Glaze Cracking around Handles when Drying

[OVMI_Designs]

I am trying to work out an issue I am having with a cone 6 matte base glaze. Here is the recipe:

--------- G2934Y

Frit 3249: 10%

NephSy: 10.5%

Wollastonite: 21.5%

Talc: 14%

EPK: 20.5%

Calcined EPK: 10.5%

Silica: 13%

Added Zircopax 10% to make it opaque white.

 

I have added 10% of a glossy clear glaze to this after it has been mixed. This was recommended by Tony Hanson, whom formulated the matte glaze.

---------

I am using a slipcast whiteware clay body: Miller NS-4 Dover. I am applying the glaze in two section. Pouring the inside, Drying overnight, and then dipping the outside. If I dip for say 1 second, the glaze application looks fine when it drys, but is too thin when it fires, resulting in a rough/dry surface. If I dip for 3 seconds I get a thick application that cracks around the handle attachment sight, and around the interior where the foot meets the sidewall. When fired the glaze looks great, silky/waxy smooth, but crawls around the spots it cracked when drying. I am firing with a KilnSitter to Cone 06 for Bisque, and to Cone 6 for glaze. Any ideas?

I have tried reducing the SG to 1.5 from 1.45 and using darvan 7 to thin it out.  No changes.

I have tried adding more calcined EPK and less raw EPK. No improvement on cracking, but is a dust/dry application.

I have tried adding bentonite. Made the issue worse.

I have tried reducing the amount of clay 10% and flocculating the glaze with vinegar which seemed to help a little but not much with cracking. The application is dust/dry, It is currently firing so I have yet to see the results.

Thanks for the help.

[neilestrick]

It it's cracking when drying on the pot, that's usually a case of applying it too thick. If a 3 count is too thick, add more water to thin it out. In my studio we apply at a 6 count. I don't see anything in the formula that worries me- I've got several glazes that have 20% clay in them. What is in the clear glaze that you're adding to it? That could be contributing to the problem.

[liambesaw]

Try glazing the inside and outside on the same day or closer together, once the body is a bit wetter it might dry more evenly and less cracky

[Chilly]

Are you using the same glaze inside and out?  If yes, have you tried dipping and using the John Britt (I think) method of glazing the inside by violently pulling the mug up so the glaze blooshes inside?

[Min]

I use a blend of a dolomite matte with a lot of clay in it blended with a clear also, had the same issue. What solved the cracking was to use a fine paintbrush and brush water onto the handle joint and spritz water inside the mug bottom then let the mugs (or tubs etc) sit for a couple minutes then glaze. I'll do a whole board of mugs this way, dampening the areas where the glaze pools so it goes on thinner in those problem areas.

[OVMI_Designs]

I suppose the crux of the issue is that in order to get the silky smooth surface I am hoping for it has to go on thick which in turn causes the cracking while drying issue. I suppose I could try the method Min recommends, that would be a simple thing to test.

To answer others questions:

Neilestrick: I am using the G2926B. Which is a similar recipe but uses Frit 3134. This was recommended by the maker of the matte glaze.

Liambsaw: I will try glazing the inside and out same day. I have a feeling that it wont be thick enough to get the smooth result, or will still crack around the connection.

Chilly: I have tried dipping the whole piece using tongs. the glaze doesnt move much at all so the spots where the tongs hold the pot are still seen after firing.

Is there a way to perhaps flux the glaze more so that a thinner application will melt out more to smooth over the surface? Or maybe fire to cone 7 instead? I haven't tried that yet. Out of curiosity how thick should any typical glaze be when applied? If I were to take some calipers to measure it.

 

I am trying to achieve a waxy smooth opaque surface. Perhaps you have another glaze to recommend trying?

[neilestrick]

I would thin out the glaze a bit. It will give you more leeway in the application, and prevent it from puddling too thick at the handle joint. If you want it really thick, then dip it twice, letting it dry some between dips. Don't let it dry completely between dips or that will cause other problems. I need a thick application on my pots, too, and this is how I do it.

A thinner application will never cover well, no matter how well it melts. You'e gotta have enough glaze on there to do the job.

[Chilly]

I read somewhere, that the glaze should be as thick as a credit card.  Not sure if that works for all glazes.

 

[Min]

18 hours ago, OVMI_Designs said:

Is there a way to perhaps flux the glaze more so that a thinner application will melt out more to smooth over the surface?

The G2934Y matte glaze with the G2926B clear added on Digitalfire is using 9 parts of the matte to 1 part of the clear. It sounds like you are using 10 parts of the matte plus 1 part of the clear? Added to this you are adding 10grams zirco to the matte. In any event I think you could decrease the % of the matte glaze, and increase the % of the clear, to help with getting a smoother surface. I would run a line blend with the matte and the clear, go from 50 matte : 50 clear up to 90:10. To keep the zircopax as a constant, put 10 zircopax in both the matte and clear bases. Have you tried cutlery marking the glaze? Might need more of the clear gloss glaze added to it to make a good liner glaze.

