Seattle Pottery Supply - Midnight Black Clay

[Potter3ee]

Hope everyone is staying safe in these crazy times. Ive been experimenting with SPS's Midnight Black Clay. I love working with this clay and it fires to an absolutely gorgeous pitch black. The issue Im facing is with using it for functional ware. There is no glaze that is compatible with it ! Everything leaves tiny bubbles on the surface which im guessing can't be good for food safety? I wanted to make mugs from it with a white or transparent liner glaze on the inside. Anyone have any experience working with this clay?

PS the attached image is from the internet - but its exactly what my fired results are looking like. 

[Mark C.]

Most likely way to much colorants for safe food use. I could not find any info on their web site on this clay except cone 04-cone 5 which is huge range. 

your photo did not come thru

[liambesaw]

2 minutes ago, Mark C. said:

Most likely way to much colorants for safe food use. I could not find any info on their web site on this clay except cone 04-cone 5 which is huge range. 

your photo did not come thru

It's cone 5, I'm pretty sure it's porcelain with black stain.  It's a strict cone 5 because if you go higher it bloats.  The 04-5 isn't the firing range, SPS says the lower cone is the bisque/raku cone and the higher is the maturity.

[Mark C.]

LS -Many of the pure black clays are not food safe do you know about this one as this is from your closest supplier? Their web site does not have much on this body

[Potter3ee]

I fired only to cone 5 as I was told about the bloating above this cone. They list their range cone 04-5 which apparently is the bisque-maturity temperature.  In any case I didnt go above cone 5 but still cant seem to get a clean glaze. I just want to know if anyone has with manganese heavy clays.  Ive attached the picture again I hope it works this time ! 

[Bill Kielb]

4 hours ago, tanvi504 said:

I fired only to cone 5 as I was told about the bloating above this cone. They list their range cone 04-5 which apparently is the bisque-maturity temperature.  In any case I didnt go above cone 5 but still cant seem to get a clean glaze. I just want to know if anyone has with manganese heavy clays.  Ive attached the picture again I hope it works this time ! 

Looks like your glaze has issues. Have you tried several glazes on test tiles?

[glazenerd]

Being from the West coast and given the price of the clay and results. Most likely IMCO Burgundy clay which fires dark brown at cone 5. Add 3% +/- black stain and you have black stoneware. I would recommend bisq firing on slow ramp to 1800F (cone 06) If it is indeed what I suspect: IMCO is an iron disulfide clay with a fair amount of inorganic purifies. A slow bisq will burn off the impurities; which in turn should minimize the pinhole issues. Avoid any glaze with zinc added: zinc reacts strongly to iron disulfide.  If you have a picture of the bottom unglazed area of your piece: it would with identifying the clay. (Problem)

Colored porcelain (black) fires ultra clear.

 

[Hulk]

Hi Tanvi!

I'm having good results on Aardvark Clay SRF and Cassius Basaltic (red and black clays) with Wollastonite Clear (if you search this forum, you'll get some hits on that!), seems to clear the bubbles well, no clouding or frothing either. My bisque schedule isn't particularly slow, however, significant pauses at critical temps (per GlazeNerd, 752, 1063, 1500F) probably help, as well as kiln vent running throughout.

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

 

[Potter3ee]

2 hours ago, Bill Kielb said:

Looks like your glaze has issues. Have you tried several glazes on test tiles?

Hi Bill,

ive tried several glazes on test tiles with no luck. The same glazes that work beautifully with other clays. 
 

2 hours ago, glazenerd said:

Being from the West coast and given the price of the clay and results. Most likely IMCO Burgundy clay which fires dark brown at cone 5. Add 3% +/- black stain and you have black stoneware. I would recommend bisq firing on slow ramp to 1800F (cone 06) If it is indeed what I suspect: IMCO is an iron disulfide clay with a fair amount of inorganic purifies. A slow bisq will burn off the impurities; which in turn should minimize the pinhole issues. Avoid any glaze with zinc added: zinc reacts strongly to iron disulfide.  If you have a picture of the bottom unglazed area of your piece: it would with identifying the clay. (Problem)

Colored porcelain (black) fires ultra clear.

