Test Firing for Miniature Work?

[SweetheartSister]

Hi everyone,

I want to do some test firings to make sure that my jewellery pieces are reaching maturity (I have been experiencing some crazing issues). Both my slip and overglaze have a wide firing range across three cones, so I was planning on using three orton cones in each firing to see which cone is the optimum temperature to fire at.

My work is really tiny - some of the pieces are less than 1cm x 1cm. Should I test fire using actual jewellery pieces, rather than test tiles? Because the tiles would be much bigger, and the rate that they mature won't necessarily represent the rate at which the tiny pendants mature?

I really appreciate any advice or insight on this matter, thank you!

[Bill Kielb]

with respect to cones it’s typical to include a guide, firing, and guard  cone pack to know with fair certainty what cone you actually fired to. So for example firing to cone six, one would use a 5 (guide) and a 6 (firing or target) and a 7 (guard) to see if  their ware was underfired, perfectly fired,  or over fired. Using this three cone pack allows for a relatively accurate easy memorial to the firing. Truth be told, these are usually set in front of a spyhole and the kiln operator (with proper goggles) observes them during the firing and terminates the firing based on the cone bend progression.

with respect to your kiln and thermal mass, my sense is whether test tile or jewelry it is such a small mass in relation to the kiln that the kiln itself takes far longer to heat  on its own.  loaded or unloaded it seems the loaded wares are likely such a small amount of mass compared to the kiln that the rate of heating is entirely affected by the firing schedule and the power the kiln has to fire with.

with respect to maturity, clay products mature or fully melt at their rating. Firing less than the rating means not fully mature.  Bisque firing allows us to partially fire something which usually is done at a low enough temperature so the volatile organics are burned out along with chemically combined water and the ware is sintered together. It ends up bit sturdier to handle but still porous enough to receive the glaze.  Generally the glaze and body match well with respect to maturity and they are fired as such so both mature in the glaze firing to their  rated cone.

so, cone six products fully mature at six, are not fully mature at cone 5 or 4 and  are over fired at 7. As to mixing materials with different maturity points it’s extremely difficult to know their performance as a whole. We have a fully baked cake with partially baked layers in it. Will it taste good, hard to know.

I wrote all this out because your issue seems to be glaze fit and crazing which has its own peculiarities. Glazes need to match the final fired clay product with respect to expansion and contraction otherwise they can craze or shiver. Glazes themselves mature  at a specific temperature so combining a mix of maturities makes prediction of success really difficult since some of these things are not fully baked in the end.

You may find a sweet  spot by testing. My suggestion is  test your final cone firing temperature first and see if this provides success.  Bisque firing to 04 or 05  usually only makes a difference in  how well the piece takes to glaze application not its ultimate maturity.

having said all that it may be easier than it seems. Assuming you cannot see cones during your firing and assuming your wares are so small with respect to the kiln they hardly add thermal mass  You can do what automatic controllers do to get close to firing to cone using temperature or perhaps just firing time and power setting for  your kiln. Which of course takes a few test fires of your kiln with cones and power settings to establish a base line.

Just a word of caution, Bisque fire to cone over roughly ten hours to give time to burn out the organics unless you can assure your body needs less time and glaze firings can reach cone in about five hours again based on result. Each firing should end at the designed cone though. Adding holds starts moving into another cone value  which in your case could affect or complicate the glaze fit issue.

Right now I suggest developing schedules that end at a desired cone in a certain amount of time. Test tiles or not you need some starting baseline.

