On 03/03/2019 21:42, D.M. Procida wrote:
I have some ceramic electrical insulation, that is a bit discoloured and
I would like to clean up.

It has a rough unglazed surface.

What do you suggest?

Thanks,

Daniele

Ceramic insulation is normally glazed at least on the external surfaces.
Is it still in service as an insulator? If so I would be cautious about
trying to clean it chemically or mechanically.

Obsolete insulators sometimes acquire corrosion products from metallic
parts, either green from copper or brown from steel. As someone else
said, brick acid should get rid of those, perhaps with some brushing.