Crazing - Commercial Glaze

[cbarnes]

Problems with Crazing.....

I'm using Amaco HF-9 clear claze (haven't made the leap yet to learn to make my own glazes. still learning all of the other pottery skills).  I'm using a porcelain 4-6 rated clay, slow bisque fire to 04, then I paint with underglaze and bisque fire again to 04 to set it (I was having problems with smearing).  then I dip in the Amaco HF-9.  let it dry to the touch (about an hour), then fast glaze fire to cone 5 (also tried 6).  I get crazing about 80% of the time (usually larger items more than the small ones, and platters seem to do ok).  I have a 5 gallon bucket of this stuff and 100 lbs of the clay and several made pieces so I'm hoping I can make these work together. 

 

I've purchased a hydrometer to make sure my glaze thickness is correct.

 

my husband thinks that changing the ramp down time on the kiln will allow the clay body and glaze to cool down at a better rate together and prevent the cracking (would this work?).

 

any other ideas?

 

I've read to add silica to the clay?? or something to the glaze? to help but since its a commercial glaze I wasn't sure of the formula.

 

any advice would be appreciated.  I'm so frustrated putting the time into pieces and having them fail.

 

also....can you refire a crazed piece at different temp? rate? to fix it?

 

 

[neilestrick]

Are you buying the glaze as a dry mix?

[cbarnes]

Are you buying the glaze as a dry mix?

yes,  then added recommended water and then using a paddle attached to the drill to mix.  I've added some more water because it was definitely to thick .. wouldn't dry quickly after dipping.  but now it "seems" the right consistency.

 

[MatthewV]

If it takes an hour to dry to the touch -- I really worry about the thickness of something.

 

Are your pieces a "normal" thickness? Not eggshells and not bricks, right.

 

Have you been wetting the bisque before applying glazes?

There are many ways to apply glaze to get excellent results. My preference has been getting the bisque just a tad wet with a squeezed out sponge. I find it helps the glazes absorb into the body more.

 

But what I really wonder about is how thick the glaze has gotten on. Clear glazes in particular should be "about fingernail thick".

 

Many glaze problems are hard to pinpoint. Try robooting the bucket by sieving it :-)

[cbarnes]

If it takes an hour to dry to the touch -- I really worry about the thickness of something.

 

Are your pieces a "normal" thickness? Not eggshells and not bricks, right.

 

Have you been wetting the bisque before applying glazes?

There are many ways to apply glaze to get excellent results. My preference has been getting the bisque just a tad wet with a squeezed out sponge. I find it helps the glazes absorb into the body more.

 

But what I really wonder about is how thick the glaze has gotten on. Clear glazes in particular should be "about fingernail thick".

 

Many glaze problems are hard to pinpoint. Try robooting the bucket by sieving it :-)

 

it only takes about 30 seconds- minute to dry to the touch.  its about a fingernail thick I think but i'll check again. 

 

are there any "problems" caused by a clear glaze that's to thin? other than it just not looking as glossy?  I could add water then and not be as worried about thickness. 

 

I haven't tried sieving it so I will do that also.

 

my pieces are about 1/4"-3/8" thick.

 

I will also try wetting them.  I have a base coat of underglaze on these though, that I have fired to 04 to set so it doesn't smear, will that make a difference?

[High Bridge Pottery]

Pottery is the art of putting a lot of time into failing.

 

You could add silica to try and lower the expansion of your glaze or some clay. 

[glazenerd]

Pics are always helpful: more so close up of the crack patterns. Patterns talk!!

Porcelain COE would be 5.25 up to 5.75 typically, so modifying the dry glaze would be the easiest route. If this clear glaze runs the usual specs: it has a higher COE. Silica is a possible solution: but the lack of alumina in a glaze can also causes problems.