18 hours ago, OVMI_Designs said:

Out of curiosity how thick should any typical glaze be when applied? If I were to take some calipers to measure it.

With all the clay in the matte recipe it's going to appear quite thick compared to say a heavily fritted glaze with little clay. If you dip 2 test tiles with 1, 2 and 3 layers of glaze then when they're dry scratch through the glaze on one of the tiles so you can see the thickness, don't fire this tile. Glaze the other test tile the same way (with out the scratch) and fire. Keep the unfired tile as a reference to glaze thickness.Mugs below are dipped in my white glaze with 73 matte: 27 gloss (plus tin and zircopax).

The glaze is still quite matte, it does crack on the handle joins if I don't use the wet brush technique before dipping. 

[OVMI_Designs]

Thanks Min. That's some solid advice. Ill give that ago next time in the shop. I have some test tiles ready to go.

 

That  glaze looks great by the way. Not too shiny, but not too flat. Nice work!

[OVMI_Designs]

I tried the wetting with a brush method last night. It did not seem to help unfortunately. I also tried the pour / dip on the same day, which seemed to allow for the glaze to be thick enough but still having the cracking issues in the same spot.

I did some test tiles with the 1,2,3 dip method. I also tried to reformulate the glaze with a few different materials and have a test tile for that as well. We shall see how it goes. The reformulation didn't crack on the test tile in a spot I would have expected it to which is a good sign. Hopefully the quality of the surface turns out well. I used insight-live to calculate it. The numbers are pretty close. Here is the reformulated glaze in case you are wondering:

Ferro Frit 3249	15.000
Minspar 200	33.000
Wollastonite	13.000
Silica	10.000
EPK	19.000
Talc	10.000

This takes into account the mixture of the matte and glossy glaze combining it into one formula that matches the numbers of the other combined formulas.

[Bill Kielb]

OK, I looked at this glaze and I am not sure why you prefer it. From a chemistry aspect it has a non durable RO ratio (0.07:0.93) and has a 6:1 silica to alumina ratio which is more glossy than matte so likely not true matte. Boron for cone six melt is fairly low and from a CTE standpoint the alumina would appear low for very low expansion bodies like porcelain. Having said all that, are you open to trying another matte and customizing the matte (to personal preference) with silica addition and additional melt with boron? I am sure this glaze has some redeeming qualities but it appears to be a boat load of unnecessary to me.

If you are open to a glaze change I have attached a  true matte that you could possibly work with. It has a durable RO, very clear, no zinc and plays well with underglazes and low expansion bodies. It has been tested from hard matte to absolute gloss and we have a formula in the studio that we call Plus 6 for the added silica and slight softer matte / satin look along with enough boron to melt at 4-1/2, yet tolerate a cone 7 firing. This glaze does not move much if at all.

I have attached your glaze in the Katz UMF spreadsheet of which I have permission to modify and share. We originally did some visual basic modifications on this sheet which he freely provided and he provided permission for us to freely distribute the modified sheet. If you like UMF, then this will be revealing. If not - please disregard.

Best of luck!

 

 

[oldlady]

bill,  i have enlarged the post you submitted but still cannot read all of the info.  my eyes are very good but my brain is not as fully saturated with chemistry as yours.  everything under the title Marcia's silky clear true matte is invisible no matter how much i enlarge it.

could you please list the ingredients in normal type and explain that first one, i read frit 3249.   can find no reference to this number.  did i read it wrong or is it some unusual product that is only available in some other place?  and Amtalc is also a puzzle.  i assume calcined kaolin would be EPK calcined but assumptions are unsafe as we all know.

i have been looking for a zinc free translucent matte glaze for years so i can use green slip under it without the final color being grey.  this might be it if i can clear up the ingredient list.

thank you

[Bill Kielb]

The Plus six recipe: (By hand)

• Neph SY - 40 %
• Gerstley - 20%
• EPK - 42.5%
• Talc - 18.5
• Wollastonite - 11
• Flint - 11.57

The recipe attached has slightly more silica and Gerstley than the base on Glazy. The reason for the addition: More Silica = less matte (More satin), more Gerstley = slightly earlier melt to protect against studio under fire.

This glaze has been progression tested from + 2 to + 20 Silica. At about Plus 10 - 20 it appears as a very glossy clear

The base Glaze appears on Glazy at https://glazy.org/recipes/19734

I have a full progression with pictures in a June newsletter, it is just too big to place here so if we figure a decent way for you to receive it I will forward at your request.

 

Enjoy!

