100% ball clay projecting black dots after firing at 550 °C

[bijoy]

After  firing at 550°C for 20 min the 100% ball clay surface of the product some black spots of diffrent size and shape (2-15mm). Can anyone please tell me  why it is happening. 

[Bill Kielb]

Likely organics (Carbon), why are you firing at 550c (cone 024 ish) for 20 minutes?  Just curious, what s intended with firing at this low temperature, removal of combined water?

[neilestrick]

Yep, organics. You'll start to see the spots below red heat, like at 400C-ish until it's hot enough to burn them out.

[bijoy]

2 hours ago, neilestrick said:

Yep, organics. You'll start to see the spots below red heat, like at 400C-ish until it's hot enough to burn them out.

Thank you for the reply.  Is there any alternative to remove them(Organics) without crossing 600°C? Because i don't want my product to be very hard.

[bijoy]

7 hours ago, Bill Kielb said:

Likely organics (Carbon), why are you firing at 550c (cone 024 ish) for 20 minutes?  Just curious, what s intended with firing at this low temperature, removal of combined water?

Thank you for the reply.  Yes. I just want to remove the combined water. By not making it very hard. But facing problem with the black spots. Do you have any suggestion  how i can solve this problem?

[Bill Kielb]

550 c may get it done, just more time may be the secret and safest thing to try. I think even my self cleaning oven needs a few hours time to turn the simple organics to white ash at about 500c. I would try a 30 or 40 minute hold first, or even a slower ramp, say 100c per hour, then if no improvement gradually increase temperature. I see references to 630c as a common temperature to dewater.    I don’t think you will see  significant clumping issues until above 700c. So in the end, likely a balance between temperature and time you are willing to wait.

As always, test to know for your clay, kiln, firing technique………etc

If you do test, try and return here to post what works for you. It may help someone in the future.

[bijoy]

46 minutes ago, Bill Kielb said:

550 c may get it done, just more time may be the secret and safest thing to try. I think even my self cleaning oven needs a few hours time to turn the simple organics to white ash at about 500c. I would try a 30 or 40 minute hold first, or even a slower ramp, say 100c per hour, then if no improvement gradually increase temperature. I see references to 630c as a common temperature to dewater.    I don’t think you will see  significant clumping issues until above 700c. So in the end, likely a balance between temperature and time you are willing to wait.

As always, test to know for your clay, kiln, firing technique………etc

If you do test, try and return here to post what works for you. It may help someone in the future.

Thank you kielb. I am planning to supply co2 gas on the oven when temperature is above 200°C to 550°c prevent burning of the organics. What do you think? Will it work?

 

 

 

[Bill Kielb]

44 minutes ago, bijoy said:

am planning to supply co2 gas on the oven when temperature is above 200°C to 550°c prevent burning of the organics. What do you think? Will it work?

Seems like the opposite of what one would want to do. Not sure why you wouldn’t want to burn out the organics?

[bijoy]

It will used for marking purposes. So if i increase temperature higher then it is loosing its efficient marking capabilities. 

[Bill Kielb]

16 minutes ago, bijoy said:

It will used for marking purposes. So if i increase temperature higher then it is loosing its efficient marking capabilities. 

Yeah, I am not following actually.. you are using this clay to make something and it needs to be fully dewatered or calcined but you would like the organics to stay?  Sounds like  you will need to experiment with time, temperature etc…

[Hulk]

Please be careful supplying CO2 - such that your O2 supply is not displaced.

[Callie Beller Diesel]

@bijoy What are you making? There may be more than one way to get the results you want.

[bijoy]

8 hours ago, Bill Kielb said:

Yeah, I am not following actually.. you are using this clay to make something and it needs to be fully dewatered or calcined but you would like the organics to stay?  Sounds like  you will need to experiment with time, temperature etc…

It is not matter whether the organics stays or not. I just don't want the black spots.  I tried to 600°C for 40 min according to your suggestion. But they are still there (. And also supplying co2 didn't helped me today.  They are still appearing from 400°C to 600°C (.

 

1 hour ago, Hulk said:

Please be careful supplying CO2 - such that your O2 supply is not displaced.

Can you please tell me  why?

[bijoy]

1 hour ago, Callie Beller Diesel said:

@bijoy What are you making? There may be more than one way to get the results you want.

@Callie Beller Diesel i am making marking chalk.  Which will be able  to mark easily but will not break easily.  Any kind of suggestion is very welcome about this).

[Bill Kielb]

10 hours ago, bijoy said:

I tried to 600°C for 40 min according to your suggestion. But they are still there (. And also supplying co2 didn't helped me today.  They are still appearing from 400°C to 600°C (.

I believe they will take time and temperature to burn out. Everything I read says no major clumping till 700c so, I think you could test lower temperature for more time,  or move on up towards 700c If adding time does not Help. Remember, I said it takes hours to clean the stove to white ash at about 500c. So if it were me and since you have no clumping issues at 600c, what happens if you hold for  2 hours, 4 hours? If it’s an improvement then continue on that path. If it’s not enough then you will need to go higher in temperature and / or lengthen your time.

You might consider Plaster of Paris or cornstarch easily make nice chalk as well. No kiln required.

Plaster of Paris video:

https://youtu.be/r07-m70t34A

 

[PeterH]

10 hours ago, Hulk said:

Please be careful supplying CO2 - such that your O2 supply is not displaced.

