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Glaze Squirt On Kiln Shelves


Cadaola

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Hi everybody!

 

I have 2 problems, maybe interconnected one with the other...

 

- The first problem appears when I open the kiln: almost all objects are ok, but some of them leave glaze spots on the kiln shelves, as you can see on the pictures I post.

 

I had this problem in the past with a floating blue (cone 6) that contains 20% boron that I applied too thick (but it was required from the recipe!)

 

Now I have the same problem with 2 glazes that don't contain boron or only a little bit.

One is a chun blue (2 glazes: one on top fo the other) and one is a blue-green glaze, both cone 6.

 

I'm wondering if the problem has to do with firing cicle maybe too short, or the thickness of glaze application, or the bisque firing cicle too short and impurity not burned away?

The strange thing is that the problem doesn't occur on all objects, but only on some, and the size is no matter, in fact pieces with the same size can cause or not this problem.

 

- The second issue has to do with "almost all objects are ok", in fact some of them are "rejecting" the blue-green glaze and leave some parts of the surface without glaze, while others work well (see picture).

why? I cleaned all of them with air compressor and the biggest ones I also sponged with little water in order not to absorb too much glaze.

 

It is really a mess, I don't understand: I test the glaze, it seems ok. I fire some objects, they come out ok, I fire others, they are not ok! Same kiln, same clay, same glaze, same thickness, same temperature, same all!

 

Any advise? Thanks in advance for your help :-)

 

Paola

 

 

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The kiln shelves look like the glazes are spitting during the firing, which could be a sign of firing too fast for the glaze you are using.  Our studio had a really great blue that spit -- and would contaminate any pot on the shelf next to it.  Same for the pot that rejects glaze in parts -- firing too fast (and maybe glaze too thick) to allow a good melt.  Glazes bubble on the pot surface when you are at top temperature.  Try a slower glaze firing, especially the last few hundred degrees before you reach top temperature. 

 

Test tiles are great, but they don't often show how the glaze acts on a full surface, curved surface, etc.

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Small spots on the shelf are likely from spitting, and can be caused by the boron. Large spots on the shelf and bare spots on the pots are different. When the glaze pulls away leaving bare clay we call that 'crawling'. It can happen when glazes are overlapped if the second dip is applied when the first dip is still too wet, or too dry. It can also be caused by the surface tension of the glaze during the firing, glaze thickness,  firing parameters, etc. Floating blue can be prone to lots of different problems, including crawling.

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Is it possible that there is a combination of two things going on. I agree with the spitting symptom, but wonder if the cause is firing the ware too fast after glazing. I have found that if you glaze pieces and fire them too quickly, the water in the piece turns to steam, and forces some glazes to flake. I counter this by adding a water smoke in the first hour of glaze firing. Seems to help especially if the last few pieces were just glazed. The one bowl definitely is crawling from too thick a surface.

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Thanks for your answers!

I realize that I have never paid much attention to firing shedule, because i've never had problems except for those 2 glazes.

I post my bisque and glaze curve I used for this (and all my) objects:

 

BISQUE:

60°C (140F) /h to 120°C (248F), soak 30 min.

100°C (212F) /h to 600°C (1112F)

80°C (176F) /h to 980°C (1796F), soak 30 min.

 

GLAZE:

120°C (248F) /h to 600°C (1112F)

135°C (275F) /h to 1230°C (2246F), soak 30 min.

 

Do you think those firing cicle are to brief? Do you recommend any particular firing schedule?

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Guest JBaymore

Do you have the recipes for the glazes in question?

 

Yes... the glaze fire profile is quite fast.

 

best,

 

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

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Blue-green cone 6 (causes CraWling and spit a little on kiln shelves)

 

Ferro frit 3195------20

Wollastonite---------29

Neph Sye------------4

EPK------------------30

Silica----------------17

 

Chun Blue cone 6 (first layer Tenmoku, second layer chun glaze. Spit on kiln shelves)

 

Tenmoku

Feldsp potash---43,2

silica--------------26

Whiting-----------21

EPK---------------10

red iron ox.------20

bentonite---------1,8

 

Chun

Feldsp potash---56

dolomite----------6

zinc---------------6

Whiting----------10

EPK--------------2

silica-------------30

bentonite-------1

lithium-----------1

copper ox.------0,5

rutile-------------3

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  • 2 weeks later...

I refired the small object adding some glaze on the bare surface and it came out well (not perfect, but ok).

 

I fired this second glaze kiln with a different curve in order to avoid the spitting problem and it worked well only in part: for the tenmoku-chun blue it worked well (no spit on the shelf) even if the surface quality changed a lot, but it is still pleasant (and I changed the temnoku glaze with an iron red glaze). (see small cup picture)

 

I also changed application methods: I dipped or poured instead of brushing, and I added some water to the glaze in order to obtain a thinner layer of glaze on the pot. That was useful.

 

I had a problem with another glaze (with boron content), but I already posted about it, so I don't say again here.

 

Here the firing schedule:

 55°C/h to 110°C hold 30 min

200°C/h to 1080°C no hold

85°C/h to 1200°C hold 15 min

275°C/h to 1000°C no hold

70°C/h to 760°c no hold and cool naturally

 

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post-4274-0-75454500-1430822970_thumb.jpg

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I refired the small object adding some glaze on the bare surface and it came out well (not perfect, but ok).

