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Seattle Pottery Supply - Midnight Black Clay


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4 hours ago, tanvi504 said:

@Bill Kielb do you think a pre programmed slow bisque will suffice? I read in another article to introduce 30 min hold times at three critical temperatures - 752, 1063 and 1500F. What would you suggest?

I would try the slow bisque, it’s pretty suitable for most bodies really and has a built in 2 hour dry out period built in. . At this point we really don’t know if the bisque schedule caused any issues. What I think we know about this clay is they made it with black mason stain so not a bunch of organics to burn out. As always, I could be wrong but arbitrarily increasing bisque schedules without knowing often leads to wasted energy on the planet.  This doesn’t seem to be a high iron clay so probably not necessary. Having said that, if you have access to a test kiln then it would be pretty  easy to conduct some tests without getting into  the conflicts of shared kiln use.

At some point it might be the only likely solution which I suspect would make this clay not worth using for you. Right now, to me the glaze  looks under fired and that’s what I would test for. As always, I could be wrong though.

Edited by Bill Kielb
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I don't think it's an under-firing issue. I've had glazes do that on Standard 266, a similar black clay. It's got something to do with the clay body, either from manganese or iron. A slower bisque is definitely worth a try, and I don't think one test firing at a slower schedule is going to hurt the planet as much as firing and throwing out pieces that don't work.

@tanvi504 what color is the clay in raw form?

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1 hour ago, Bill Kielb said:

Right now, to me the glaze  looks under fired and that’s what I would test for. As always, I could be wrong though.

@Bill Kielb, can you show me the image that you are referring to when you say @tanvi504's glaze, G2926B is underfired? Again, the one the op posted is one taken from the internet, not their pots.

We don't know what the SPS Midnight claybody is composed of. @glazenerd is speculating it's a IMCO Burgandy clay plus stain, @liambesaw thinks it's a stained porcelain. The schedule I linked isn't much longer than the Skutt slow fire program, if it saves a bunch of pots then I would think that offsets the extra couple hours firing insofar as walking softly on the back of Mother Earth.

 

 

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Yeah I only think it's stained porcelain because it threw like porcelain when I tried it. Could be something else entirely.  I doubt it's IMCO because I don't see any other imco clays/materials on their website.

 

By the way, there is someone there, you can call and ask them for suggestions and such.  They called me today asking if I still wanted an order back in February for some backordered stuff.

Edited by liambesaw
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1 hour ago, Min said:

@Bill Kielb, can you show me the image that you are referring to when you say @tanvi504's glaze, G2926B is underfired? Again, the one the op posted is one taken from the internet, not their pots.

Yes,

We really don’t know but then the OP said this is exactly how hers fired when she posted the look alike pictures.. Maybe she will try a glaze and it will look better, don’t know. Maybe she will bisque longer.  Just an idea She can try.

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1 hour ago, liambesaw said:

By the way, there is someone there, you can call and ask them for suggestions and such.

Good idea Liam, one long distance call later...

I was told it's a stoneware body coloured with manganese dioxide, bisque slow to cone 04, vented kiln, glaze fire no hotter than cone 5. They wouldn't give me any glaze recipes but said their opaque white glaze fits this body.

@tanvi504, you mentioned that your kiln isn't vented. Manganese fumes from the kiln are hazardous to you and must be avoided. Is the kiln outside in a separate kiln shed or ? There must be a heck of a lot of manganese in this body to stain it black!

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2 hours ago, Min said:

Good idea Liam, one long distance call later...

I was told it's a stoneware body coloured with manganese dioxide, bisque slow to cone 04, vented kiln, glaze fire no hotter than cone 5. They wouldn't give me any glaze recipes but said their opaque white glaze fits this body.

@tanvi504, you mentioned that your kiln isn't vented. Manganese fumes from the kiln are hazardous to you and must be avoided. Is the kiln outside in a separate kiln shed or ? There must be a heck of a lot of manganese in this body to stain it black!

So,

Good find! Seems like slow bisque Is fine and there are glazes that work with that claybody, just don’t exceed cone 5.

