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I've noticed some subtle differences in the finished glaze between the inside and the outside of pieces like cups and mugs.  For instance, the outside of a mug will have pinholes but the inside surface does not.  I am assuming that the inside of a cup will get hotter than the outside and that this explains the phenomenon. 

Thoughts?

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13 minutes ago, Rick Wise said:

Thoughts?

Might get hotter, good chance it doesn’t though as it’s a function of radiation and conduction and the firing rate. So I am sure there are many times the outside is hotter than the inside until conduction does it’s thing.. The oxygen thought seems to be a good possibility though. In reduction, the inside of covered wares tends to be more reduced because they tend to trap the CO for reduction and are less influenced by the dynamics. A wind screen if you will.

The oxygen idea seems most valid IMO

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3 hours ago, liambesaw said:

I figured the inside of a cylinder would act as a reflector, I almost always get a more fluid melt inside cylinders.

All good points actually. Is the material conduction slow enough to influence the cool down so it is outside in at a meaningful rate? I think time to throw some open cylinders set on a cookie and fit cones inside.

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34 minutes ago, Bill Kielb said:

So for a covered cylinder or lidded jar it was less heatwork inside?

Ok, Min just doubled your experiment :rolleyes: Some open cylinders (wide tumbler cups, no need to put a handle on) and some lidded jars (need not be fancy) with cones inside. I can see how a lidded vessel would have some lag to the interior, after all that is a programmed "feature" of L&L kilns with the ceramic sheath over the thermocouples. But is there heating lag to the interior of an open mug or is it a cooling lag that results in a different glaze outcome inside vs. outside?

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20 minutes ago, Dick White said:

Ok, Min just doubled your experiment :rolleyes: Some open cylinders (wide tumbler cups

Sounds crazy but maybe include thin walled, medium walled and thick walled. I have a hunch that most cups and bowl surfaces are fairly even overall as the exterior has slightly more but the interior includes the bottom as radiant so overall potential cooling surface might be greater inside than out for open shapes. This may end up as a mass issue actually, or at least that could have a big influence.

Seems  like a cool useful experiment.

Edited by Bill Kielb
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2 minutes ago, Dick White said:

So, Bill, you have a student who needs a senior research thesis?

I don’t but that would be a perfect task ! Need to work an oxygen probe in there as well to cover Liam’s thoughts. Neil might have someone from his advanced class with interest.

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25 minutes ago, Bill Kielb said:

Neil might have someone from his advanced class with interest.

I wish. My students have very little interest in the gritty technical side of ceramics. They just want to make nice pots, which they manage to do while avoiding this type of stuff.

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12 hours ago, Dick White said:

Ok, Min just doubled your experiment :rolleyes:

Sorry Dick but things are a bit hectic here, however if you come do my drywall sealing and painting I'm supposed to be doing this week I'll do the test. (it's only 40 sheets of drywall we put in) :D

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Oh fun! I'd like to help - a bit too far though.
All done with taping, topping, and texturing? ...skipping those tips... 
May I suggest: wool (actual sewed leather) roller covers, longest nap you can put with; vertical/hanging roller screens in five gallon buckets; telescoping roller handles, such that lower hand is near your hip, and bending at the waist is eliminated; quality paint, quality sealer/primer; Flood Floetrol; rock music. If you're spraying finish, rolling on primer/sealer over a fresh drywall job, ah'd recommend that.  

 

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I have handled to much drywall to want to help. I like simi smooth wall. After taping and sanding then sealing with drywall sealer I like to roll a thicker than cream mud nap super light texture with a 3/8 nap roller then paint.

On the cup inside pinhole topic 99% of pinhole for me is the insides of any form from cups to bowls-the lack of atmosphere as well as temps are the causes I had felt. Now I'm firing in reduction but it does not matter as lack of oxygen or atmosphere difference is what causes this and temp can play a part as well. It very complex and longer firing can help or more saturation of temp if you are a computer electric person or a slower gas fire at high temp and slow cool etc.

When you think you have mastered it,it will rear its ugly head once again as this is ceramics not for the weak of heart

 

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29 minutes ago, Mark C. said:

On the cup inside pinhole topic 99% of pinhole for me is the insides of any form from cups to bowls-the lack of atmosphere as well as temps are the causes I had felt. Now I'm firing in reduction but it does not matter as lack of oxygen or atmosphere difference is what causes this and temp can play a part as well. It very complex and longer firing can help or more saturation of temp if you are a computer electric person or a slower gas fire at high temp and slow cool etc.

Mark  What do you mean by "the lack of atmosphere"?  

There will always be an "atmosphere" in an open area even if the atmosphere is just a perfect vacuum.  The composition of the atmosphere inside a cylinder is likely to be somewhat different than the more open and connected volumes.   movement of atmosphere in a combustion kiln is different in many ways from the atmosphere in an electric kiln.  Composition for one; constant movement another.   

My experience with "pinholes" at cone 3 oxy and cone 10 reduction have been application issues not firing conditions.  glazes bubbles and blisters are glaze composition and local melt temperature problems.    (our differences may be in the definition of "pinhole").  

My answer to Rick's question:   The temperature on the internal surface of a cylinder will always be lower than the cylinder outside surface during the firing period.  During the "hold" time the temperatures will tend to come closer.  During cooling step the internal surface will tend to be hotter than the outer surface since the heat stored in the ware must migrate to the kiln insulation and then to the outside environment.  The differences between the outer and inter surfaces temperatures depend also on the local thickness of the cylinder and the properties of the materials of both the clay body and the glaze(s) used.  

LT
 

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2 hours ago, Magnolia Mud Research said:

During cooling step the internal surface will tend to be hotter than the outer surface since the heat stored in the ware must migrate to the kiln insulation and then to the outside environment.

Hmm, all may be true but if the interior surface (radiant) actually is larger than the exterior surface unless cooling by convection it’s definitely can be more complex for sure. Energy movement by radiation is pretty straight forward and very fast.  Most open top vessels have a radiant inside bottom surface which could actually create more interior surface than exterior.  Mass of the bottom, contact and conduction through the shelf ........ all complicate things greatly. I am not sure there are any absolutes here. As far as the speed of conduction, I suspect its fairly high and for normal thickness wares a reasonable heating rate is likely well below the conduction rate so maybe holds are good or maybe not usually necessary. Test results I think could be enlightening. General speculation, I think difficult really. Personally, I have no idea which way the results would go, if there is a trend.

Edited by Bill Kielb
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MMR

I have felt this way about it as you said

(The composition of the atmosphere inside a cylinder is likely to be somewhat different than the more open and connected volumes.   movement of atmosphere in a combustion kiln is different in many ways from the atmosphere in an electric kiln.  Composition for one; constant movement another.   )

That this is more the root cause of inner pinholes say in a mug just down inside the cup near the lip-thats where I see this in reduction fires. I have a very standard application of liner glazes using a glaze jet for 99% of liners. The application is very controlled so my thought is its the atmosphere moving over the cup form. its always the same are near the top on the inside of say a bowl or cup. If I am really going slow at cone 10-11 they are far less so temp plays a part as well. This is more of a feeling for me after firing so many kiln loads over 45 years.

one last not its more where two glaze join as well but not always so thats a 3rd issue as well

My last fires two weeks ago had only 2 forms with pits in many many hundreds of pots in a total of 47 cubic feet of reduction wares with rutile glazes-those fires where very slow at top end-I did the same thing in yesterdays two kiln fires very slow top end movement up temp.

Since my main glaze is rutile based I live or die by pits and have had 45 years of living with the why and why not of it in reduction (no experience in rutile oxidation).

 

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