The enamel powders you buy from the hobby store are clean.
You can use them direct without any washing.

Enamel powder you buy in a second hand store or obtain from a friend is advisable to test them if they are dirty.
If they are, then you can wash them.

Washing enamel powder is not difficult.
However, you must have a lot of patience and have to work very carefully.
Depending of the pollution you have to wash the enamel powder multiple times.

Below you see the sequence of washing

• Fill a small glass with enamel powder and add water to it.
• Stir with a spoon
• Let is stand for at least 1 hour or till the water starts to be clear.
• If you see some foam or dirt at the surface of the water then you will know the powder is dirty.
• Pour the water carefully and slow out of the glass.
• Watch if you do not pour color particles into the sink, let some water over in the glass.
• Repeat the above process till the surface of the water in the glass is clean 

Repeat this process 1 time more however use demineralized water to fill the glas.
When it is ready to remove the water wrap some plastic household foil partially on the top of the glass.
Now you can pour all the water out of the glass without spilling color particles into the sink.

Put all the wet enamel powder into a bowl and set it at top of your kiln to dry quickly.
Stir the enamel powder occasionally with a steel spoon till it is completely dry.

Put all the powder on a piece of paper or tile and mill the powder with the backsite of a spoon till all the clumps are removed.
At the end of this process you will have clean enamel powder and you can put it back into a clean jar.

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