Problem with Stoney Matte Glaze "Disappearing"

[Rick Wise]

I need glaze advice. 

I have a white stoney matte glaze that is usually reliable and produces a nice .... well ... stoney matt finish.  The recipe is posted below.   But on seemingly random occasions -- and mostly on small flat pieces rather than, say, mugs -- the glaze seems to more or less disappear in the glaze firing.  They come out looking as if there was no glaze applied .... or with some amount of variation between some glaze and none at all.  It is as if the glaze just burned off leaving a bare rough surface.

Would that be the case perhaps?  If a glaze firing heats a pot's surface up more than intended would you expect a stoney matte formulation like this to burn off in a Cone 6 electric firing?

 

Material	Amount
Nepheline Syenite	42.5
Kaolin	24.5
Whiting	17.5
Talc	14.2
Silica	1.3
Total	100

[Dick White]

Can you show us a picture of one or a few of these failures? Kinda hard to say without seeing it.

 

[Magnolia Mud Research]

57 minutes ago, Rick Wise said:

I have a white stoney matte glaze that is usually reliable and produces a nice .... well ... stoney matt finish.  The recipe is posted below.   But on seemingly random occasions -- and mostly on small flat pieces rather than, say, mugs -- the glaze seems to more or less disappear in the glaze firing.  They come out looking as if there was no glaze applied .... or with some amount of variation between some glaze and none at all.  It is as if the glaze just burned off leaving a bare rough surface.

Would that be the case perhaps?

How thick is the glaze applied to the "small flat pieces"?  
several of my cone 5 and cone 10 glazes require a thicker application to not look like no glaze at all;  there is a detectable sheen on the fired surface but the coating looks clear as water.    Before going on a revamping of the recipe of the glaze, test using various thickness of the application:  One dip, two dip, three dip, four dip, five dip, etc. and get some data to work with before going further.     break the fired pieces and look at the cross section to how the thickness builds up.  

LT
 

[Min]

Is this a cone 10 recipe that's maybe been fiddled with a bit? Odd to see a cone 6 glaze with zero boron or zinc. 

"But on seemingly random occasions -- and mostly on small flat pieces..." and "They come out looking as if there was no glaze applied .... or with some amount of variation between some glaze and none at all.  It is as if the glaze just burned off leaving a bare rough surface."

Wondering if it's simply under melted. 

[Crooked Lawyer Potter]

I want to emphasize that 98 out of 100 times it comes out great. The problem seems random but i suspect is somehow related to excessive heat. I always glaze by dipping and it is submerged long enough to pretty much max out (say, ....maybe a 15 count). 
Below are pictures. The pics are from left: pic of the 3 examples, closeup of the one on right, closeup of middle one, and close up of one on left. Center and right did fine. They have the feel of a smooth river stone. The one on the left is a problem example. Look at its closeup — see how its bare except for some whiter  streaky areas where some glaze remains. 

[Bill Kielb]

@Crooked Lawyer Potter
Wow, I like a count of three, one - one thousand .......  a count of fifteen seems fairly long, do you establish a specific gravity for your glaze that works well for you and if these are fairly thin glazes, say 160 SPG, any chance they sometimes really need some aggressive stirring action before  every dip?

[Rick Wise]

I keep SG at between 140 and 150.  And they are always well stirred and flocculated.  I like the long count on the theory that it takes up whatever it can and therefore I dont have issues about was it "too long" or "too short".   Also, when I have to dip twice to cover a larger piece I get less problems with overlap. (That's my thinking anyway.)

I was under the impression that a glaze may change from gloss to matte (or vice versa) depending on temp its fired at.  Is that not correct?  If correct, what about accidental over-firing as the reason it goes from stoney matte to STONE!

[Min]

48 minutes ago, Rick Wise said:

I was under the impression that a glaze may change from gloss to matte (or vice versa) depending on temp its fired at.  Is that not correct?

If the glaze is matte because it's simply underfired then yes a gloss glaze can appear as a matte glaze. Correctly formulated and fired mattes will stay matte even when overfired (to a degree). I think the white streaks in the first pot are simply from a heavier application where you have glaze drips.

Is the extreme crazing I'm seeing an issue for you? Not surprising considering about half the glaze is made from Nep Sy plus theres a fair amount of whiting in it. Still think this glaze is underfired for cone 6. There is no boron and no zinc. Calcium (from the whiting) and magnesium (from the talc) with no boron or zinc are high fire fluxes, need some boron or zinc to have them work at cone 6.