Nerd

[neilestrick]

I would try adding equal parts silica and kaolin in 2% (each) increments until the crazing stops. To do this: take 200g of dry glaze and add 4g each of the silica and kaolin, add water and mix well with a stick blender, then dip a tile in it. Then add another 4g of each, blend and dip a tile. Continue until you get up to 10% (20g total each) of each. Fire them all off and see where the crazing stops. You could do this by just adding silica, but it will alter the silica to alumina ratio. By using both silica and kaolin, the ratio should stay pretty close to the original. Without knowing the formula of the glaze, there's not much else that can be done.

 

Is the crazing big spaced out cracks or little close together cracks?

[cbarnes]

Here are some samples.

 

I've mixed all the glaze so I can't add to the dry mix, can I add silica & kaolin to the already mixed glaze? in small batches?  or maybe I just toss it and start over   spendy mistake but that's pottery I guess.  do things slow and experiment... I keep forgetting that.

 

or maybe try different non porcelain clay body?  is it that porcelain is more difficult?  I thought I just needed to find clay that was rated the same cone range as the glaze. 

 

thanks for all the help!!!!! the science side of this is not easy for me.

 

 

 

 

 

[neilestrick]

You can do the same test by adding the silica and kaolin to a specific amount of wet glaze, then calculate it out for the big batch. I would take a pint of wet glaze and test increments of 5g each silica and kaolin up to 25 grams each. That will likely fix this batch, but you'll need to run new tests with dry material for the next batch.

[High Bridge Pottery]

Porcelain shrinks less cooling down than most stoneware clay so needs a glaze that does the same otherwise it cracks. Are you firing to cone9? I really don't think it would cost much to buy the raw materials to mix a clear yourself. No time like the present and if you are buying silica and clay you are 2/3 the way there. Can still test this glaze too with additions. Great pots 

[MatthewV]

Your application appears fine -- there is a mismatch between the glaze and the body. Which is too bad because this the harder to fix!

[Marcia Selsor]

I agree with Neil about adding small amounts of silica and Kaolin to the glaze. I would even start with as little as 1% each. You might also try dampening the bisque before glazing so it doesn't absorb the glaze as thickly.Try/test separately.

 

 

Marcia

[glazenerd]

CB:

COE/ crazing is something I have studied extensively for years because the glaze I use is prone to it. Here is a simple, but good rule of thumb in determining the differential COE between the clay and glaze. The closer the cracks are together: the further apart the COE values are between the clay and glaze. The further apart (openly spaced) the cracks are: the closer the COE  values are.

The crack pattern shown in your example is closely spaced: some would classify it as a crackle glaze. I refer to it as clay/glaze interface checking. When I see this pattern I automatically assume there is a minimum of 2 point COE differential. To apply this: clay would be 5.50 COE and the glaze 7.50COE. These are not true numbers, given as an example: but also to indicate there is a serious differential between your clay and glaze. The advice Neil and Marcia gave is the path to correct this problem, although I would start at 5% personally due to the severity of the COE offset. If the crack patterns were spaced much further apart, then I would start at 1-2%. I am a big fan of NZ kaolin as a glaze fix, but I also know it is not readily available to everyone. NZ is much cleaner, and is a great source of alumina.

AL/SI ratios are the heart of any glaze, although several other disciplines also have to be met. As you know silica is the principle glass former in any glaze: but alumina gives glaze its strength. To illustrate from the glass industry: 70% silica and 30% sodium is soda glass: the windows in your house are soda glass. Take that same formula and remove 15% sodium and replace it with 15% boron and you have pyrex. Add additional alumina and you have structural glass used in skyscrapers. Alumina is the unseen structural support in your glaze and your clay. Too little alumina weakens the glaze and further compounds the crazing issues. So yes to silica, but also yes to alumina additions to cure this problem. For more information, contact my website at: www. long-winded-answers,com.

Nerd

[Min]

I agree about adding silica and alumina to fix crazing, if the base can support the amount needed to do so. Otherwise you need to change the fluxes, obviously not possible with a commercial glaze.

 

I think there is another way of looking at coe figures and their relationship to crazing. That being to look at glaze “families†or “systemsâ€.

