Clay Body And Glaze Issues. Help!

[tbaa]

i tested some new glaze recipes recently, and the results were awful. A potter friend who looked at some of the glazed pcs, thought the clay may be the problem, not the glaze. The glazed piece he checked was still porous at the base, and apparently this means there wasn't enough flux in the clay body. I am using fine stoneware clay, and understand feldspar could be added.

How does one do this? Same as adding grog? I can certainly test different percentages.

The bisque firing was done at 940-950 deg. (1750 ish Fahrenheit) The glaze at cone 6. Would it help to re-fire the bisque at a higher temperature? Or use higher temp glazes on the existing pieces?

While I can try adding more flux to the clay body, and test that for new work, I am trying to work out how I can use glazes effectively on the pcs I have.

I hope this makes sense. I am quite new to much of this. Although I've used ready clay and glazes, I am new to mixing raw material for glazes, and have not made my own clay mixture (and hope I never have to!)

[oldlady]

you have not mentioned the most important variable in all this. at what temperature does the clay mature? there are so many things to consider but this is basic.

[Callie Beller Diesel]

There are a lot of ways glazes can go wrong. Can you tell us what it did, or better yet, post a picture? What was it supposed to do? Did you get your clay out of a box? If so, what brand is it? If it's a clay from a class setting, you can ask your teacher what kind it is. Do you have the glaze recipe, or the brand off the bottle, if it's a pre-mix?

 

I would not endeavour to try wedging feldspar into an already mixed clay. The level of hassle would in no way be worth it.

 

If your clay is still porous after firing to cone six, refiring to bisque temperatures, which are lower, won't help. It will likely partially melt the glaze again, leaving it rough to the touch. If you take your current piece to cone 9 or 10, your cone six glaze will likely over fire and run off the pot. This one might not be fixable, but we can help you troubleshoot what happened for the next project.

[glazenerd]

TBAA

 

The glazed piece he checked was still porous at the base, and apparently this means there wasn't enough flux in the clay body.

Actually three primary causes: 1. under-fired  2. under fluxed 3. large particle size clay was used.  Issues 1 and 2 are the most likely suspects. Nep SY is the better choice for wedging in additional flux: plus you get a full melt at cone 6. Sodium fluxes will work at cone 6, but work better at cone 10. Potassium fluxes work very well at cone 6, but off gas alot as well. Nep SY is reliable and easy

Nerd

[Biglou13]

Wow that's a lot of what ifs? For first post.

 

I'd fire a test bar. Bisque then to temp. Weigh it. Now soak it in water for 24 hours. Now pat dry and re weigh. What percentage difference. How vitreous is a good start......... Most of the time (not always) it's best to be vitrified if not near vitrified. It's possible to have a glaze mature on immature body, but this situation is not optimal.

 

Chemistry of clay and glaze can have dramatic effect on appearance of glaze. We would have to know recipe of clay and glaze to give you better advice.

 

If you don't want to make clay.... You won't like adding flux to made clay body.. While it is possible I don't suggest it. I screwed up a clay mix once and had to add more flux / nephsy ( left a whole bag out of recipe). Luckily I own a pug mill / clay mixer.

 

Now determine what temperature your intended glazes are supposed to be used at.

[Marko]

All the above. I stay with tried an true commercial clay. It just makes one less headache to think about when you have it already made. As for glaze, you can find plenty of good recipes here on CAD. I'm just getting started on glaze mixing. With a little help from my friends on PC, CAD, and YouTube. Posting some picture really helps. Best wishes.

[tbaa]

To 'old lady'......I'm not even sure what your question means. sorry! 'Diesel', many thanks. At least I know what not to do! Glaze nerd, I will try nep sy. I agree it's likey under fired AND under fluxed. Big lou and Mark, thank you too. I'll try the soaking etc. even though this is making me wish I was still doing this out of a studio somewhere else! Some more details below in case it helps understanding all the 'ifs'

 

First of all I am working in India, and am just getting a studio set up. Raw material, if available does not come with any assurance of quality.

