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Because they are Raku fired besides their unique shape.

Raku firing is an ancient Japanese technique:
After the first firing, a ceramic piece is glazed with a special glaze and fired in a gas kiln,very fast (1.30 - 2 hours).
When the temperature reaches 900° C the kiln is opened and the pieces are taken out with tongs, placed in metallic containers, covered with sawdust and sealed.
That causes the reduction. It happens because the flammable material catches fire from the red hot ceramic, but due to the sealing the flames seek oxygen to keep burning and the only available is in the oxides of the glaze. So the coloring oxides losing their oxygen particles, turn to pure very thin metal.
Especially the copper colored glazes give all kinds of bronze, blue, turquoise, green effects. Always unique!

No! due to the thermal shock, from 900° C to ambient temperature they are more sensitive.
Also the bronze copper color over time, slowly turns to a nice blue - turquoise or green!
This is nothing to worry about, it is just the procedure of the continuing reduction, as oxygen particles from the air replaces those lost during the past procedure.
So your piece may continue to change and this is part of its beauty!
However, the nice black crackles will remain the same. They are made from the smoke which gets into the glazes shocking crackles and intensifies them.
So Raku means always different, always unique, always spectacular ...but sometimes a disaster!
That causes the higher price of a raku piece.

Yes, here are useful tips for the proper and safe use:

- Be aware that the ceramic oil burner gets hot when it is lit. So touch carefully.
- The best option is to use tea light candles as they offer security against leaking wax.
- Never leave the burner lit when leaving the house.
- Make sure the burner is out of reach of children and pets.
- Ensure there is liquid or wax melt in the bowl before light the candle, or it could crack.
- Frequently clean it from the previous oil residues with soap.