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oh looks like I missed a lot here in a day ! the clay is a deep red in its raw form (with a lot of black settlement when wet which im guessing is the manganese)

@Min thank you for calling SPS and getting an answer for me ! too kind. I'll try the slow bisque program on the skutt to cone 04 and see what comes of it. Ill fire with my current glaze after that and see if it fires right. If not, I might buy the one from SPS that they claim works.  The kiln is in our studio but we leave the door and windows open when we fire and do it when we aren't around so no one has to breathe the toxic stuff.

Thanks everyone for some great advice. Ill fire next week and post my findings here - in case anyone is interested. 

 

 

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( high in manganese body) My thought from the beginning but it could have been other heavily loaded colorants . That body will be hard to get a good glaze fit with many glazes.

These are a few made like this in the SF bay area and they love to bloat and are hard to glaze fit.

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  • 2 weeks later...

Just wanted to post my results here and see what everyone thinks. The smaller test tiles in the images are the new ones (slow fired bisque to cone 04 and glaze to cone 5) and the bigger ones are my old test tiles with a regular bisque to cone 06. There is only a very small improvement if any. The last test tile is the new recipe I tried posted by @Bill Kielb  While it appears to be the best one in terms of microbubbles, it has given a kind of brownish tint. Im not sure if any of these glazes would work for food safety (liner inside mugs).  Thoughts? 

FED0D44A-BC86-4643-B645-5AEC12B2F768.jpeg

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A10C7888-4F1F-4D5A-82AF-CD892A3FE752.jpeg

9F062010-35A9-4F32-8E4D-ABEC7C746E61.jpeg

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17 minutes ago, tanvi504 said:

Just wanted to post my results here and see what everyone thinks. The smaller test tiles in the images are the new ones (slow fired bisque to cone 04 and glaze to cone 5) and the bigger ones are my old test tiles with a regular bisque to cone 06. There is only a very small improvement if any. The last test tile is the new recipe I tried posted by @Bill Kielb  While it appears to be the best one in terms of microbubbles, it has given a kind of brownish tint. Im not sure if any of these glazes would work for food safety (liner inside mugs).  Thoughts? 

 

 

A10C7888-4F1F-4D5A-82AF-CD892A3FE752.jpeg

9F062010-35A9-4F32-8E4D-ABEC7C746E61.jpeg

It’s hard to tell from your pictures If this was a melt issue or not. The glaze I gave you should melt closer to cone five and our theory was the previous glaze pictures appeared to be not fully melted.
With respect to color, it may not be a glaze you can use or might be iron from your natural materials. It’s a Gerstley recipe with EPK as I remember so maybe worth reformulating with Fritt in hopes of clearing it further.

It is a glaze that is durable, meaning it exists in a flux range that is generally durable but to have confidence it needs to fully melt and be as its chemistry says which means glossy. If all that is true, it is fully melted and glossy then you could move on to some simple preliminary tests and ultimately send it off for testing to be reasonably sure nothing leaches.

It was Specifically designed to be a durable Zinc free transparent Gloss glaze when fired to maturity..

So, maybe we proved this is a melt issue, maybe not. Close up pictures of the surface would help. Did one glaze melt better than the other would be my first question and if yes it might be worth figuring out the color change issue or using a different formulation clear that does not change the color. This glaze is used over very bright white porcelain so the coloration issue In yours is interesting but there is a reason.

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Hi again Tanvi! Thanks for posting back with your progress - looks bubbly; my opinion, would want to see the bubbles "healed over" such that there's no crater down to the clay body.

I'll get some pics up of aforementioned Wollastonite Clear (Bethany Krull) over Aardvark Clay's mid fire black stoneware this afternoon...

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Wollastonite Clear on the right; the surface is definitely somewhat "orange peel" - however, looking with magnifying lens in bright light, looks like the "pores" are healed shut.

Middle is Lakeside Clear Blue; the blue is mostly gone - in the two dip triangle there's a bit of blue.

Left is Bill Van Gilder's Rutile Green; this one froths a bit, and most of the green colour disappears - it does look cool though.

Wish I knew why the Wollastonite Clear isn't retaining the bubbles, lays down flatter, and is much smoother than the other clears I've tried on this clay (Aardvark Clay's Cassius Basaltic) and the red clay I'm using (Aardvark SRF).