 

This is from digital fire:

Results are determined by the set of expansion numbers (different values are available from different sources) and the method of additive calculation method chosen (based on formula or mole%). Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of variations, the calculated expansions will give you an indication of which variations have higher and lower expansions. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system. Also, some oxides, like Li2O or B2O3 do not impose their expansions in a linear fashion, thus they do not calculate as well.

https://digitalfire.com/4sight/glossary/glossary_calculated_thermal_expansion.html

 

I use a clear with a coe of 5.76 (using Insight). It is bombproof and does not craze on my clay, have used it for many years. Using the same clay I recently ran some tests for a friend with a glaze that has a coe of 7.20. I tested it on the same porcelain clay that I use the low expansion glaze on. To my amazement it does not craze either. I ran exhaustive crazing tests on it and all samples are craze free.

 

Neil E. got me thinking about this when he posted his clear glaze for porcelain a few months ago. At the time it didn't seem logical to me why it didn’t craze since the coe came in fairly high. Now it makes sense. 

[cbarnes]

Thank you everyone for the suggestions.  it turns out "hopefully" that it was an easier fix.  we decided to just try a higher cone temp and slower fire.  I refired the crazed pieces (two tests now) with good results.  no apparent crazing on previously crazed pieces.  I'm going to do some shock testing to see if they hold up.  previously fired to cone 5 at a "fast glaze" setting on my kiln (5 hour fire I think), refired to cone6 with slow glaze setting, took about 7 1/2 hours but the cool down took a lot longer.    before and after shots attached.

 

 

[High Bridge Pottery]

Interesting to see if it passes the shock testing. I would think maybe you have slowed the problem not rid the problem but I am always surprised in ceramics. Maybe the cone6 instead of 5 has changed the expansion of the glaze.

[cbarnes]

Interesting to see if it passes the shock testing. I would think maybe you have slowed the problem not rid the problem but I am always surprised in ceramics. Maybe the cone6 instead of 5 has changed the expansion of the glaze.

 

I will let you know.  I assume the best testing would be

> dishwasher run several times - like over two weeks, just leave it in there?

> freezer for 1/2 hour then boiling water?

 

for now, I will advertise these as not microwave or oven safe. my intention was for them to be decoration or light use, but you know people wont listen. 

[Min]

 

Interesting to see if it passes the shock testing. I would think maybe you have slowed the problem not rid the problem but I am always surprised in ceramics. Maybe the cone6 instead of 5 has changed the expansion of the glaze.

 

I will let you know.  I assume the best testing would be

> dishwasher run several times - like over two weeks, just leave it in there?

> freezer for 1/2 hour then boiling water?

 

for now, I will advertise these as not microwave or oven safe. my intention was for them to be decoration or light use, but you know people wont listen. 

 

 

I use a slightly harsher version of the oven / cold water shock test to test for crazing. It’s a good indicator if the glaze is going to have delayed crazing. I also do porosity / absorption tests for the clay (before doing the crazing tests)  If the clay is too porous the glaze will have delayed crazing as moisture soaks into the pot over time.

 

This is what I do, test pieces in oven at 300F for around 20 minutes to make sure they are heated all the way through then plunged into cold water. If there is no visible crazing I bump the temp up to 310F and repeat then repeat again at 320F. After the last cycle I brush calligraphy ink on the test pieces then rinse them off, hopefully no crazing lines. I know others say going to 300F X 3 cycles is enough so I might be a bit extreme.

 

The freezer test is for shivering, glaze in compression. Thin walled cylinder, glazed thickly on the inside only, right up to the top and just over the rim. Pot in coldest part of freezer overnight then put in the sink and filled with boiling water. If the pot splits open or shivers of glaze come off the edge of the pot the coe is too low. [(glazes in compression are far less common than ones in tension (crazing ones)]

 

Yup on using the pots for a while and seeing how they standup, a couple months in the dishwasher is good too for checking glaze durability.