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"I heard" that gray lines forming on porcelain indicate usage of aluminium in the coating. I would primarily like to verify this information.

Bowl #1 and bowl #2 have been used both for several months and were bought in different shops. The same observation goes for other dishes. Some don't feature lines, other dishes feature those after days already. The dishes who tend to display those patterns have been bought all at the same place (a large chain of stores specializing on furniture). The lines are most likely caused by cutlery as in cups the same lines are found forming ellipses at the bottom from the stirring. Again some cups don't feature those patterns others do.

Bowl #1 with no lines despite usage for a while:

enter image description here

Bowl #2 with lines:

enter image description here

Bowl #2 with lines - picture taken HDR mode:

enter image description here

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  • $\begingroup$ Could it merely be due to differences in washing-up methods? In other words for one bowl a metal scourer was used to clean it as opposed to a washing-up cloth or brush? $\endgroup$ – user7232 Sep 7 '14 at 23:48
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If the two bowls look different although treated similar, the material must differ.

Your bowls are almost fully covered with a glaze - except for the ring on the bottom on which the ceramic was standing while fired in an oven.

Here, water can enter the ceramic core. If the ceramic material of one bowl is more porous, water can be taken up easier. Upon heating, as in a dishwasher, the water absorbed expands. The glaze can't withstand the tension build up and breaks.

This results in the cracks that you observe.

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  • $\begingroup$ maybe I should have mentioned that somebody told me this aluminium-theory but I cannot track down a source for it $\endgroup$ – Raffael Sep 7 '14 at 18:26
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    $\begingroup$ @Raffael I think it is a misunderstanding. There is aluminium in your dishware, but not in metallic form. It's just that kaolinite, the clay mineral from which porcelain is made, is a silicate with aluminium cations in the crystal lattice. (Aber jetzt: Fußball ;) ) $\endgroup$ – Klaus-Dieter Warzecha Sep 7 '14 at 18:40
  • $\begingroup$ that would mean that both bowls feature aluminium cations and hence the aluminium is not involved in forming the lines? $\endgroup$ – Raffael Sep 8 '14 at 7:51
  • $\begingroup$ @Raffael Yes, exactly. Refining aluminium metal from alumosilicates is almost impossible. $\endgroup$ – Klaus-Dieter Warzecha Sep 8 '14 at 10:48

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