Any ideas why these mugs look streaky after holding liquid?

[TaraB]

Hello!

I've just made a new batch of mugs fired to cone 7. (It is walkers stoneware 10 clay which can be midfired ^7-8 or hi fired ^8-10) .

I made a coffee in one so that I could do some product photography, however once I tipped the coffee out there were streaks of discolouration on the inside of the mug. Initially I thought that they may be non-vitrified, so I poured water into several of the other mugs and the same issue is happening. My question is why would these be non vitrified if the correct cone was used and the sitter dropped at the right temp? The firing was no different to any other...? The only new thing I have done with these mugs, compared to my usual firing, was add a gold lustre. I read something about poor ventilation during a lustre causing 'smoking' to the ceramics but I'm not sure what that actually means. I had both peepholes open as well as the top vent hole. I was so excited to launch these mugs but now I'm unsure if they're fit for functional use.

Please let me know if you have any ideas what may have happened and if its possible to sell these mugs! Thank you kindly.

Images show before liquid (no streaks) and after liquid (streaks).

Tara

[Bill Kielb]

I can’t help wondering ….
I assume it’s glazed and can’t help wondering if this isn’t just a coffee stain that comes off with a little scrubbing similar to my glass coffee pot which gets a nice cleaning with a mr clean eraser about once a week to remove the light staining. Having said that cone ten clay fired to less than cone ten usually ends up less vitrified and more absorbent. Looking at the graph for this clay though it’s roughly 1/2% from cone 8-10 so not bad.

For the clay you can always check by doing some absorption tests. My question would be more of why is the glaze staining? Of course the glass in my coffee pot stains and it is pretty durable glass. If anything I would make sure the glaze is a reasonably durable composition which can be tested as well. Folks often make durable liner glazes just to be sure their food wares are as safe as practical.

If the stain scrubs off, then is the glaze durable and are we scrubbing off some glaze or is it as durable as my glass coffee pot? if the stain is embedded under the glaze, the glaze is likely not very durable.

Edited November 4, 2022 by Bill Kielb

[TaraB]

Thanks for taking the time to reply, Bill. I see what you’re saying with the glaze… It’s just odd because I’ve used this same glaze several times before and never had this issue. It’s not a stain as such, because I get the same streaking in the remaining mugs with plain cold (or hot) water. It does seem to be a vitrification problem. I have had them filled and sitting on paper towel for the past few hours and I’m starting to notice very slight dampness on the paper . Unfortunately I cannot retire them as they have gold lustre which I am assuming will not do well in hifire!? 
Can batches of the same clay vary? ie, could this particular bag of walkers 10 be slightly different to my previous bags? I am so stumped as to why this has happened. 

[TaraB]

Re-Fire*

[Bill Kielb]

19 minutes ago, TaraB said:

Can batches of the same clay vary? ie, could this particular bag of walkers 10 be slightly different to my previous bags? I am so stumped as to why this has happened. 

It could be different, the only way to know is test fire  and do an absorption test. I still don’t understand why it is making it through the glaze though. Maybe some really fine crazing but I would expect to see that in the stains. Do they use witness cones to confirm? My only thought is the glaze and clay likely are not mature. Maybe someone here will have a better thought.

Edited November 4, 2022 by Bill Kielb

[TaraB]

The witness cones were bent as normal too. I think I may have thought of a possible cause... The update is, the paper towel became slightly damp, which suggests porosity. The mugs also dried completely streak free after a few hours (they look normal!)... again, suggesting porous glaze.

Then I remembered that the glaze I used was nearly empty. I used the very last drop (brush on clear glaze) and then finished the last few mugs with a new batch of glaze. I have no idea which mugs had the old versus new glaze but I am wondering if I fill them all with water, I may find there are a couple that don't streak. I guess its worthwhile testing this theory tomorrow if it means I may be able to save a couple of mugs. Not sure if this theory is right but maybe the composition of the glaze changed or it became too thin as the glaze/container ran low.

I am assuming I cannot sell the porous mugs?... They aren't microwave safe due to the lustre but will they stain on the raw base overtime and possibly lose shape? If so, they might just have to find their way into my own personal mug collection lol.

Thanks again

[PeterH]

Have you tried an indian-ink test? Easy enough to wipe off if there are no cracks.

[Callie Beller Diesel]

1 hour ago, TaraB said:

I am assuming I cannot sell the porous mugs?

I wouldn’t. No they won’t loose shape. It’s fired ceramic and isn’t subject to dissolving or anything like that. But if moisture can get into the pores of the clay, or even just get stuck between clay and glaze layer, bacteria can grow. Pieces can and do mildew stain like that. If they were mine, I’d toss them. 

