Bowl turns black with heat! Strange physicochemical phenomenon!

tsuwal

I have a bowl in my kitchen that turns black when heated (when it goes to the microwave oven or the washing machine). It is driving my crazy so I started to investigate. Here are the results:

This is how it looks after cleaning it:



After going to the microwave it becomes like this:




The covered part of the bowl also became dirty so the black stuff seems to come from the inside of the bowl.
Now it started to investigate at which temperature it becomes black. I put it in the oven and set the temperature to 120º C (248º F) and nothing happed. At 150º C (302º F) the black stuff started appearing at the bottom:



Then took the oven to 220º C (428º F) and it stayed the same.

Please help me solve this mistery! I have other bowl equal to this one that also suffers from this stange disease.

tsuwal

I may add that the black thing is hard and sticks well to the bowl, it is not easy to get it out. After pressing it, it sticks to your fingers.

tripleM

What do you use to clean it? Glycerin based soap? Alcohol? Bleach? Try using different types of cleaners that will act as solvents for the material. Observe which works best.
The ceramic itself will have no negative reactions to heat, so it is in either the glaze or the coating. I suspect the coating.
If it is easily cleaned with alcohol, it is possible it is a polar compound, whereas if you are cleaning it with an acid or base easily, it may be difficult to pinpoint exactly which type of material it is. The black color looks like a phase change is occurring, so it may be possible to isolate this compound before it reaches this critical temperature. After heating to 120C and removing from the heat source, do you notice any of the substance?
Also is it only on the outside of the bowl or on the inside as well?

Borek

Is it always covered with this black substance in exactly the same places, or is the pattern different each time?

tsuwal

the pattern seems to be the same each time. The black stuff also appears on the inside but with less intensity:



This picture was taken at the same time as the second picture above.

Alcohol and glycerin based soap I don't have at home. It doesn't dissolve very well on dish washing detergent. What could bleach tell me?

Borek

My bet is that the ceramic contains some partially volatile impurities, which try to escape when heated - and they escape through the cracks of the glaze. If so, it should stop after several uses.

Velikovsky

Be careful, some glazes can be quite toxic if they break down and release their opacifier's and colourants, more so if the bowl was sourced from China!

turbo

I suggest throwing that bowl away.

tsuwal

Borek, but if it was volatile it wouldnt become stuck between the cracks, right?

Velikovsky

Concur Turbo.

Borek

Depends. It can be liquid at elevated temperatures, leak outside, and oxidize there.

Some time ago someone posted similar question and pictures, but I am not sure if it was here or at another forum. Things that they observed were different (white and fluffy), but otherwise the situation was similar - it grow on the outside of the heated ceramic vessel.

chemisttree

Likely a dark colored bowl with a semi-opaque coating (opaque from crystallization) or a dark pigment in a matrix of microcrystalline material. When cool the coating is highly reflective (diffuse reflectance). When heated the crystallites melt producing a clear matrix that allows light to penetrate more deeply into the coating where the light is attenuated... hence black.