Oops, almost forgot

To answer your question about the 3249, that was the analysis of the OVMI glaze

 

 

 

[Hulk]

Thanks Bill!

[Min]

On 11/15/2018 at 8:20 AM, OVMI_Designs said:

Ferro Frit 3249	15.000
Minspar 200	33.000
Wollastonite	13.000
Silica	10.000
EPK	19.000
Talc	10.000

If you take the revised recipe that OCMI_Designs posted and drop the silica and bump the EPK to 26, and keep everything else the same you will get a very similar glaze to the one Bill posted but with about 1/2 the LOI  than the Plus Six recipe. (part of the EPK will likely need calcining) Just another way to approach it.

[Bill Kielb]

True but the resulting glaze has a less than desirable RO ratio of 0.16 : 0.84, relatively high Boron (.21) and  6.79:1 silica to alumina ratio which means more glossy than matte.

It may work fine and experimenting is always great. LOI is usually not my primary concern but there are many ways to do this

 

[Min]

Try it with 

25 minutes ago, Min said:

drop the silica and bump the EPK to 26

 

[Bill Kielb]

Ok

maybe, many ways to do this . The solution I gave is tested, good chemistry, no calcined hassle and works for us. This may be fine as well

[OVMI_Designs]

The matting mechanism of the glaze I am working on is MgO based and doesn't rely so much on the Silica to Alumina ratio. With that said I don't know enough about glaze chemistry to know what makes a glaze good aside from seeing and testing the end results. I am open to trying other matte base glazes for sure. Whats appealing to me with this glaze is the low LOI and low expansion. In the past using the clay body and kiln I have, I have had issues with crazing on some glazes and blisters on others so I tend to try and stick to glazes with 6.5 or lower expansion.

I will try some test tiles of both, and try bumping the EPK and lowering the silica on the one I posted. I was attempting to lower the amount of epk used as a means to try and reduce the cracking of the glaze around the handle issue, which perhaps is being caused by something else. Is there something I need to know about the amount of clay in a glaze that pertains to durability?

Thanks Y'all.

-Matt

[OVMI_Designs]

Also, oldlady asked about Frit 3249. It isn't a typically used frit, but you can source it from some ceramic supply shops. Its chemistry is useful for this glaze because it contains some important oxides as compared to say Frit 3124 or 3134. Here is the breakdown:

CaO 3.5

MgO 12.2

B2O3 28.9

Al2O3 13.3

SiO2 42.1

[Bill Kielb]

The glaze I gave you is a a magnesium matte but I understand if you prefer experimenting. Always fun.

with respect to clay 10% or greater to keep the glaze suspended, else each have their own advantages in composition chemically so I have do not have a generic go to on this with respect to the mixed hydrated  glaze and its drying tendencies.

with respect to low expansion, the fired  glaze will have its own CTE, not necessarily in correlation with prefired expansion calculation.

loi is what it is so when looking at relative concentrations it is interesting and in my view of lesser interest with respect to off gassing when we look at  actual temperatures during firing but I get it is a cause for concern for many.

I would suggest you look at work done by Sue McLeod and Anna Burke. The  test show a  strong correlation with alumina level and flux ratio and were used to correct Marcias matte clear for application on low CTE porcelain.

finally a very reasonable indicator of durability is flux ratio. Durable (tested) measured ratios extend from 0.3 : 0.7 to as low as 0.2:0.8. Outside this range, durability  in the dishwasher suffers greatly. It is an easy way to make an initial assessment of a glaze and likely durability.  Let’s face it, no one is out there testing what they create regularly and getting a glaze in this flux range is fairly easy so unless artistic, non functional or sculptural why not strive for known durability?

just my thoughts, there are many

 

[Min]

1 hour ago, OVMI_Designs said:

I have had issues with crazing on some glazes and blisters on others so I tend to try and stick to glazes with 6.5 or lower expansion.

Calculated COE figures don't work with any glaze with a crystalline structure, including the magnesium mattes. 

If you still get cracking with either of the above recipes try calcining part of the kaolin. For now I would just look how the glaze looks / tests after firing.

If you are using the same glaze as a liner I would really suggest trying to cutlery mark it, might have to compromise between the degree of matte to gloss to prevent it. I used Dover White throwing body years ago, used low COE gloss glazes, had to get the COE below 6 to prevent crazing for the clear.

[OVMI_Designs]

Thanks for the info. I will continue tweaking the glaze, adjusting material percentages to find a good balance. I'll post some results with photos so you all can see the progress.

[OVMI_Designs]

Here is the result of my glaze reformulation. Thanks for all the help!

Here is the recipe:

Ferro Frit 3249	14.000	g
Minspar 200	35.000	g
Wollastonite	13.000	g
Silica	9.000	g
EPK	18.000	g
Talc	5.000	g
Calcined Kaolin	6.000	g