 

 

[SweetheartSister]

13 minutes ago, Bill Kielb said:

with respect to cones it’s typical to include a guide, firing, and guard  cone pack to know with fair certainty what cone you actually fired to. So for example firing to cone six, one would use a 5 (guide) and a 6 (firing or target) and a 7 (guard) to see if  their ware was underfired, perfectly fired,  or over fired. Using this three cone pack allows for a relatively accurate easy memorial to the firing. Truth be told, these are usually set in front of a spyhole and the kiln operator (with proper goggles) observes them during the firing and terminates the firing based on the cone bend progression.

with respect to your kiln and thermal mass, my sense is whether test tile or jewelry it is such a small mass in relation to the kiln that the kiln itself takes far longer to heat  on its own.  loaded or unloaded it seems the loaded wares are likely such a small amount of mass compared to the kiln that the rate of heating is entirely affected by the firing schedule and the power the kiln has to fire with.

with respect to maturity, clay products mature or fully melt at their rating. Firing less than the rating means not fully mature.  Bisque firing allows us to partially fire something which usually is done at a low enough temperature so the volatile organics are burned out along with chemically combined water and the ware is sintered together. It ends up bit sturdier to handle but still porous enough to receive the glaze.  Generally the glaze and body match well with respect to maturity and they are fired as such so both mature in the glaze firing to their  rated cone.

so, cone six products fully mature at six, are not fully mature at cone 5 or 4 and  are over fired at 7. As to mixing materials with different maturity points it’s extremely difficult to know their performance as a whole. We have a fully baked cake with partially baked layers in it. Will it taste good, hard to know.

I wrote all this out because your issue seems to be glaze fit and crazing which has its own peculiarities. Glazes need to match the final fired clay product with respect to expansion and contraction otherwise they can craze or shiver. Glazes themselves mature  at a specific temperature so combining a mix of maturities makes prediction of success really difficult since some of these things are not fully baked in the end.

You may find a sweet  spot by testing. My suggestion is  test your final cone firing temperature first and see if this provides success.  Bisque firing to 04 or 05  usually only makes a difference in  how well the piece takes to glaze application not its ultimate maturity.

having said all that it may be easier than it seems. Assuming you cannot see cones during your firing and assuming your wares are so small with respect to the kiln they hardly add thermal mass  You can do what automatic controllers do to get close to firing to cone using temperature or perhaps just firing time and power setting for  your kiln. Which of course takes a few test fires of your kiln with cones and power settings to establish a base line.

Just a word of caution, Bisque fire to cone over roughly ten hours to give time to burn out the organics unless you can assure your body needs less time and glaze firings can reach cone in about five hours again based on result. Each firing should end at the designed cone though. Adding holds starts moving into another cone value  which in your case could affect or complicate the glaze fit issue.

Right now I suggest developing schedules that end at a desired cone in a certain amount of time. Test tiles or not you need some starting baseline.

 

 

Thanks so much for your response Bill, this is extremely informative.

I think what has thrown me off and muddled me is that my earthenware casting slip and my clear overglaze both have wide firing ranges.

My earthenware slip's recommended firing temp is 1080 - 1140 celcius. Which is Cone 03 - Cone 2. So how can I know which cone is the best to fire to, or is this suggesting that the kiln will reach maturity within this range?

Similarly, my transparent glaze has a firing range of 1000 - 1140 celcius. In the past I fired the bisque to cone 01 and the transparent glaze to 05 and this worked for me, however, when I included some larger items in the kiln, that is when crazing occured. So I wonder if the 'fit' was not good enough with this combination, and it needs adjusting. How can I know that a glaze and clay body will fit each other well? Particularly as I am using commercial/ready made slip and glazes.

Sorry if these questions have obvious answers - I have been doing a lot of research but still don't feel like I understand entirely.

 

[Bill Kielb]

No don’t be sorry your situation although simple is actually extremely complicated to figure out. Bisque firing to something near maturity  generally makes it close off enough to become difficult to apply glaze. As a guess I think you want everything  to end up as mature as practical for strength so it appears cone 2 is your limiting factor.

Trial and error is going to be the only way to solve this with reasonable certainty as even pretty closely matching body and glazes can and will craze over time. How much time? We still are trying to figure out a foolproof test for that.

I still think getting some baseline schedules down pat will help, then you can fire  with a single cone later just to confirm you are still hitting your final desired cone temps. From there I believe you will see what is successful and what crazes suddenly more easily. Right now, lots of moving parts likely makes this tougher to find the sweet spot for your jewelry.

