White raised bumps after glaze firing

[k77]

This is my first time posting here.  I would so appreciate some input. 

I have been firing using my own electric kiln for a few months now and have been having problems with pin holing.  I've previously used witness cones to verify the temperature of the kiln (I found it does fire hot and have taken that into account), I have slowed down my bisque firing and separately added a hold and slow cooling.  In short, I've tried to follow the advice I've read on this and other forums and despite using various new firing schedules, haven't fully resolved the issue (though it is a little better).   

Today I have a new issue.  A bunch of plates that came out of the kiln are covered in raised white bumps.  Is this pin holing? Or something else?  

For these plates I used Witgert white stoneware clay with 25% fine grog : https://auxcouleursdargiles.be/terres-blanches/1751-witgert-11-sf-blanche-2502-10-kg-4250536100804.html

I glazed using Mayco Stroke and Coat Seabreeze (3 coats applied by brush)

My bisque firing schedule is 60 ph to 200, 100 ph to 700 then 40 per hour to 980. 

My glaze firing schedule is 250 ph to 1160, then 100 per hour to 1260 with a drop to 1160 and a ten minute hold. 

The pots were quickly rinsed in water the day before firing (as they came out of the kiln) and were lightly wiped with a sponge immediately before applying the glaze. 

Can I save these by sanding and refiring at a lower temperature? 

Thanks so much in advance for any input you might be able to offer. 

K

 

 

 

 

[Hulk]

Hi K,

Welcome to the Forum!

Edit/Add: the spots are not craters, they are bumps/lumps!
Scroll down to agglomeration...

Are the white spots "craters" - where the center of the spot is bare clay?
If so, could be that there are particles in the clay that are off-gassing.

Helpful in clearing up fine bubbles (fizzing), cratering, and pinholes:
  Adjusting peak glaze fire temp
  Drop and hold after peak
  Bisque fire adjustments (hotter, longer, with oxygen)

1260 C may be a bit too hot*?
Some clays are sensitive to over firing, others are more forgiving.
If the clay will fully vitrify at a lower temperature, that may help.

A smaller drop, ~55C, then a longer hold - try 30 minutes - may allow the glaze to "heal over" the craters.

Bisque fire with oxygen, extended hold at 815 C may also help, especially if the defects are more prevalent where the clay is thicker.

I'm not finding shrinkage and absorption details on that clay body.
The linked page indicates max 1300 C - not sure what "max" means in that context - could be the point where the clay begins to slump?
The hottest sample is labelled 1240 C (1260 C is much hotter than that!)
Technical data sheet here: Ready-made plastic bodies - Arno Witgert seit 1820 (witgert-tonbergbau.de)

Any road, consider placing cone packs on each shelf (a half a cone hotter/cooler can make a big difference), keep detailed notes.
Please do post updates!

*Looking at the "Water Absorption %" numbers, the 2040A clay, for example, is is 11.1% at 1100 C, 1.5% at 1200 C, 1.4% at 1300 C
In that case, likely not worthwhile to fire over 1200 C

The 11 (plastic body) is 2.8% at 1200 C, .6% at 1300 C
In that case, firing to ~1260 C may be about right.
 

Edited March 26, 2023 by Hulk
lumps/bumps, not craters, my bad

[k77]

Hi Hulk, 

Thanks so much for taking the time to answer (and in so much detail!)   

Although the clay is not poking  all the way through the glaze, my sense is that the bumps are coming from the clay - rather than from the glaze.  In which case, off gasing seems likely.  I will try and change the firing schedule somewhat and add a hold in the bisque as you suggested.

The possibility of it being too hot makes sense to me, it crossed my mind when I saw the results.  However.....

Having read your reply, I am now somewhat confused about the correct technical data for this clay.  In fact I bought the clay from Cigale and Fourmi that lists the water absorption as 1.3% @ 1300. https://www.cigaleetfourmi.fr/terres-a-gres/3996-gres-blanc-w11-chamotte-25-0-02-mm-sac-de-10-kg.html

Confusingly, when clicking on the link to the technical data on that page, the information is different.   (2.7% @1300).  

