have both in our supply!).

AMT      MATERIALS

27.6      MAHAVIR POTASH FELDSPAR

26.5      SILICA 325

15.3      FERRO FRIT 3134

18.4      EPK

12.2      DOLOMITE

   2.0      COBALT CARBONATE

102.0   TOTAL

 

 

AMT      MATERIALS

29.3      G-200 FELDSPAR

26.2      SILICA 325

13.9      FERRO FRIT 3134

16.4      EPK

12.2      DOLOMITE

   2.0      COBALT CARBONATE

100.0   TOTAL

[Min]

@davidh4976, good job on subbing the Custer out. I like how much silica and alumina are in the formula, will make for a good durable glaze. I could see a bit of iron in the recipe if you ever wanted to tone down the blue a little.

[davidh4976]

32 minutes ago, Min said:

@davidh4976, good job on subbing the Custer out. I like how much silica and alumina are in the formula, will make for a good durable glaze. I could see a bit of iron in the recipe if you ever wanted to tone down the blue a little.

Thanks! Everyone in the pottery LOVES the color, so I am not tweaking the color at all!    :-)

I general, I am finding that Al2O3 of about 0.4 and SiO2 of about 3.5 (UMF) is working well for our "workhorse" cone 6 glazes.

I may play with increasing the B2O3 flux a bit to see if I can get the glaze to smooth out on the Chocolate clay body without detrimentally impacting the color and coverage on the other clay bodies. 

Edited February 3 by davidh4976