 

Glazenerd,

that’s an interesting suggestion. I dont have access to Imco clays here (vancouver, canada) but I might try a different brown body with stain and see what comes of it. Thank you for suggesting that. 
 

37 minutes ago, Hulk said:

Hi Tanvi!

I'm having good results on Aardvark Clay SRF and Cassius Basaltic (red and black clays) with Wollastonite Clear (if you search this forum, you'll get some hits on that!), seems to clear the bubbles well, no clouding or frothing either. My bisque schedule isn't particularly slow, however, significant pauses at critical temps (per GlazeNerd, 752, 1063, 1500F) probably help, as well as kiln vent running throughout.

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

 

Hulk,

that is a great suggestion ! SPS carries Aardvark clay so this might be something I could try once Aardvark opens again - i see they’re closed due to covid.  
Thanks a lot for sharing that. 

[JohnnyK]

3 minutes ago, tanvi504 said:

 SPS carries Aardvark clay so this might be something I could try once Aardvark opens again - i see they’re closed due to covid.  

Thanks a lot for sharing that. 

Have you tried giving them a call to see if you could order and pick up your materials? Alpha Fired Arts is doing just that here in the Sacramento, CA area, just to stay in business...

[Min]

@tanvi504, have you tried the Coffee clay from Plainsman? I've got a friend who uses it with no issues. Tony Hansen notes both Alberta and Ravenscrag slip glazes work well with it. Greenbarn is still open, you phone or email in orders, prepay then pick up outside the store under a tent.

[Bill Kielb]

47 minutes ago, tanvi504 said:

ve tried several glazes on test tiles with no luck. The same glazes that work beautifully with other clays. 

My curiosity makes me view this as the added stain become refractory and your glazes just wont melt. Add the fact that you can’t exceed cone five makes it even tougher. My thought just for a quick trial would be to add some boron to your glazes (Gerstley, Fritt, 1%, 2%, 5%) to see if this improves things. If yes, then the suspicion that the stain is refractory has some credibility and you could  then move to design glazes to work with this.

[Potter3ee]

2 hours ago, JohnnyK said:

Have you tried giving them a call to see if you could order and pick up your materials? Alpha Fired Arts is doing just that here in the Sacramento, CA area, just to stay in business...

Unfortunately the canada-usa border is closed so pick up is impossible too. Will just have to wait until this passes i suppose. So heartbreaking to hear the struggle other businesses are facing. 
 

1 hour ago, Bill Kielb said:

My curiosity makes me view this as the added stain become refractory and your glazes just wont melt. Add the fact that you can’t exceed cone five makes it even tougher. My thought just for a quick trial would be to add some boron to your glazes (Gerstley, Fritt, 1%, 2%, 5%) to see if this improves things. If yes, then the suspicion that the stain is refractory has some credibility and you could  then move to design glazes to work with this.

my clear glaze is actually a plainsman recipe that uses frit 3124 for boron. Thats the one I tried and it bubbled. 
 

2 hours ago, Min said:

@tanvi504, have you tried the Coffee clay from Plainsman? I've got a friend who uses it with no issues. Tony Hansen notes both Alberta and Ravenscrag slip glazes work well with it. Greenbarn is still open, you phone or email in orders, prepay then pick up outside the store under a tent.

ive tried it and it fires a dark brown. Im after a pitch black. I might actually try adding black stain to plainsman coffee to see what happens. Thanks for suggesting it

[Bill Kielb]

9 minutes ago, tanvi504 said:

my clear glaze is actually a plainsman recipe that uses frit 3124 for boron. Thats the one I tried and it bubbled. 
 

Understood, but these look under fired. I don’t know the recipe but ...  it sounds like you could increase the Boron to see if this melts pretty easily. (More 3124)

Approximate range

• 0.15 Boron generally gets you to cone six
• 0.25 Boron generally gets you to cone two

It would be something that you can try. We have seen similar issues with glazes over heavy underglaze made with stain. The areas become very refractory and many glazes won’t melt or heal, they just look bubbly or even dry, bumpy, bubbled, pitted, most get the impression they are overfired. Add more boron and voila, smooth.