Another of my suppliers has the same product but lists the water absorption of the clay as 1.2% at 1260.  https://www.ceram-decor.fr/en/ARGILES/8989-GRES-BLANC-CHAMOTTE-25--002---W2502-.html  I'm certain this is the data I relied upon when choosing the firing temperature.   Knowing that my kiln fires hot, I chose 1260 in order to have proper vitrification and achieve porosity of around 1%.   The technical details on that page list the water absorption as:

                      1000°C : 13.1%
                       1100°C : 7,2%
                       1200°C : 2,4%
                       1300°C : 0,7%

I can't find the technical details for this clay on the Witgert link you sent.  It seems I need to resolve this, as possibly a lower firing temperature might be the answer (subject to achieving vitrification).

I'd welcome any other guidance or suggestions.  

Many thanks!

Katherine

 

[Callie Beller Diesel]

Hi and welcome!

Raised bumps are not pinholing, no. Unless the bumps are hollow, they’re not from off-gassing.

The first thing that I’d check is if there’s anything that could have fallen into the pieces from the kiln shelves above them. If you’re using sand to place pieces on, or if there’s kiln wash dropping from above, that’s the first place I’d look.

It took me a bit to decipher the spec sheet because of the brochure style layout, so I’ll transcribe here for the rest of the discussion.

EDITED TO ADD:  this clay body seems to be the one without grog, not the one with. This will affect water absorption and probably shrinkage. 

Assuming it’s plastic body #11 on page 6, then the specs are:

Analysis: 

SiO2: 73.4

AlO2: 21.6

Ti.02: 1.2

Fe2O3: 1

CaO:  0.1

MgO2: 0.3

Na2O: 0.1

K2O: 2.3

LOI: 7.0

(If anyone wants mesh numbers, let me know, they’re on there, but I don’t think they’re necessary yet.)

Plasticity: 24.4, Modulus of rupture 4.6,

Dry Shrinkage 6.2

Fired Shrinkage:

1000C: 2.1

1100: 4.5

1200: 6.1

1300: 6.6

Water Absorbtion:

1000C: 11.8

1100: 7.2

1200: 2.8

1300: 0.6

 

If the bumps aren’t something that’s falling from above, the only other thing I can think of is a reaction between your clay that matures at 1300 C (cone 12) and your chosen glaze, which was only formulated for 991 C but happens to not run at all. Stroke and Coat is a weird animal. 

It could also be reacting with pieces of grog that are close to the surface on your pieces. Did you trim the surface of your plates or was there some grog on the surface that wasn’t burnished in?

[k77]

Hi Callie, 

Thanks so much for taking the time to help me!

I will check the kiln shelves, but I would be surprised if this is the issue, it hasn't been an issue before......

I didn't trim the surface of the plates as such (I hand build), but...... I did use a metal rib to smooth out the clay.  Now I think about it, this did expose the grog on some occasions (which I did smooth out with a rubber rib - but maybe not enough?)  Perhaps I should only use a rubber rib for this clay?? I usually use clay without grog so this is something I perhaps haven't been paying enough attention to. 

I'm certain this clay is the one with grog.  It is part of the W11 series.  W11 is without any grog.  Then W11 25 02 has 25% grog at a a size of 0.2. 

Seems I need to pay attention to how I'm making the plates so as not to expose the grog.  But do you have any other thoughts on the firing temperature.  I've been surprised to realise that the suppliers list such inconsistent results. 

Thanks again

K

 

[Callie Beller Diesel]

If it’s your forming method, see if burnishing the grog back into your clay a bit with a silicone or other soft rib helps.

Another easy test would be to see if the version of your clay without grog also causes these lumps. If it doesn’t, there’s your answer.

Another thing I’d advise, whichever clay body you wind up with, is to do a porosity test to confirm your clay’s water absorption at your chosen firing temperature. In an ideal world, it should be under 1% for functional ware. If it’s less than that, you may find that things like coffee cups will weep.

[Callie Beller Diesel]

@Hulk K mentions in their first post that the bumps are raised. 

If it’s any consolation, I spent half an hour trying to figure out what might be causing pinholes before I re-read the original. I found the reference to the possibility of the clay body drawing flux from the low fire glaze on the digitalfire article on pinholing though. Which is why I thought it might be the grog.

[k77]

They seem more like bumps to me.  As if there are grains of sand under the surface of the glaze in places.  There are however also one or two pinholes, which is something I've been battling for a while.  These white bumps seem different from the problem I usually have! 

[Callie Beller Diesel]

@k77 they could be related. According to the  digitalfire article that I linked to in the post just above. grogged clay bodies can create more off-gassing issues because of the air pockets that the grog creates. This effect can be worsened if the glaze you’re using is not very fluid and don’t release gasses easily. That last describes Stroke and Coat very accurately. Stroke and Coat glazes are surprisingly not mobile for something that should be very over-fired at the temperatures you’re using.

