Glaze Cracking And Dropping Off Pot Before Firing.

[Babs 682]

I know this has been posted before but I can't find the posts this am.

One of my glazes cracks and  drops off the pot soon after dipping. I have watered down the glaze but it is a tricky one, any drips do the above, and the rest of the glaze develops small Cracks and is friable.

On a previous clay body it was fine.

I know that the recipe would help here

Frit 3134  50

Pot Feld   20

Ball clay C  20

Mag Carb Light 10

to which I add various colourants

Tin Oxide  10 I vary this with ZIrc. for most of the deal.

1 -2 Cobalt Carb.

Presuming the thickness is ok what else can I adjust?

[High Bridge Pottery 665]

I think I remember something about it being the clay shrinking and to calcine half of it but may not be correct at this pm.

[Joseph Fireborn 1,235]

What HBP says is correct, you can calcine some of your clay(OM-4) and it will help with cracking and falling off, both on the application and the crazing in the glaze after fired. I have tested this on a glaze before. 

[Min 1,752]

10 magnesium carb is a fair bit in a glaze, my 2 cents is that is likely to be part of the problem. Is this a ^6 glaze and is it for functional pots?

[oldlady 1,967]

  ball clay OM4 is probably not available in australia.

[Babs 682]

Well, I added sodium silicate, and dampened my pots more than I would in normal glazing routine, this helped but NO room for drips or touch ups...but the overall cracking is greatly improved.

It is a cone 3-5 glaze but I've taken it above this., it is a shiny functional glaze. Doesn't crack after firing.But with this clay, it certainly doesn't like to be applied as before. I can no longer get the clay I used to use.

Ball Clay OM4, ? don't know old lady but I have a great supply of Ball Clay C and QA which this glaze requires and don't want to purchase any more.

So calcine the Ball clay??

Min if I were to alter the quantity of Mag Carb, can you help with this as I am no chemist. Mag carb is a crawler right?

[oldlady 1,967]

OM4 is a common ball clay here but it is called a "dirty" clay and looks beige when you open the bag.  i use a different, white, ball clay for most glazes.  i think the name refers to the Kentucky mine number 4, an old one.

[Babs 682]

Right, my different ball clays are the dirty looking ones!! May look better after calcining...ok to do this to bisque temp? ie  for me cone 06, temp around 1000deg C?

[jrgpots 240]

Some of my glazes are very touchy if they are thick.  They crack and fall off if applied too thick.  Try thinning a bit especially with that much Mg carb.

 

jed

[neilestrick 2,697]

It's a lot of magnesium indeed. It's way above limits, as is the boron. The alumina and silica are at the very bottom of limits. 10% tin and 2% cobalt are also a bit excessive. This is not a good glaze for functional work. It's not forming good glass.

[TJR 361]

OM4 is a common ball clay here but it is called a "dirty" clay and looks beige when you open the bag.  i use a different, white, ball clay for most glazes.  i think the name refers to the Kentucky mine number 4, an old one.

The name OM4 refers to the name Kentucky Old Mine number 4.

Don't ask me how I remember this.Art school over 40 years ago.

TJR.

[rakukuku 122]

we used to put gum arabic into glazes to help them adhere.  Also can add a little Karo syrup but after  a couple of days it will stink horribly like barf.   I think there is other stuff you can add but don't know what its called.    Rakuku

[Min 1,752]

Just to answer your question from earlier on, to replace some or all of the magnesium carb in a recipe talc and or dolomite can be used, it's not a 1:1 swap though. I don't have the chemistry for your ball clay but would guess even with accurate figures this glaze is a no go for functional pots like Neil said. Must be an expensive glaze to make in Australia? 

[MatthewV 259]

Gum arabic can be used (add a tiny tiny (0.01%) bit of copper carbonate to prevent the stinkiness mentioned before). Copper is an industrial fungicide and the small amount will not be enought to change the final color.

 

But the much easier way is adding ~1-2% bentonite. The tip with bentonite is mixing it into other dry ingredients or it will clump terribly. Bentonite helps keeps glazes in suspension and also with the dry strength.

 

Finally, watering down a glaze is really the last thing you want to do. Peeling of a glaze usually happens because the bisqued body cannot take in more water.

[Babs 682]

Well, I have been using this glaze with ok results for years as I said with different clay.

That being said, I didn't think 1-2 Cobalt Carb is a high quantity, the tin I substitute with ZIrc., or parts thereof. as the price of tin has ascended!

As you guys state that this is non functional because of what is stated above, I'll stop using it for this, I've eaten off plates with this for years!!! No scratching, high gloss,.. Hmm just left me wondering, most of the time I use Cobalt Carb I use 2-3 oparts, dropping to 0.5 or 1 forhte oxide. 