 

My clay is quite fine, and seems a little less plastic than it should be. (Rolling a small amount and bending it around a finger, it has small cracks, or 'breaks' more than it should. ). This is premixed and not of any known brand. It's good though, for throwing and slab work. Heavy for building. Some test tiles were brittle, but the bisque pieces were good. Glazed pcs have a good ring to them, so don't sound dull or hollow.

 

I tried 9 glaze recipes on tiles, which I tested for shrinkage before. Shrinkage was minimal when bisqued. And minimal again when glazed. I am working with cone 6 glazes.

 

Test tile Glazes were all mixed in small quantities. The only one that worked was a transparent glaze. All the others were off: Not the colors shown in the glaze recipe pictures, and many of them rough and dry on the surface.

 

Five finished pcs were fired using premixed amaco glazes. Two mugs had green glaze inside, and clear, (over velvet underglaze painted design) on the outside. The glazes were good. The underglaze colors were off. The 3 other pieces were mostly dry, & rough. Some Patches where the glaze had melted, but mostly not.

 

what I'd like to do is fire the rest of my bisqued pieces - 50 or so. I'd like to figure out what I can do with these. I will test the glazes again on fragments broken bisqued pcs as well as on clay mixed with neph sy.

 

Any idea how much neph sy to add?

 

THANK YOU FOR YOUR PATIENCE AND HELP.

[tbaa]

 

 " Nep SY is the better choice for wedging in additional flux: plus you get a full melt at cone 6. "

 

Since I'm working with cone 6 temperatures, I'd like to start with neph sy. Any suggestions as to how much to add per lb of clay?

[glazenerd]

Start with 3-5% and test it for absorption.    Something else I am noticing:

 

The only one that worked was a transparent glaze. All the others were off: Not the colors shown in the glaze recipe pictures,

 

The glazes were good. The underglaze colors were off.

TwIce you mentioned colors being off. Makes me believe your clay has high iron or high magnesium content: or both. By chance do you notice a light gray, light rose, or almond color after bisque firing?  Think you have a clay issue here, as well as flux issues.

 

The 3 other pieces were mostly dry, & rough.

Clay body does not have enough flux.

 

Some Patches where the glaze had melted, but mostly not.

Fluxes are off in your glaze as well.

 

Nerd

[Min]

I don’t think I would try and fix this clay body. tbaa mentions that “some test tiles are brittle†and yet it is porous. It also has low plasticity.  Was it sold as a midrange functional claybody?

 

I’m wondering how it was determined cone 6 was fired to? Temp alone or cones used? This might explain the glazes not melting if kiln was fast fired to temp alone. Glaze recipes would help for the ones scratch mixed.

 

Don't think there is much you can do with the 50 or so pieces already made with this clay. I would go back to basics and test this and other bodies before starting to use for functional pots. Get some samples of ^6 clays and do like BigLou suggested with absorption testing, plus I would measure shrinkage and slumping. Instructions for doing this are here http://ceramicartsdaily.org/ceramic-supplies/pottery-clay/testing-123-how-to-test-clay-bodies-to-find-the-right-sculpture-or-pottery-clay-for-your-work/  Also, use cones to verify firings (if they are available to you). Clay is cheap, your time spent fixing it is not, I would spend the time testing a new clay then getting back to work.

[tbaa]

Nerd, this is VERY helpful. I agree it's worth checking the clay. The clay is a dark-ish gray, but fires to a slightly rosy or almond white. No flecks of any sort. I'll have the clay analyzed for iron and magnesium. I have more than a couple of hundred lbs.....so worth the effort. Min, I HAVE to work this out. If I get another batch of clay, who knows what I'll have to contend with next..... And they only deliver by the truckload.

 

Thank you both.

[tbaa]

Shoulda paid attention in chemistry class.....who knew?!! Now I have to figure out what's magnesium and what's that to iron....

[Min]

Nerd, this is VERY helpful. I agree it's worth checking the clay. The clay is a dark-ish gray, but fires to a slightly rosy or almond white. No flecks of any sort. I'll have the clay analyzed for iron and magnesium. I have more than a couple of hundred lbs.....so worth the effort. Min, I HAVE to work this out. If I get another batch of clay, who knows what I'll have to contend with next..... And they only deliver by the truckload.