In North America, it’s best practice to match the glaze maturation point with the clay body you’re using because it prevents a LOT of problems. If you had been local to me, I’d have said to not use that clay/glaze combination at all. But we also have more stoneware clay bodies that are formulated to mature at 1200C/ cone 6. We notice here on the forum that suppliers in most other places tend to tell their customers to just use their high fire stoneware clays at medium range. It’s fine to do IF you are making sculpture and don’t have to worry about food grade durability, OR if you’re willing to do a lot of glaze troubleshooting to make it work. But it’s a lot less work to find a similar looking glaze that will mature at the same point as your clay, and fire both to that top point. 

[k77]

Yes Callie, this makes sense.  

It's interesting what you say about Stroke and Coat being less mobile - I hadn't thought about that.   Compared with other commercial dipping glazes I have tended to find that it pinholes less (which is why I started using it -  even though to be honest, it isn't very economical!) However, that was on a different clay body (no grog).

You are absolutely right about the lack of choice for clay bodies here in Europe.  I'm British, but learnt ceramics in California.  As a result, I tend to rely on lots of resources in English.  Here in France, I've had the task of trying to translate everything, but also have discovered some things just aren't available.  I haven't found any clays that mature at a lower temperature.  The choice appears to be between Stoneware (gres) and Earthenware (faience) only.  Nothing in between. 

The plates I've already made, can they be saved? Sand and refire to lower temperature? 

 

[High Bridge Pottery]

1 hour ago, Hulk said:

looks like concave pits to me, particularly the lower right one

I think it's just an optical illusion making them look like craters. If you look at the rim in the lower right the light source is also coming from that direction and the rim is casting a shadow onto the plate.

 

They don't particularly look like blisters to me. I am guessing you would have noticed all the bumps when glazing if it was some chunks in the glaze but worth sieving through 120mesh just to make sure. Maybe you caught a kiln brick putting a shelf in and knocked some debris onto the ware? You could try sanding them, might give you an idea if they are solid chunks or filled with air?

Edited March 24, 2023 by High Bridge Pottery

[Callie Beller Diesel]

The only way to know is to try it on one and see what happens. When you sand them down, look at the inside of the lumps to see what that tells you. Is it hollow? Does it look like a grog spec? See what you can see.

In terms of picking a clay body, I can tell you what I’d do. That Witgert brochure is translated to English and Spanish it has a LOT of really handy information that I wish all suppliers gave out. I think there could be some candidates if you can get your hands on the right one. I’d suggest getting a hard copy of that sheet and lining it up properly though, lol!

If you don’t go with a Witgert clay, look at the spec sheet from the manufacturer, not your supplier. Especially when translations are involved or the supplier is using their own product ID numbers that may differ from the maker, the manufacturer is probably more accurate. Check the top temperatures and the absorption rates of the earthenware clays. There are a few on the Witgert site that seem to be labelled as earthenware, but have absorption rates listed at <1%  at 1200 C. Those are your candidates for cone 6 tableware. You could maybe fudge 2%, but make sure your glaze passes a hot/cold shock test if you don’t want coffee mugs with slow leaks. You may also be able to push a clay that’s close but not quite, but be sure to take precautions so you’re not wrecking kiln shelves when you test. 

Stroke and Coat is meant to be very user friendly for kids and other teaching situations. It goes on like paint and doesn’t move. The colour you’ve chosen is pretty easy to replicate in a lot of different glaze bases.

[Min]

7 hours ago, k77 said:

Then W11 25 02 has 25% grog at a a size of 0.2. 

The clay has 25% grog that you are using? If this is correct that sounds more like a sculpture body than one suitable for plates. 

[Kelly in AK]

It’s odd, for sure. The only times I’ve had something like this is when some material in my glaze was not ground fine enough or not dispersed well enough. Sieving it well fixed the issue. I’ve had tin and wollastonite do this. But you’re not making your own glaze, that’s what catches me off guard. For the people considering this problem, could materials agglomerate or crystallize in a glaze that’s sat around long enough and cause this? 

[k77]

Thanks again Callie. That's a really helpful suggestion in terms of finding clay bodies. 

Hi Min,  Yes it has 25% grog but it is really fine (0.2mm).  It is called 'impalpable' on the website.  I switched to this clay after finding that my usual clay body (without grog) was tending to crack.  I hoped that this would give a little more strength and prevent the cracking.   Am I mistaken in doing this?  