[Hulk]

Per Glazy.org, recipe from Louis Katz' HyperGlaze db, range indicated 5-8. I haven't checked calculated loi; my guess being loi (burn off) is low.

The problem pic doesn't include bare clay (for comparison); the surface looks dimple-y and shinier than typical bare clay. Could be a thin coating? How do you check/verify glaze thickness (after application, before firing)? The edge of the dip would likely be much thicker on the gravity side, and much thinner at the "top"; I place a small square of tape somewhere in between. An eighth turn makes it a "diamond" - easily picked off when the glaze is mostly dry. I'll either wax before picking the tape, then fill with an accent colour, or just leave the bare spot.

Looks like that glaze doesn't quite fit - medium craze pattern on the grenade and mug pieces, looks same on the problem piece where thick(er). Are you lining with the same glaze?

Are the problem pieces thinner? From same bisque as non-problem pieces? Same clay, finished same (burnishing, etc.)? From same shelf - in glaze fire - as non-problem pieces? Are you using cones?

You might have some luck testing to recreate the problem case...

[Min]

43 minutes ago, Hulk said:

Per Glazy.org, recipe from Louis Katz' HyperGlaze db, range indicated 5-8. I haven't checked calculated loi; my guess being loi (burn off) is low.

With all due respect just because a glaze is posted from a database on Glazy doesn't mean the info contained (ie firing range) is correct or the recipe is suited for making a durable glaze.

[Hulk]

Agreed.

The posted recipe is in Katz' "all glazes" list. That's a long list - looks familiar, likely was researching a glaze name; lots of cone ten glazes, some have notes, which could be helpful. I might see it again via string seachin' a glaze name. Any road, it's a match, so "Stoney Matt" may have come from there, or shares common predecessor.

GlazeMaster calculates 12.42 for loi, bein' curious and on the couch so far today, which for reference, doesn't seem terribly high, for my two clears are 7.5 and 9.1, others from as low as five, up to over fifteen. ...silica to alumina looks low, expansion calculates high - not for my studio, "crazed" enough. 

There are few glaze recipe sources that I trust, even then, testing required. I started out comparing local JC ceramic lab mid fire recipes against reading in Tony Hansen's site, Hesselberth's site, this forum, Susan Peterson's book, Van Gilder's book, several other books and magazines ...many tests and trials yet to do, have found some well behaved, attractive, durable glazes, and some not so much, so far.

[Rick Wise]

What is loi?

[Magnolia Mud Research]

3 hours ago, Rick Wise said:

What is loi?

In normal ASTM industry analytical jargan "loi" means "Loss On Ignition"; meaning that this amount of mass is converted to vapor when heated to the temperature of the testing; I haven't a clue as to the meaning of the studio ceramics glaze software output jargon.   

LT
 

[Hulk]

Reflecting on, (dreaming through) the glazing, err, my glazing process, when the bisque is dipped, the clay absorbs moisture from the glaze, hence, there's a layer of glaze right next the clay that's been dried a bit - it behaves differently on account of, yes? There's a clingy/sticky bit there, yes? Then, the longer we wait, more moisture is absorbed by the clay, and the clingy layer gets a bit thicker. From there, there's a point where the clay is saturated, hence the clingy layer might be maxed out around there. Yah, but wait, am thinking I don't want to get to that point, on account of when I withdraw the ware, I want the clay to still be pulling moisture, so glaze sheets off the work, then, sha-zap, the layer begins to gel, then, it's gel! In a few more seconds, the sheen starts to fade - nice even layer of glaze. I believe there's a timing thing there. I wait for the liner to fully dry before doing t'other side as well, same idea.

OK, back to "wait" - soaking the ware in the glaze well after the point of clay saturation, the clingy layer reverts to same wetness as the bucket, pick up the ware, the layer - not clingy anymore - sheets off, leaving a thin wash. I'll give this idea a test, but not today - not glazing today. May I suggest verifying your thickness.

I'm guessing the glaze layer is thin.

LOI: loss on ignition, i.e., how much stuff burns away in the firing - it's a calculated value based on the formula. One could weigh/test to determine actual loi, however, that would involve separating from the clay, or - less accurate - accounting for loi of the clay.

[Bill Kielb]

21 hours ago, Magnolia Mud Research said:

haven't a clue as to the meaning of the studio ceramics glaze software output jargon.   

 

To me it seems often to end up to be nearly the  water and chemically bound water.

