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When the yarn is used to make a rug, it is dyed and goes through a process of being set called the mordant process (from the Latin 'modere', to bite). This involves using a metal salt which helps the dye to bind with the fibres. The wool is then rinsed to remove any excess dye which remains in the yarn. The lack of running water in some areas of the Middle East means that some rugs will contain wool which has not been rinsed properly and can therefore bleed.
Some rug dyes will not be colourfast and may migrate when exposed to moisture or a wet cleaning solution (most painted rugs fall into this category). Some rugs may even dry crock (colour transfer by rubbing in a dry state). We therefore need to test each and every rug upon collection and prior to cleaning. This will help us to determine the safest way to proceed. Rugs which bleed may still be wet cleanable if we are able to set the dyes. This is usually done with a mild acetic acid. If we are unable to make the dyes stable then we will either clean the rugs in a way that allows us to safely remove the excess dye or use a low moisture method of cleaning. A small number of rugs which we test will not be wet cleanable at all, and so the only option is for us to use a dry dusting process and stop at this stage.
We use ph testing for two very important reasons.