 

Thank you both.

 

I guess we are spoiled in other parts of the world where we have access to better suppliers and can buy smaller amounts that a truckload at a time.

Good luck with your testing, hope you get it sorted out. 

[glazenerd]

 

The clay is a dark-ish gray, but fires to a slightly rosy or almond white

Magnesium (MGO( would show up as a shade of gray. Rose "ish" and almond are a combination of iron and titanium. Almond white would put TiO2 at +2.50%. Rose tint would be iron, with TiO2.So now I have another issue: if you are getting both a rose and a almond out of the same batch of clay: would have to conclude it is very poorly blended. Would also explain why some are rough and porous and others are fine.

Nerd

 

(old age edit)- forgot something> High levels of potassium will cause an almond appearance at bisque. Standard cone 6-10 clay bodies will have 3.5 up to 4.00 % of batch weight of potassium (if it is the primary flux.) Levels above 4.50 start to show up as cream, and get deeper in tint as the level rises. Not mentioned before because you lack flux: so this is not even on my radar in this case.

 

run me a test please: make a 4 x 4 tile and fire it to cone 6 without glaze.  Want to see something... maybe.

[tbaa]

Sounds good. I'll test a tile and post the result. Don't hold your breath though....my test kiln just arrived, and I'm still waiting to get the gas fittings installed. I may need a pugmill after all..... Thanks for taking so much time over this. Much appreciated. Yes studio space in Boston was much easier!

[tbaa]

May to August.....and still no solution. But i threw and fired just fine while in the US. I am setting up to run a bunch of tests as I now have a test kiln, and have worked at trying to understand what could have gone wrong at the glaze course.

I think the problem may have been poor attention to temperature while firing. And casual substitutes for Frits.

One last question before I run tests. The clear glaze is in fact a little milky, and Though I am looking for other recipes to test, I wanted to check if that may have affected underglaze colors? (The milkyness that is....) ?

[Tim T]

Is the milkyness from colouring in the glaze, or is it because of lots of tiny bubbles? Have a look with a magnifying glass. If, as I suspect, it is bubbles, then the gas given off by the clay and/or glaze during the firing isn't escaping the glaze.

Some things to try:

- A slower firing , especially between 500 and 1100C, will allow more time for the gas to escape (from glaze and clay) before the glaze melts and traps the bubbles - this is roughly the temperature range in which carbonates change to oxides, organics burn off etc.

- Related to this, a slower bisque firing to a higher temperature, to get gas out of the clay - I bisque fire to 1050 which is a bit higher than many - though this can make the bisqued clay less porous and so it may take up less glaze, so you may need to thicken your glaze

- Until the glaze has melted the gas can easily escape through the gaps between particles, so slower firing below the melting point or less flux in the glaze to make it melt at a higher temperature may help

- If the glaze is runnier and less viscous the gas can bubble through the glaze more easily, so try less alumina or more flux. This sounds contrary to the previous item, but you could try more of a higher melting point flux like magnesium, and reduce the amount of low melting point fluxes, depending on your firing temperature

 

Looking back to your original posts, if you are committed to the clay you may find it easier to change the firing temperature than to modify wet clay. I suggest making up some test bars that are say 12cm x 2cm x 0.5cm and doing the following tests at a range of temperatures say 50C apart to investigate how the clay behaves:

- scratch a number into each bar e.g. 1100-1

- weight the bars as soon as cut out, and also make 2 marks exactly 10cm apart (I have a piece of wood with 2 protruding nails at the right separation to make this quick and easy)

- dry till absolutely bone dry (in the summer my workshop can nudge 40C and the air is dry so I just let dry naturally, but in the winter when pretty well dry I put them in the kiln at 100C for a few hours)

- weigh again and measure the length. This gives moisture content and also shrinkage wet to dry, and acts as a reference for the fired tests

- fire at each temperature and let cool

- measure the length. This will slowly reduce until the clay starts getting over fired, when it will start to increase as gases are emitted in the clay

