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Cheers!  I thank you very much if anyone can consult.
 I don't know why in the black or red ceramic paste (which I have high temperature),
  my faldespatico base enamels do not cover this clay. The enamels come out with bubbles.., although in white clay they work well for me.
 I understand that black clays contain ox iron and manganese..

Edited by Uliana
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Thanks for answering. I don't understand why enamels react so differently in the different types of body.  And how can I configure the enamel with different paste.

I kiln in an oxidizing atmosphere  / cones 8-9 

1.          100*c/h  - 500*C - 0:00

2.          5h  - 1070*C - 0:00

3.          3h -  1257*C - 0:20

4.          End

IMG_20200531_155632__01-3.jpg.289a8b7a4b1675c01fbe8f97aca3afc5.jpg

Clay I use for high temperature:

  • 1.    White chamotte refractory paste (gray refractory paste 0-0.2 mm)

SiO2      ox of silisio                63,92

Al2O3   ox of alumina         26,48

Fe2O3  ox iron                        1,06

TiO2      ox titanium              1,52

CaO       calcium ox                 0,14

MgO    magnesium ox          0,18

Na2O   sodium ox                  0,24

K2O     potassium ox            1,60

MnO    manganese ox        <0,01

L.O.I  ….. 4,26

Plasticity (Atterberg)

L.L    …..…34

I.P    ……..16

  •  2.  Black chamotte refractory paste (black refractory paste 0-0.2 mm)

SiO2      ox of silisio               41,62

Al2O3   ox of alumina         30,17

Fe2O3  ox iron                       13,86

TiO2      ox titanium              1,49

CaO       calcium ox                 0,28

MgO    magnesium ox          0,62

Na2O   sodium ox                  0,22

K2O     potassium ox            1,37

MnO    manganese ox          0,36

MnO2 manganese ox          2,79

L.O.I………7,37

Plasticity (Atterberg)

I.P.........22

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The two dark clay test tiles with the large bumps, are the bumps intentional or did they occur in the firing? 

With dark clays containing lots of iron and manganese like the one you are using it's really important to bisque fire them in a well vented firing.  How are you doing your bisque firing? Also, what is the temperature or cone number your dark clay is rated to fire up to? 

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11/5000

Thanks Min!

The bumps they occurred in the fairing. Is not intentional!
Range of this black clay is: 1200ºC-1240ºC
My bisque is 1010ºC
The faring wis glaze is at 1257ºC


I am a beginner in the firing, I thanks for any info on that topic.
Most of my glazes in that clay rich in iron and manganese get pitted or bubble.., although in white clay I don't have this problem!
I don't know why? how can these defects be avoided?
It may be that I need to add something in glaze ? I am thinking about Bentonite ?

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8 hours ago, Uliana said:

Range of this black clay is: 1200ºC-1240ºC
My bisque is 1010ºC
The faring wis glaze is at 1257ºC

By firing to 1257C you could very well be overfiring the clay, this can be one cause for the bumps (called bloats) in the claybody itself.

The bisque firing schedule is really important with dark iron/manganese claybodies. The kiln needs to be well vented and not too crowded with pots. There needs to be time for the organic materials to cleanly burn off in the bisque. If you have a controller on your kiln then you need to slow the firing down to 100 C an hour from 150C up to 325C and a very  slow 50C an hour between 700C and 900C. These are the temperature zones that the carbons need time to burn out. If they don't then what happens is the gasses that didn't burn out in the bisque try to escape during the glaze firing but they can't so the clay can get bloats and the glaze has problems like you are having. Do you have a controller and a vent on your kiln?

I hope this is clear for you when you translate it. 

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Min, thank you very much for the reply!

Yes, I have the temperature controller. I'm going to change the settings of the program.
That topic is very interesting to me, do you know any book to know more about it?

Through love for clay I will improve English =)
I hope I will be successful, and here I will leave the photos of my experiments.

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Your English is just fine, I was trying to avoid words that might be difficult to translate.

You can read about body bloating here, this is an excellent website with a huge amount of information on many ceramic subjects. Page 166 from "The Ceramic Process" which is available here as a preview also mentions body bloating.  Lastly, there is a really good thread on firing high manganese clay here

For the bubbles and blisters search those terms in this forum and also on the first one I linked to above.

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