As I said, just something you might try.

[Potter3ee]

22 hours ago, Bill Kielb said:

Understood, but these look under fired. I don’t know the recipe but ...  it sounds like you could increase the Boron to see if this melts pretty easily. (More 3124)

Approximate range

• 0.15 Boron generally gets you to cone six
• 0.25 Boron generally gets you to cone two

It would be something that you can try. We have seen similar issues with glazes over heavy underglaze made with stain. The areas become very refractory and many glazes won’t melt or heal, they just look bubbly or even dry, bumpy, bubbled, pitted, most get the impression they are overfired. Add more boron and voila, smooth.

As I said, just something you might try.

This is the recipe I use - 

https://digitalfire.com/recipe/g2926b

Sorry Im relatively new to glaze chemistry so Id love your advice on this - do you think increasing the frit might help in this case? My clay goes only upto cone 5 and ive fired this glaze on other clay bodies to cone 5 with no problems.

 

 

[Bill Kielb]

3 hours ago, tanvi504 said:

This is the recipe I use - 

https://digitalfire.com/recipe/g2926b

Sorry Im relatively new to glaze chemistry so Id love your advice on this - do you think increasing the frit might help in this case? My clay goes only upto cone 5 and ive fired this glaze on other clay bodies to cone 5 with no problems.

 

 

Yeah, normally I would say raise the Fritt but looking at this chemistry I would say this has an  unusual amount of silica helped along by an unusual amount of Boron. So for this I would normally lower the silica a whole bunch, just  because I view it as a waste of natural resources.  This should still be very glossy with silica lowered  to 10 and  that should help it melt sooner but is not an ideal adjustment. 
Recipes  are very individual so that is  not to say this is good or bad.

Here  is one you can try as well, it was designed to melt at cone five over heavy underglaze and the Gerstley can be increased a few points to lower its melting temp further if needed.

• nephsy 21.88
• silica 23.44
• gerstly borate 15.63
• epk 20.31
• wollastonite 18.75

At some point You might decide on another clay if pursuing this is not your thing. The glaze above Is a high gloss clear that can serve as a nice base as well.

 

[Potter3ee]

1 hour ago, Bill Kielb said:

Yeah, normally I would say raise the Fritt but looking at this chemistry I would say this has an  unusual amount of silica helped along by an unusual amount of Boron. So for this I would normally lower the silica a whole bunch, just  because I view it as a waste of natural resources.  This should still be very glossy with silica lowered  to 10 and  that should help it melt sooner but is not an ideal adjustment. 
Recipes  are very individual so that is  not to say this is good or bad.

Here  is one you can try as well, it was designed to melt at cone five over heavy underglaze and the Gerstley can be increased a few points to lower its melting temp further if needed.

• nephsy 21.88
• silica 23.44
• gerstly borate 15.63
• epk 20.31
• wollastonite 18.75

At some point You might decide on another clay if pursuing this is not your thing. The glaze above Is a high gloss clear that can serve as a nice base as well.

 

That’s great. Thanks for sharing Bill ! Ill give the recipe a go and see what happens. Like you said, I might end up having to switch bodies but I will try everything I can before I do.  

 

[Min]

@tanvi504, what is the bisque schedule you used for this clay, do you use a vent, and how tightly packed is the bisque load? 

Hansen's G2926b is going to be an early melting glaze due to the high levels of boron and sodium, which in effect seals off the body early on in the firing and makes expelling any  off gassing from a dirty claybody problematic. Also it is quite high in both silica and alumina, this is going to contribute to a stiff glaze which will exacerbate the issue of not healing over broken bubbles from off gassing. If you have a really clean bisque firing this glaze just might work but you would probably be better off looking for a more fluid glaze and following a bisque firing for "dirty" claybodies.

[Potter3ee]

4 hours ago, Min said:

@tanvi504, what is the bisque schedule you used for this clay, do you use a vent, and how tightly packed is the bisque load? 