[Min]

I believe 0.20 mm is a 70 mesh grog. Like others have said I would sand down or break a pot open and see what is below the bump. I've used clay that has 70 mesh silica sand added and found a few bits that are much much larger than what would go through a 70 mesh screen so it might just be some larger bits of grog that are protruding from the fired clay as the clay shrinks during the glaze firing. If you have a glaze sieve I would also do as others have said and run the glaze through the sieve and see what is left in it.

[Min]

18 hours ago, Kelly in AK said:

For the people considering this problem, could materials agglomerate or crystallize in a glaze that’s sat around long enough and cause this? 

I've had glazes with wollastonite agglomerate in the bucket over time, I usually find it had done this when I go to dip and pot and find a raised hard bit on the surface. The bits are little, about the size of a pinhead. If the first pot shows one of these then I re- sieve the glaze and usually find a few more. I would hazard a guess that they won't fire out but remain as a hard little pinhead sized bit on the fired glaze if they aren't removed.

[k77]

Hi all, 

A quick update.  I sanded the pots and refired.  The bumps were white and hard in texture.  Somewhat difficult to remove.   I was able to cover them with glaze, but the finished results (where my brush strokes are visible) are not to my liking.  

I have put a fresh load in the kiln today (that I glazed a few days ago). I noticed when putting them in that there were some hard dots on one of the plates.  These wouldn't bother me in a dipping glaze (they would melt and blend in) but I'm wondering if in Stroke and Coat, with its lack of movement, these are the culprit?  Maybe they are bits of dry glaze that I've picked up during a long painting session? Maybe they are small pieces of dry glaze from the bottle (I do try and get every last bit out!) 

So, my working theory for now is that these lumps may actually be from the glaze. This time I've rubbed them out with my finger before firing.  Hopefully I'll see better results after this firing! 

K

[Roberta12]

@k77 I have had a similar issue over the years with clear.  I think my problem was what @Min described.  I now sieve everytime.  And if I still have an issue I use a dremel with a diamond tip to grind that bump down.  Then I apply more clear and refire.  I have been successful with that method 99% of the time.  However, that was with a clear glaze.  With Stroke and Coat, you may not have a successful result.  But I did have to grind the bump down to the surface of the plate.  

Roberta

[PeterH]

On 3/25/2023 at 3:03 AM, Kelly in AK said:

For the people considering this problem, could materials agglomerate or crystallize in a glaze that’s sat around long enough and cause this? 

This thread may be of interest.

 

[Callie Beller Diesel]

While there are some commercial glazes that have lumps out of the bottle that will melt and smooth out, Stroke and Coat isn’t one of them. So, picking any visible bits off any unfired glazed pots is a delightfully simple solution to this problem!

If the bottle has got a lot of dried stuff clinging to it, depending how bad it is you might want to clean it off or reincorporate the bits back into the bottle. You might have to sieve it, or spend some time breaking up the lumps with the back of a spoon.

Stroke and Coat was designed to be used in teaching settings with children, or in paint your own ceramic places. Scenarios where you need something colourful and beginner proof. Most people aren’t used to working with how unintuitive glaze application is but they’ve worked with paint, and in these intro classes you need to get good results in a short timeframe. So the manufacturer adds dye for a what you see is what you get experience, and the glazes get glossy but don’t run when someone puts it on too thick. It was discovered pretty quickly that a lot of the colours could be successfully fired at higher temperatures without too much colour loss, so that’s why Mayco shows info on how to use some of them at cone 6.

The people I now IRL that use stroke and coat on the regular use them at earthenware temperatures. My friends who own a paint your own ceramics place add an extra coat of clear glaze on top as insurance for customers, and the professional artist who uses them does the same to smooth out any appearance of brushstrokes. But I have to stress that’s at cone 06 (1000 C-ish). If you try this solution yourself at the temperatures you’re working at, put a waster slab under a test piece, and check the results for durability if you’re making kitchen ware for sale. 

[Kakes]

I've experienced raised bumps like you describe from what I assume is a clay/glaze mismatch. I use Standard 710 with Mayco alabster glaze (I think - will need to check on this when i'm at the studio. i'll check and will also take a photo) This doesn't happen if I use Standard 420 which is a very groggy sculpture clay. Go figure.

I don't have anything helpful to add. Since i don't make functional ware, i've just decided to use it for the bumps.