 

8 hours ago, bijoy said:

Can you please tell me  why?

At low concentrations, gaseous carbon dioxide appears to have little toxicological effect. At higher concentrations it leads to an increased respiratory rate, tachycardia, cardiac arrhythmias and impaired consciousness. Concentrations >10% may cause convulsions, coma and death.

A few of the sites I used to visit had equipment  rooms with CO2 flooding in case of fire, and took the dangers quite seriously. I think somebody had died in an incident at another company.

[bijoy]

9 hours ago, Bill Kielb said:

I believe they will take time and temperature to burn out. Everything I read says no major clumping till 700c so, I think you could test lower temperature for more time,  or move on up towards 700c If adding time does not Help. Remember, I said it takes hours to clean the stove to white ash at about 500c. So if it were me and since you have no clumping issues at 600c, what happens if you hold for  2 hours, 4 hours? If it’s an improvement then continue on that path. If it’s not enough then you will need to go higher in temperature and / or lengthen your time.

You might consider Plaster of Paris or cornstarch easily make nice chalk as well. No kiln required.

Plaster of Paris video:

https://youtu.be/r07-m70t34A

 

@Bill Kielb Thank you for your suggestion). Can you please tell me how i can increase the ball clay green strength.  As ball clay provide the best marking quality on my uses and plaster of paris is not quite smooth at marking. 

[Russ]

You might try a binder to hold the clays together but something that will not be so hard that it wont mark. Cmc gum comes to mind . Or you could just use pressure to bind the clay such as how they make talc tile with minimal moisture and is just held together by pressure alone.. similar to a pill manufacturing press. I kno they use these machines for candy also.. just have a mold in the form of your chalk stick and press.

[Hulk]

21 hours ago, bijoy said:

Can you please tell me  why?

 

23 hours ago, Hulk said:

Please be careful supplying CO2 - such that your O2 supply is not displaced.

Why? O2 is required.

At my Dad's work, a man working in a tank displaced enough air (shield gas for heliarc) that he passed out; his friend and coworker tried to help. I was five, however, likely I'll never forget how devastated my Dad and his work friends were.

There was a near incident at the steel mill where I worked. Two workers made their way to the top of the furnace during an outage; it had been venting for a while, however, the plume of warm nitrogen was enough to drop the first guy. The second guy ran back the way they'd come, figuring there was some oxygen in that direction, took a few deep breaths, then jogged over to drag the downed guy back - saved his life. Both were suspended without pay for violating the procedures; the second guy was suspended longer - for putting his life at risk.

[Min]

@bijoy, does it have to be ball clay or could you use a cleaner burning kaolin instead?

[bijoy]

14 hours ago, Russ said:

You might try a binder to hold the clays together but something that will not be so hard that it wont mark. Cmc gum comes to mind . Or you could just use pressure to bind the clay such as how they make talc tile with minimal moisture and is just held together by pressure alone.. similar to a pill manufacturing press. I kno they use these machines for candy also.. just have a mold in the form of your chalk stick and press.

@Russ how about sodium silicate and 5 % bentonite as inorganic binder?considering the shelf life of inorganic binder.

[bijoy]

14 hours ago, Hulk said:

 

Why? O2 is required.

At my Dad's work, a man working in a tank displaced enough air (shield gas for heliarc) that he passed out; his friend and coworker tried to help. I was five, however, likely I'll never forget how devastated my Dad and his work friends were.

There was a near incident at the steel mill where I worked. Two workers made their way to the top of the furnace during an outage; it had been venting for a while, however, the plume of warm nitrogen was enough to drop the first guy. The second guy ran back the way they'd come, figuring there was some oxygen in that direction, took a few deep breaths, then jogged over to drag the downed guy back - saved his life. Both were suspended without pay for violating the procedures; the second guy was suspended longer - for putting his life at risk.

Thank you for the information).

[bijoy]

13 hours ago, Min said:

@bijoy, does it have to be ball clay or could you use a cleaner burning kaolin instead?

@Min @Bill Kielb @Callie Beller Diesel @Russ Ball clay and bentonite are preferable. How about sodium silicate and 5 % bentonite as inorganic binder with ball clay for better strength?considering the shelf life of inorganic binder. Or can i use 50:50 Ball clay & bentonite or 100% bentonite for getting more strength with only 400°C? Because beyond 400°C organics are getting oxidized and creating dark spots which is highly  undesirable.  Or can i use hydrogen peroxide to oxidize the organics without making dark spots showed in the picture?

[Bill Kielb]

6 hours ago, bijoy said:

Ball clay and bentonite are preferable

This was an interesting read http://www.madehow.com/Volume-1/Chalk.html how conventional chalk and pastel chalk is made. Calcium carbonate and or calcium sulphate. Clay consistency is mentioned  but it looks like it is done without clay and at much lower temperatures. Maybe worth reading,

[bijoy]

2 hours ago, Bill Kielb said:

This was an interesting read http://www.madehow.com/Volume-1/Chalk.html how conventional chalk and pastel chalk is made. Calcium carbonate and or calcium sulphate. Clay consistency is mentioned  but it looks like it is done without clay and at much lower temperatures. Maybe worth reading,

Thank you @Bill Kielb. But my chalk is for marking cloth.  It should be smooth like soap. The only problem is the dark spots.  I kept it on 600°C for 30 min today and please see the result below (..