Cassius 1.JPG

Edited by Hulk
whey -> why
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This definitely helps ! Thanks for explaining the ‘healed’ shut concept. I looked again and it looks to me like the white and the new clear recipe might be okay. I tried the lemon leach test and nothing changed so thats good too. Ive attached closer pictures here. Would love to hear your thoughts.  Im still thinking about getting Aardvark Basaltic because it looks like it might have more options for glaze. 

Is this the wollastonite clear you mentioned?

gertsley borate 21

wollastonite  8

neph sy   30

EPK  10

silica 325     31

 

         total 100

D5A89ED3-360C-47F8-BB62-21C6A424FD27.jpeg

8DA45609-1619-4C2F-A97D-6178B0107BB4.jpeg

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Hi Tanvi!

I bought a bag of the black clay for a commission - not sure if I'll get more. I do like the red clay though - definitely will continue with red - and the Wollastonite Clear is the only clear I've tried (so far) that works on it. It also looks good on the buff clay, it applies well (at sg ~1.41 and gelled to a "three" with Epsom salts*) and wipes off easily.

Am glaze firing this weekend, will post more Cassius Basaltic pics afterward.

 

Here's a thread on the Wollastonite Clear; Bill posted some analysis there:

https://community.ceramicartsdaily.org/topic/20620-does-anyone-recognize-the-source-of-this-glaze-recipe/

and one of my posts on that thread:

 

Recipe you posted is exact match for "Wollastonite Clear" from a book I borrowed last year - not sure which one now* - credited to Bethany Krull; I see same recipe in a few other places...

Any road, I did choose the glaze as one of the four clears to try out.

I'm planning to keep it (pending the results of more testing), particularly for Aardvark SRF, as it clears the micro bubbles better (much better - no bubbles at all) than the other clears I'm trying. SRF has a high absorption rate, yes, however, the colour is beautiful, it throws and walks really well; it's on my keeper list!

The other clears I selected, a variant of Kitten's Clear and two of Tony Hansen's mid fire low expansions, don't clear the bubbles on SRF; all four craze over the bmix and lighter red clay. The lower expansion crazes less - more work to do there, or phase out the bmix and light red clay.

*The book, as I recall, had pictures of many artists' work, and recipes for all glazes; I liked the look of the glaze ...hmm ...our library doesn't retain checkout history, hmm ...my guess is that it was this book!! Glaze-The Ultimate Ceramic Artist’s Guide to Glaze and Color by Brian Taylor and Kate Doody

 

* sg (specific gravity) is repeatable, given graduated cylinder, accurate scale, and good technique. "Gel" is a bit harder to gauge, so, I turn the glaze with a kitchen whisk until the whole mass is spinning at a quick "one two three" per turn, then watch it slow to a stop and bounce back where there are about three turns to a full stop, and significant bounce back.

If you're adjusting thixotropy, add just a smidge of Epsom salt (dissolved in water) or vinegar at a time, mix thoroughly, and observe the behavior changes - how the mass shears as it slows down, how many turns it takes to stop, what happens when it stops ...and how the glaze behaves on your work!

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I noticed something interesting when looking at these recipes. @Hulk, can you confirm I've got the right recipe for Wollastonite Clear below? If it is the correct one then have a look at that formula versus the Hansen G2926B. Slight difference in the K2O and Na2O but the overall KNaO is the same, other fluxes and boron are identical. The Wollastonite version has less silica and alumina so it's going to be a more fluid glaze than the G2926B.

134421048_ScreenShot2020-05-12at8_56_16AM.png.30d005f5b79a161f319d6d44ae7ba5bd.png

Have a look at the LOI for the Wollastonite Clear, it's 7.9  Since we know that reducing the LOI helps with bubble reduction then it seems logical to replace the gerstley borate with a frit. Recipe is going to be very similar to the G2926B but with less silica and alumina (since I think it's based on the Wollastonite Clear). The one below is using Ferro 3134, LOI has dropped down to the same 2.9 as in the G2926B. In theory it should melt sooner and more than the gerstley version this recipe with less bubbles. So following this train of logic you could reduce the silica and alumina, while still be in the well supplied range, to make an even more fluid glaze, tradeoff would be the COE is going to raise so more chance of crazing.