[SweetheartSister]

49 minutes ago, Bill Kielb said:

No don’t be sorry your situation although simple is actually extremely complicated to figure out. Bisque firing to something near maturity  generally makes it close off enough to become difficult to apply glaze. As a guess I think you want everything  to end up as mature as practical for strength so it appears cone 2 is your limiting factor.

Trial and error is going to be the only way to solve this with reasonable certainty as even pretty closely matching body and glazes can and will craze over time. How much time? We still are trying to figure out a foolproof test for that.

I still think getting some baseline schedules down pat will help, then you can fire  with a single cone later just to confirm you are still hitting your final desired cone temps. From there I believe you will see what is successful and what crazes suddenly more easily. Right now, lots of moving parts likely makes this tougher to find the sweet spot for your jewelry.

I suppose that doing test firings is the best way to hit that sweet spot, as you say. I've been reading that freezing and then pouring boiling water over a finished, glazed piece is the best way to see if it's a good fit on or/will be long lasting. So I think I will try that with each batch of tests, until I don't get crazing anymore.

Something else I read though, is that earthenware always remains a bit porous, and therewill be inevitably craze at some point in time? If this is the case, would it be advisable for me to try porcelain instead in the future? I have been reading that porcelain has a very high density, and due to it being a high fire ceramic, it is less likely to craze over time (so long as the clay and glaze are a good fit, etc)

I really hope I can crack this! The enjoyment for me is in the designing, sculpting and making, but obviously getting the kiln temps and timings right is so important in creating a chemically stable, beautiful piece.

[Bill Kielb]

My wife uses porcelain for beads and earrings and other Jewelry Fantastic stuff, hand painted underglaze cone six frost semi translucent.  All glazed with a clear gloss or matte clear that we designed and fit to the clay body.

crazing tests by shocking cold to hot work but there is no standard as to predictability. They are inferential and definitely better than no test for sure.

You will find your sweet spot, enjoy the journey and create cool stuff. Heck ya never know what it might lead to.

[SweetheartSister]

On 6/27/2019 at 11:37 PM, Bill Kielb said:

My wife uses porcelain for beads and earrings and other Jewelry Fantastic stuff, hand painted underglaze cone six frost semi translucent.  All glazed with a clear gloss or matte clear that we designed and fit to the clay body.

crazing tests by shocking cold to hot work but there is no standard as to predictability. They are inferential and definitely better than no test for sure.

You will find your sweet spot, enjoy the journey and create cool stuff. Heck ya never know what it might lead to.

Lovely work! I'd be interested to know which underglazes your wife uses on her porcelain?

[Bill Kielb]

She has used them all, Amaco, Mayco,  Duncan, (underglazes) and even mason stain mixed in a propylene glycol base when colors need to be just right while she is painting. Her first love was painting and brush art work or portrait art. Since clay, she loves throwing, sculpting, and does quite a bit of custom jewelry using a very white semi translucent cone six porcelain called frost. My last picture was a bit limited but what I had in arms reach that showed underglaze.

what was significant for us and probably relevant to your story is we really had to solve the glaze dilemma that matched the claybody and would be very clear, very glossy in the case of the gloss, or clear and smooth melt in the case of the matte. Both glazes were designed to melt well over heavy underglaze applications which was a significant hurdle to achieve.

As you can see, it has led to all sorts of jewelry and other items and even lends itself to China paint accent and adornment with Swarovski crystals, you name it. All things being equal, even carved pure white jewelry items are a thing using just the frost clay body and a super smooth glaze application.

so ya never know where clay will take you. She loves the creativity it allows, hopefully you will enjoy  the journey and make great pieces as well.

 

[Chilly]

I'd say you need to test with the same size as your actual items.

[Bill Kielb]

On 6/30/2019 at 1:12 PM, SweetheartSister said:

Lovely work! I'd be interested to know which underglazes your wife uses on her porcelain?