- weigh, and the difference with the dry unfired weight will give the loss on ignition (LOI) - this all comes off as gas, so seeing how the LOI increases with temperature will tell you the temperatures where most gas is given off from the clay

- soak the weighted piece in water for exactly 1 minute, dab off surface water with a towel and weigh again. This gives the porosity - less than 1% increase in weight and the clay is generally deemed impervious, so firing above this point is only useful if you want a higher temperature colour in the clay. The further below this point you go, the less vitrified the clay is, so it isn't as strong and is more reliant on the glaze to make it waterproof

- at the less vitrified temperatures, a simple test for comparative strength of the clay is to snap one of the test bars in your hand (or not!) and see how much effort is required

- if your clay colour varies significantly with temperature, it may be worth mounting the test pieces on a labelled board (perhaps after applying a clear glaze to one end of the pieces and refiring) both as a reference in deciding on a firing temperature, and also as a visual check to see if pieces come out of the kiln under or over fired

 

From these tests yo should be able to decide on the best firing range for the clay - apart from any colour you may want, there is no point firing above vitrification asyou are more prone to slumping and bloating; you can see the danger temperatures for gassing; you can make sure you don't over fire the bisque so it loses too much porosity. The only limits on how low to go with the firing are obviously the porosity increases and strength decreases as the firing temperature drops, but that is a design choice rather than something that has a right/wrong answer.

[tbaa]

Tim, thank you.  Between you, Glazenerd and John Britt, I was able to resolve basic issues.  Eventually just poor firing at the place I went learn about glazes.  

 

[tbaa]

That went off too quickly.  I wanted to say how much I appreciate the help and advice.  I am going to see what to do with improving plasticity when I recycle my first batch of clay....and may be coming back for more help.  

[glazenerd]

tbaa:

 

Normally I would say add some very fine ball clay for plasticity. However, you seem to be having some clay flux issues as well. So I will make an exception and say: start by adding 5% Plastic Vitrox (PV) clay. Not only will it add plasticity, but also 5% potassium as well. See if we can kill two birds with one stone here.

 

Nerd

[Guest JBaymore]

I guess we are spoiled in other parts of the world where we have access to better suppliers and can buy smaller amounts that a truckload at a time.

 

 

Yeah, we are.  And we tend to make assumptions when someone post on the forums that the person posting is in "our" country.  Colors the responses.  We are seriously spoiled by our "supermarket society" here in the USA... and can easily get materials from all over the world.  And those materials are reasonably well controlled in their consistency.

 

tbaa .... if you can get it.. .... a good book that can help in less developed countries and for more "primitive" and "back to the real roots" types of ceramic situations is "Pioneer Pottery" by Michael Cardew.  (Note: not "Pioneer Potter" .... which is ABOUT Michael Cardew.) 

 

The "by the truckload" comment hints at maybe it sounds like this clay you are getting is more of a natural deposit that has been cleaned up a slight bit (or not at all) rather than a blended clay "body" (additions to make it work and fire better).  Do you know if this is true? 

 

Do you know who ELSE they are supplying?  And how they are making with it and firing it?

 

best,

 

...............john

[tbaa]

will check with the supplier to see if PV is available here. If so,  'Ill try both in small batches.  Assuming the clay flux issue the effect on the glaze colors?  (That is to say how did you know there is a clay flux issue??)  Thanks a ton though.  It will be a couple of months before I get to do this....but that means more time to source PV.....

[glazenerd]

 

The glazed piece he checked was still porous at the base, and apparently this means there wasn't enough flux in the clay body. I am using fine stoneware clay, and understand feldspar could be added.

PV is potassium based: should have no effect on color.

 

Nerd

[tbaa]

J Baymore - i saw your note about primitive pottery, though it doesnt show here.  Sent a reply but some glitch with checking if I was a robot or not....  Long reply with picture posted....but no idea where it went.

 

Holey moley!

 

So, here I go again:  Thanks, I will look for the book. I am now trying to work out firing schedules.  Mainly at what rate to increase temperature in my kiln.  How many degrees over how much time - from whatever the loaded kiln reads at before I light it.

[tbaa]

John, Oh boy, your post shows now.... anyhow, .....