I would play around witht e dolomite option but this does nothing for the Boron contemet

[MatthewV 259]

The boron is coming from the very high amount of Frit 3134. A more normal range for this frit is 20%.

 

 

But looking at the picture, the easier way might be to just try a new recipe.

This recipe will give a similar result. It worked with New Zealand ingredients just fine at ∆5-6 and I suspect it would be fine at ∆4. It was very popular in Wellington. And it should be cheaper to make!

 

Silica 23
Kaolin 3
G. Borate 13
Potash Feldspar 47
Zinc Oxide 4
Dolomite 4
Whiting 6

Rutile 2.0
Copper Carbonate 1.5
Cobalt Carbonate 0.5
Bentonite 2.0

[Babs 682]

thanks Mathew, Still trying to come to terms with the logic that peeling glaze means body cannot accept more water, I thought it was accepting too much and so the glaze deposit on the surface was too thick....

Will try your glaze recipe next time.

Never know everything Hmm Like to crunch this one just because..

[MatthewV 259]

Well, take a bisque bowl and soak it in water. Apply glaze to the inside as best you can. It should take a long time to dry and peel away before firing.

 

The same principle is happening when a glaze is too wet. The clay body fills with water and then the remaining glaze adheres poorly. Your new clay has a different poroisity which is why it wasn't taking well to a glaze that worked for you before.

[Callie Beller Diesel 1,343]

Babs, OM4 and other dirty or secondary clays have a high wet shrinkage rate (Alberta slip is bad for it too.) If you're adding water to the slurry in the bucket, it's making the problem worse. If you want to keep using it on the outside of your pots, calcining half of it like Joel suggested will help.

[Min 1,752]

Babs, if you want this recipe with talc instead of magnesium carb this is what I came up with.

Ferro Frit 3134............. 48.30
Potash Feldspar............. 19.30
Ball Clay FX (CA1).......... 19.30
Talc........................ 13.00
=========
99.90


Silica is up a bit from original recipe. Silica and alumina still below "limits" for functional glaze and boron still high. I used data for FX (CA1) ball clay, I'm guessing that is what you have but I'm not sure. I'm know your materials are going to be different form mine half a world away but this should get you close to your original glaze. If you use zircopax for the opacifier it is going to up the silica levels a bit. Adding 10 zircopax brings silica up to just below "limits" minimum for durable glazes.

 

Oxide   Formula  

  CaO       0.43*    

  MgO       0.27*    

  K2O        0.10*       

  Na2O      0.20*       

  TiO2       0.01     

  Al2O3     0.20     

  B2O3      0.40   

  SiO2       2.28   

  Fe2O3     0.00     

 

                        Cost:        

              Calculated LOI:    2.03

                 Imposed LOI:        

                       Si:Al:   11.15

                      SiB:Al:   13.11

           Thermal Expansion:    7.78

[Babs 682]

Thanks so much Min, will give it a crack.

haven't used this glaze for a while so before I throw it away, just wanted to make something of it.

Colour is a French blue, bright and cheery outside snacking plates..

No more, hmm

[neilestrick 2,697]

There are a lot of glazes out there that look good and people have been using them for years, but they're technically not a great glaze. if you had it tested you may find that it's leaching cobalt, and if you put copper in it, it's very likely to leach.

 

Intense blues can be had from 0.5% to 1% cobalt in most glazes. 2% is usually overkill and 3% is most certainly too much. If it's taking that much cobalt to give blue, then it's probably because there's something wrong with the glaze.

[Babs 682]

Thanks Neil. I appreciate your advice greatly.

Because this is giving trouble and with your advice, I'll be giving it the flick for functional, it also has the tendency to Crak, not a crackle but in odd spots, it looks like the glaze cracked and moved during firing.

Non of this showed in my previous clay.

Anyway, I'll be trying another glaze for the bowls etc I have promised a poor unsuspecting person....

Facetious  here, I live on a property which is low in cobalt, we have to inject our livestock with a cobalt sup. so perhaps if I grind down my blue dishes and spread them over the broadacres, I will be enhancing the health of my stock, or perhaps these bowls are jus the thing for people living in this area!

[Min 1,752]

Babs, why don't you post a picture of the glaze, maybe someone has something similar and they would share their recipe?

[Babs 682]

have done back a page, Min, not a true repro. of the blue, streaks save it from being  boring, can get a mauve tinge, the mag. carb. Still thinking of what Neil said about too much cobalt, always thought 2-3 ok for the Carb. and 0.5-1 for the oxide, if time gets aplenty I will mix a  bit with Neil's recommended cobalt carb quantities  and check the colour. I loooked thro' a couple of books last night and the cob.carb quantities do vary from the oxide, but his point about it being too much cobalt for food safe glazes i take on board.