Hansen's G2926b is going to be an early melting glaze due to the high levels of boron and sodium, which in effect seals off the body early on in the firing and makes expelling any  off gassing from a dirty claybody problematic. Also it is quite high in both silica and alumina, this is going to contribute to a stiff glaze which will exacerbate the issue of not healing over broken bubbles from off gassing. If you have a really clean bisque firing this glaze just might work but you would probably be better off looking for a more fluid glaze and following a bisque firing for "dirty" claybodies.

The kiln I fire in isn’t vented and the bisque load was quite full. I used the pre programmed bisque firing schedule on a skutt to cone 06 on medium speed. 

I see your point and thank you for explaining that so well.  Ill try and find a suitable bisque schedule and maybe also try the new recipe recommended by Bill. Looks like my clear glaze recipe only suits the plainsman body and not much else. 
 

Thanks Min ! 

[Bill Kielb]

3 hours ago, tanvi504 said:

The kiln I fire in isn’t vented and the bisque load was quite full. I used the pre programmed bisque firing schedule on a skutt to cone 06 on medium speed. 

I see your point and thank you for explaining that so well.  Ill try and find a suitable bisque schedule and maybe also try the new recipe recommended by Bill. Looks like my clear glaze recipe only suits the plainsman body and not much else. 
 

Thanks Min ! 

For questionable clays, especially high iron red clays you might want to use the slow bisque schedule. The glaze sure does not look melted in the pictures and that glaze chemistry has a gloss ratio -  si:al ratio of over 11:1. In Stull, it starts to trend toward  underfired at 11:1 so I would  expect this to underfire or be hard to melt. Just reducing the amount of silica in this glaze gets it solidly into a high gloss so I am not sure why it has so much to begin with.

[Min]

4 hours ago, Bill Kielb said:

The glaze sure does not look melted in the pictures and that glaze chemistry has a gloss ratio -  si:al ratio of over 11:1

Bill, I don't think the image Tanvi posted is of the G2926B glaze, the image is one taken off the internet. 

On 4/28/2020 at 10:18 PM, tanvi504 said:

PS the attached image is from the internet - but its exactly what my fired results are looking like. 

 

4 hours ago, Bill Kielb said:

Just reducing the amount of silica in this glaze gets it solidly into a high gloss so I am not sure why it has so much to begin with.

Probably because Hansen made this glaze to use on Plainsman 370 which is a difficult claybody to get a non crazing clear to fit. The glaze can take the high amount of silica (and alumina) and still melt well, by doing so it reduces the crazing on this claybody. 

@tanvi504, here is one article on a bisque firing schedule for dark clays.

[Potter3ee]

@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?

[Potter3ee]

@Min I did read that article when you suggested a slow bisque but it requires candling of 12 hours increasing the entire bisque schedule 3x. That’s not something I can afford unfortunately (I rent a kiln from my studio mate) what is your opinion on using a pre programmed slow bisque on a skutt kiln?  Or introducing 30 min hold times at 752, 1063 and 1500 F ?

[Min]

17 minutes ago, tanvi504 said:

@Min I did read that article when you suggested a slow bisque but it requires candling of 12 hours increasing the entire bisque schedule 3x. That’s not something I can afford unfortunately (I rent a kiln from my studio mate) what is your opinion on using a pre programmed slow bisque on a skutt kiln?  Or introducing 30 min hold times at 752, 1063 and 1500 F ?

Go down to the 7th row in the Comments section of that firing chart, the 12 hours is for "The amount of hold time at this temperature varies depending on water content and thickness of the ware." You definitely don't have to hold this long, or indeed at all, if your pots are bone dry. 

This is the Skutt slow program going to cone 04, compare it to the one I linked to above and you can see the differences.

 

[Callie Beller Diesel]

I have some personal experience with Plainsman Coffee, and I can say that it does create a very subltly pinholed texture in glazes that are not a problem with my  usual M390, although not nearly to the same extent you have shown in your original images. I used it with a Ravenscrag based glaze with added silica. My normal bisque is about 11 hours in a packed kiln. I've only gone through a box of it so far, but Coffee seems like a very nice clay to use, and I think a small addition of black stain would push it into that true black you're looking for. If it's something you want to really jump into, Plainsman sells most of its clay bodies in 50 lb dry bags, so percentage additions like that would be very easy to make.