[Min]

In addition to the LOI from dehydroxylation there is also LOI from carbonaceous and tramp materials burning out plus carbon dioxide gas. In this glaze the biggest contributer to LOI is going to be the calcium carbonate (aka whiting) as it has an LOI of nearly 44%. The LOI from the whiting can be significant if the glaze also employs early melting frits or gerstley borate. Since calcium carbonate doesn't start to off gas until after the early melting frits  and g.b. have begun melting there can be issues with glaze bubbles and blisters depending on glaze viscosity and fluidity.

[PeterH]

16 hours ago, Min said:

In addition to the LOI from dehydroxylation there is also LOI from carbonaceous and tramp materials burning out plus carbon dioxide gas. In this glaze the biggest contributer to LOI is going to be the calcium carbonate (aka whiting) as it has an LOI of nearly 44%. The LOI from the whiting can be significant if the glaze also employs early melting frits or gerstley borate. Since calcium carbonate doesn't start to off gas until after the early melting frits  and g.b. have begun melting there can be issues with glaze bubbles and blisters depending on glaze viscosity and fluidity.

Digitalfire's page on LOI has a graph of %decomposition vs temp https://digitalfire.com/glossary/loi

[Bill Kielb]

On 2/11/2021 at 9:13 AM, Rick Wise said:

Also, when I have to dip twice to cover a larger piece I get less problems with overlap. (That's my thinking anyway.)

I was under the impression that a glaze may change from gloss to matte (or vice versa) depending on temp its fired at.

Your method is what works for you, but I would try decreasing the dip time, just in case my pot was becoming so saturated with water that I was losing the larger suspended components of the glaze resulting in an application where some areas of the pot did not include all the glaze components evenly. Might be worth a try and some tests.

with respect to gloss / matte my understanding would be for true mattes then si:al ratio is a good indicator of what the chemistry should fire to. So in general under UMF or extended UMF  Si:al ratios in the 5:1 or lower range ought to fire matte and even over fired would become runny mattes. So these would be true mattes. Complicating things though would be underfired glazes appear matte and slow cooling to give time for crystal growth also can create the matte look. The latter being very popular with many potters. A true matte, generally is always matte and goes from matte to gloss just by progressively adding more silica to the recipe.

[Rick Wise]

Thanks Bill. I guess i need to re- think this. 

[Bill Kielb]

2 hours ago, PeterH said:

In addition to the LOI from dehydroxylation there is also LOI from carbonaceous and tramp materials burning out plus carbon dioxide gas.

Just an offhand question, Any chance you have a compositional apportionment on the LOI? I have seen tests but can’t seem to locate a source and it’s been quite some time.

[Hulk]

11-FEB-21 post, I was supposing:

OK, back to "wait" - soaking the ware in the glaze well after the point of clay saturation, the clingy layer reverts to same wetness as the bucket, pick up the ware, the layer - not clingy anymore - sheets off, leaving a thin wash. I'll give this idea a test, but not today - not glazing today. May I suggest verifying your thickness.

I'm guessing the glaze layer is thin.

Got around to glazing last week - I had a few test tiles (trying a new clay) to glaze, so tried holding one in the glaze for about twenty five seconds (right, single looong dip). The glaze layer came out thicker than the layer I get by my typical test tile dip, which is just a few seconds, two angled dips, such that the middle is a shade thicker than the ware gets, and the edges thinner (left). Perhaps holding the dip even longer would cause the layer to sheet off, idk. I did try wetting a test tile with water, waiting a few moments, then dipping, which did produce the "thin wash."

The triangle is where I'd stuck a bit o' tape - once the glaze has set up a bit, easily removed, revealing thickness of the glaze layer.

I've used the square o' tape (turned 1/8, it's a diamond!)  the few times I've sprayed glaze. For dipping, I'm looking to get dialed on test tiles and a test piece or two before running the "good" pieces through th' process.

Just following up.

[Hulk]

Aand fired

Typical test tile, left, where the overlapped portion in the middle is about the thickness I'm looking for.
Extra thick test tile in the middle - good sign that my liner glaze behaves here*.

The blue tile - dipped a few moments after wetting the corner on the left - didn't catch the light just right; the look is dry and scabby where the glaze is thinnest.

*Trying out a white stoneware, am liking it so far. The clear liner is nth iteration of low expansion, thanks to Min and several others for help. This glaze is going on well, comes out smooth, doesn't mark, seems to be holding up well, and no craze.