Clear glaze that does not craze on MB

[Christy Ann]

I am looking for a recipe for a clear dipping glaze (Matt or shiney) that plays well with MN clay MB.  The recipes we now use craze for everyone who uses them at cone 6 or 10.  The crazing also occurs with multiple brands of underglazes. I would love it if anyone could share a good clear recipe that works with this clay. 

Edited September 18, 2023 by Christy Ann

[Pres]

I moved this post here from the ICAN area so that @Christy Ann would get more traffic, and it would be more searchable.

 

best,

Pres

[Min]

9 hours ago, Christy Ann said:

MN clay MB.  The recipes we now use craze for everyone who uses them at cone 6 or 10

Not sure what MN clay MB is. Is it this? https://www.mnclay.com/AddToCart.aspx?ProductCode=CLMMB6W 

Are you using the same clay for both cone 6 and cone 10?

9 hours ago, Christy Ann said:

The recipes we now use craze

What are they?

[High Bridge Pottery]

How bad is the crazing? If you post a photo and the recipe it might be easier to tweak what you have if it is close to something you like minus the crazing.

[Christy Ann]

I hope you can see the crazing here. I do not have the recipe because I use the glaze called shiny clear in a studio, and I do not make the glaze.. I use it with a clay called MB from Minnesota Clay at cone 6. The same thing happens at cone 10 with MB, although I then use another clear glaze suitable for cone 10. 

[Christy Ann]

On 9/18/2023 at 10:11 AM, Min said:

Not sure what MN clay MB is. Is it this? https://www.mnclay.com/AddToCart.aspx?ProductCode=CLMMB6W 

Are you using the same clay for both cone 6 and cone 10?

What are they?

I use both MB6 and MB10 and have the crazing problem with the two different clear glazes I use over underglazes. You can see a photo below of the crazing at cone 6.  Other users of these glazes have the same problem on other clay bodies. I do not have the recipe because I am at a studio where glazes are mixed by the head of the studio. I would like to suggest another clear that will work with our clays from Minnesota Clay. B Mix doesn’t serm to have this problem but it is not sold locally. 

[neilestrick]

The absorption rate on the MB6 is way too high. For functional work it should be under 2.0, preferably 1.5. So it's not an ideal place to start. Not sure how they made the cone 6 version have higher absorption at cone 9 than the cone 10 version...

Without knowing the formula of the glaze(s) in question, we can't really offer a definitive solution unless someone here is actually using those clay bodies.

[Kelly in AK]

Neil, you kind of beat me to this, but since I already wrote it...

Minnesota Clay's website says the range of MB6 is cones 6-9. At cone 6 they say it's "vitreous and porcelain like." Also at cone 6 the absorption is listed as 3.7%. Huh? At cone 9 absorption is 2.1%. Their cone 10 version "MB Stoneware," lists absorption of that body at cone 8 as 1.4%. Yeah, the high fire clay is more vitreous at cone 8 than the mid range clay at cone 9. Something funky there. 

Back on topic: The studio head, if they are mixing glazes, uses a recipe. Ask them for a copy. If they are using premixed dry glaze you should be able to learn the supplier and the supplier will have some data on the glaze, though they don't often share formulas. It's possible with a recipe to adjust expansion of a glaze to fit a clay body (not always, but generally).

This clay body though, I'm not so sure about it. Doesn't look "mid range" at all on paper. I do have issues with suppliers who bill clay as mature over a span of four cones, but I'll stop the rant here.

Asking this question directly to the folks at Minnesota Clay would probably be the most direct solution. I suggest, however, you get some clay that is actually vitreous at cone 6 if you're making functional pots in that range.  

[Christy Ann]

I have the cone 6 clear formula our studio is using with MN Clay MB. It is for Shiney Clear:

wallastonite 2250 grams

frit 3195. 2250 grams

epk 1500 grams

flint 1500 grams

if you are able to help us revise the recipe or find a new recipe that works with this clay I would be extremely grateful!

 

[Piedmont Pottery]

57 minutes ago, Christy Ann said:

I have the cone 6 clear formula our studio is using with MN Clay MB. It is for Shiney Clear:

wallastonite 2250 grams

frit 3195. 2250 grams

epk 1500 grams

flint 1500 grams

if you are able to help us revise the recipe or find a new recipe that works with this clay I would be extremely grateful!

 

This is interesting, in that the calculated COE is low enough that I wouldn't expect it to  craze  on any of the clays I use.  Regardless, my usual fix for glazes with too high an expansion coefficient is to add a bit of talc.  I'd begin with maybe 250 g talc added to the amounts you listed as a starting point and go up from there.

[Christy Ann]

We will do some tests. Thank you so very much! This helps 

[Hulk]

The software I'm using (Glazemaster) calculates COE 6.52 for that glaze, which, if all things were equal, would craze on some of the clays I've tried (and no longer use...), and maybe craze on one or more that I use now.

However, all things aren't equal!
My understanding is that:
  a) calculated COE is especially helpful when adjusting a recipe, where the ingredient ratios are adjusted, and/or a new ingredient is introduced;
  b) calculated COE is less helpful for comparing glazes that have radically different ingredients/recipes, where a glaze with a lower number may craze more than one with a higher number.

My own experience supports both notions.

I'd started over several times in my quest for a well-fitting clear.
The sodium and potassium were lowered, and lowered again; talc was raised, and raised again; lithium was introduced - via petalite, close, a bit more talc, a smidge more petalite, voila!'
I've posted the recipe here somewhere; if you're interested, OP, shout. No guarantee it would work with the clay in question! GlazeMaster calculates COE of 5.31

Edited September 23, 2023 by Hulk
correction, minor

[Min]

@Christy Ann, while you are doing your testing I would suggest doing some absorption tests with your claybodies also. Manufacturers post absorption figures but it's best to do your own testing since those posted figures can vary from actual results using our own kilns and firing practices. Info on how to do a calculated absorption test on this link about 2/3 the way down. Given the posted absorption figures are fairly high for the clays you are using and typically ^6 and ^10 work isn't glazed on the bottoms (like lowfire or earthenware usually is), this is relevant. As has already been said the absorption figures for functional ware at mid and high fire should be below 1 1/2 - 2%.

While you can get a well fitting non crazing glaze for pots fresh out the kiln they should still be stress tested for delayed crazing also. When functional pots (with higher absorption) is used it soaks up moisture from use and from doing the washing up. This moisture enters a porous claybody causing it to expand ever so slightly. (especially when heated as in a microwave) When the claybody expands two things happen: one is the clay can become very hot, as in too hot to pick up a mug by the handle, and secondly the glaze will more than likely craze. This is called delayed crazing. Delayed crazing can also happen over time even without using the pot if the absorption is quite high by atmospheric moisture.

edit: to stress test for delayed crazing put some samples (fired in different areas of the kiln) into a 300F oven. Leave them there for 20 minutes or so then plunge them into ice water. On a clear glaze if there is crazing it should show up, if unsure cover it with a felt pen then wipe it off. (or use calligraphy ink if you have it)

[High Bridge Pottery]

I would try swapping some wollastonite for talc and frit 3195 for petalite.

 

Something like - 

Wollastonite - 20

Talc - 10
Frit 3195 - 25
Petalite - 5 
EPK - 20
Flint - 20

[Morgan]

This has been a fantastic clear for me on a few craze prone clay bodies.

https://glazy.org/recipes/55849