610498155_ScreenShot2020-05-12at8_59_52AM.png.8b8046890d8d3ab06f6a36077f4ae0b9.png

 

 

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Hi Min!

That's the Wollastonite Clear recipe!

I have a bucket of G2926B, which started off per Hansen's recipe, then I tweaked it to reduce expansion, both against my "other" challenge, crazing on white, café, and Sedona red clays. I've abandoned that thread of inquiry for now. Interesting, both G2926B variants cloud over the red and black clays, and show a lot more bubbles! For now,G2926B is on the "use it up eventually where it works" end of the shelf.

Don't know why the Wollastonite Clear clears the bubbles better, it just does!

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5 minutes ago, Hulk said:

Don't know why the Wollastonite Clear clears the bubbles better, it just does!

It will be a more fluid glaze as the silica and alumina levels are lower compared to the G2926B. Stiffer the glaze the harder for bubbles to clear.

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The G2926B is actually the clear that I initially tried and it bubbled like crazy. The recipe posted by Bill fared a bit better. Ill try the Wollastonite clear next and see what comes of it.  Thanks everyone ! This has been so informative. 

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1 minute ago, tanvi504 said:

The G2926B is actually the clear that I initially tried and it bubbled like crazy. The recipe posted by Bill fared a bit better. Ill try the Wollastonite clear next and see what comes of it.  Thanks everyone ! This has been so informative. 

The clear I gave you is a wollastonite clear. With this recipe however you can increase the Gerstley to increase the boron to get it to melt sooner as opposed to the recipe you had which had a bunch of Boron already and a giant amount of silica in it.  That is why it is important to determine if it a melt issue. So, if you feel the recipe I gave you melted better, then increase the Gerstley to 20 to get it to begin melting near cone 3  And see if that improves it further.

If you can , also Test glaze something at the same time that is white as well, just to confirm that your mix is not contaminated.

If it does work even better then you have a melt issue.  So then on to solving the color change with this glaze or a different one that melts closer to cone 3. This glaze is very clear so if the brown is not the result of iron in your mixed products then we will assume some component of this glaze interacted with the clay body and move on from there.

picture of this glaze on a very white body below. As you can see, it is a very transparent  colorless smooth gloss. The wollastonite used in the recipe was a means to produce  a very durable glaze.

E722A442-839C-414B-B3F7-30C25216474D.jpeg

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1 hour ago, tanvi504 said:

The G2926B is actually the clear that I initially tried and it bubbled like crazy. The recipe posted by Bill fared a bit better. Ill try the Wollastonite clear next and see what comes of it.

I was just trying to point out in my post above with the charts that G2926B and Wollastonite clear are basically the same glaze, the latter having less silica and alumina. Looking at the formula rather than the recipe you can see this. 

I like Bill's idea of adding more gerstley to his recipe, definitely room for more in the formula. The last recipe I posted is a reworked version of Toms Wollastonite Clear (B. Krull) recipe using frit instead of gerstley.

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  • 2 months later...

Ahh, din't post the Cassius Basaltic pics I'd promised.

Top left, test tortilla warmer (the "street taco" size tortillas fit with a bit of room to spare) - the clay fires to a tight smooth finish; the blue (on the lid, Lakeside Pottery clear blue) behaves well. Top right, the Wollastonite clear, per prior, heals over quite well and clears the bubbles, although the healed over pores are still discernable.

Bottom left, again, Wollastonite clear liner, BVG Rutile Green looks rad, eh? The streak on the cup is from a rivulet of the clear I left on thar.

Bottom right, a small (and weirdly handled) test cup, Wollastonite clear liner, commercial (premix) light blue.

Several of the Cassius pieces in that load have a bloat spot, ranging from a ~2mm raised bump to ~6mm bump with rupture. The next bisque, I ran much longer at the critical temp ranges, we'll see how that works when next glaze load gets fired ...from there, may not buy any more of this clay - it does look so cool, throws nicely, fires smooth and tight, and I have some glazes that will work - on t'other hand, it's expensive, and prone to the bloaties.

 

cassius four.jpg

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