 

I fired this second glaze kiln with a different curve in order to avoid the spitting problem and it worked well only in part: for the tenmoku-chun blue it worked well (no spit on the shelf) even if the surface quality changed a lot, but it is still pleasant (and I changed the temnoku glaze with an iron red glaze). (see small cup picture)

 

I also changed application methods: I dipped or poured instead of brushing, and I added some water to the glaze in order to obtain a thinner layer of glaze on the pot. That was useful.

 

I had a problem with another glaze (with boron content), but I already posted about it, so I don't say again here.

 

Here the firing schedule:

 55°C/h to 110°C hold 30 min

200°C/h to 1080°C no hold

85°C/h to 1200°C hold 15 min

275°C/h to 1000°C no hold

70°C/h to 760°c no hold and cool naturally

I think you should rethink  about your firing schedules for bisque and glaze.  85C is quite fast for the last step of glaze firing and if you are going this fast than the top temperature should have been 1230 C, but since you  hold it for 15 min than the top temp is 1221 C

Also depending on how good the elements are it is possible that the kiln can't fire this fast.  Do you use self standing cones, or make your own cone pack?

David

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Here the firing schedule:

 55°C/h to 110°C hold 30 min

200°C/h to 1080°C no hold

85°C/h to 1200°C hold 15 min

275°C/h to 1000°C no hold

70°C/h to 760°c no hold and cool naturally

I think you should rethink  about your firing schedules for bisque and glaze.  85C is quite fast for the last step of glaze firing and if you are going this fast than the top temperature should have been 1230 C, but since you  hold it for 15 min than the top temp is 1221 C

Also depending on how good the elements are it is possible that the kiln can't fire this fast.  Do you use self standing cones, or make your own cone pack?

David

 

I took the glaze firing schedule from digitalfire:

http://digitalfire.com/4sight/firingschedule/mastering_glazes_cone_6_firing_schedule_83.html

 

I usually use a more simple firing schedule:

GLAZE:

120°C (248F) /h to 600°C (1112F)

135°C (275F) /h to 1230°C (2246F), soak 30 min.

 

I have an electric kiln. which firing schedule do you use or recommend me to use?

 

(I don't use cones, because the kiln seller told me they are not useful because the kiln is high performance and fires very precisely with the same temp all inside... but I will buy them now, that I understand I can reach cone 6 without necessarly reaching 1230°C (did I understand correectly?)

Thank you :-)

 

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Here the firing schedule:

 55°C/h to 110°C hold 30 min

200°C/h to 1080°C no hold

85°C/h to 1200°C hold 15 min

275°C/h to 1000°C no hold

70°C/h to 760°c no hold and cool naturally

I think you should rethink  about your firing schedules for bisque and glaze.  85C is quite fast for the last step of glaze firing and if you are going this fast than the top temperature should have been 1230 C, but since you  hold it for 15 min than the top temp is 1221 C

Also depending on how good the elements are it is possible that the kiln can't fire this fast.  Do you use self standing cones, or make your own cone pack?

David

 

I took the glaze firing schedule from digitalfire:

http://digitalfire.com/4sight/firingschedule/mastering_glazes_cone_6_firing_schedule_83.html

 

I usually use a more simple firing schedule:

GLAZE:

120°C (248F) /h to 600°C (1112F)

135°C (275F) /h to 1230°C (2246F), soak 30 min.

 

I have an electric kiln. which firing schedule do you use or recommend me to use?

 

(I don't use cones, because the kiln seller told me they are not useful because the kiln is high performance and fires very precisely with the same temp all inside... but I will buy them now, that I understand I can reach cone 6 without necessarly reaching 1230°C (did I understand correectly?)

Thank you :-)

 

 

Bisque  cone 06

48C ramp to 121 C hold 30 min, 149 C ramp to 538C hold 15 min., 65C ramp to 648C hold 10 min.,135 C ramp to 896 C hold 5 min., 26C ramp to 996 C hold 5 min.  The big difference is slower through the carbon burnout period.

Cone 6 glaze

93 C ramp to 93 C hold 15 min.,step to 1115 C hold 5 min., ramp 26 C to 1215 C hold 5 min., you can add your cool down if you like.  The only way to be sure you are getting a cone 6 firing is to use witness cones  after that you don't need to use them every time.  I would recommend self supporting cones for witness cones.  I hope things start to improve for you.

David

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Bisque  cone 06

 

48C ramp to 121 C hold 30 min, 149 C ramp to 538C hold 15 min., 65C ramp to 648C hold 10 min.,135 C ramp to 896 C hold 5 min., 26C ramp to 996 C hold 5 min.  The big difference is slower through the carbon burnout period.

Cone 6 glaze

93 C ramp to 93 C hold 15 min.,step to 1115 C hold 5 min., ramp 26 C to 1215 C hold 5 min., you can add your cool down if you like.  The only way to be sure you are getting a cone 6 firing is to use witness cones  after that you don't need to use them every time.  I would recommend self supporting cones for witness cones.  I hope things start to improve for you.

David

 

 I received a lot of input from all of you and I'll try to find my way. I will let you know about my results (hoping they are positive). It will take a while (maybe months) but I will tell you for sure! Thank you for your support and help! Best wishes! Paola

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