We do tend to get a lot of questions about this kind of thing here when folks use a cone 6-10 clay body, and are only firing it to the lower end of that range. Many manufacturers all over the world will sell clays with firing ranges like this, assuming you have a certain amount of technical ability, and not assuming everyone’s making functional ware with it. What I wish they’d mention more is the need to fire to the top end if you are making functional ware. I’d suggest testing some older pieces for staining or delayed crazing if you can. My bet is that it’s been happening the whole time, but these are just the circumstances that brought it to light the fastest. 

[Min]

12 hours ago, TaraB said:

My question is why would these be non vitrified if the correct cone was used and the sitter dropped at the right temp?

Like others have said, it's the firing of the clay temp.

Say you are baking chocolate chip cookies and the recipe says you can bake them to 300F or you can bake it to 400F, does that make sense? Can the cookies be baked through at both temperatures?

Back to your clay, if you are making items not meant to hold liquids and you don't mind them absorbing water then firing to the lower cone might make the clay mature enough for those purposes. But a clay can't be as vitrified as possible at two different ranges. If it is a midfire clay and you fire to highfire then chances are it will have bloating, slumping, brittleness, etc issues. If you on the otherhand fire a highfire clay to a lower range then chances are it won't be as mature as possible and will be immature and underfired. In other words it won't be as strong, will be under vitrified, likely leak and absorb water. This is a problem for functional wares that are meant to hold liquids or will be washed more than a few times. Pots will absorb water which will induce crazing, leak, get hot in the microwave, grow mildew / mold etc. 

Another thing I would look at is your glaze maturing temperature, is it under-fired too? What is the recommended cone for it? If your glaze is a cone 10 glaze then the mugs you currently have could be re-fired to just under that (given they already have had a lot of heatwork). Lustre will burn off and probably leave a purple mark but you might be able to save them. Leave the mugs that had liquid in them to dry in a 200F oven overnight to dry them out before firing them and I'ld suggest firing them on waster slabs in case the glaze runs. Nothing to loose. 

Going forward look for a body that matures at the same cone as what you are firing to. Testing your claybody can be done quickly or in a more detailed method, I'ld suggest the more detailed method whenever you test a new clay.  Link here if you need it on how to do so. 

 

 

[Magnolia Mud Research]

19 hours ago, TaraB said:

My question is why would these be non vitrified if the correct cone was used and the sitter dropped at the right temp? The firing was no different to any other...? The only new thing I have done with these mugs, compared to my usual firing, was add a gold lustre.

my thoughts are more about thinking about the clay body, the glaze, and the firing might be causing the variable appearances when the mug contains liquids such as water or coffee. 

1. if the glaze is completely covering the clay body on the mug and is completed melted and not crazed; how does liquids (water) get past the glaze and cause a change in the appearance of the glaze?  The first approach would be to make sure the application produces a good solid coating. 

2. assuming that the clay body might not be completely vitrified throughout I would fire some mugs without glaze and see if the mugs absorbs water.  @Min used the cooking of cookies analogy on what temperature to fire the clay body in the kiln.  I want to extend the analogy to point out that the mass of a mug is several more than ten times the mass of a "cone" and requires a longer time at the peak temperature than a "cone".  The correction is to either hold longer time at peak temperature, or just shoot for a higher "cone" temperature.  

3. The glaze is supposed to have a "gold luster" visual effect after firing.  Is the "gold" uniformly distributed throughout the glaze? in other words: is the differences are about the "gold" rather than about the other possible reasons?  easy test is to fire several mugs with and without the "gold" located in the kiln side by side see what happens?  

4. my ceramics professor long ago pointed out that application is 99% of the glazing;  therefore think about how application of this set of mugs was different from those made previously. 

LT 
 

[Babs]

17 hours ago, TaraB said:

Thanks for taking the time to reply, Bill. I see what you’re saying with the glaze… It’s just odd because I’ve used this same glaze several times before and never had this issue. It’s not a stain as such, because I get the same streaking in the remaining mugs with plain cold (or hot) water. It does seem to be a vitrification problem. I have had them filled and sitting on paper towel for the past few hours and I’m starting to notice very slight dampness on the paper . Unfortunately I cannot retire them as they have gold lustre which I am assuming will not do well in hifire!? 
Can batches of the same clay vary? ie, could this particular bag of walkers 10 be slightly different to my previous bags? I am so stumped as to why this has happened. 

When did you buy your previous batch?

Walkers did change sites of quarrying. But issues stated above re vitrifying and glaze maturity hold water if you'll pardon my poor choice of phrase

[Rae Reich]

Looks like your glaze is a semi-matte or matte. The most reliable liner glazes are gloss/high gloss. I’ve found that unless you fire a matte glaze pretty high, so it gets a little gloss, it will not act nicely as a liner. Even if not obviously porous, they can eventually absorb odors.