According to Derwent, the Inktense pencils are permanent and can be washed after heat setting with an iron. - on cotton.
This is a pdf guide put out by Derwent. you can download it if you like. Scroll down for the heat setting directions.
https://www.derwentart.com/a/341080
according to this, some are ink based, some are wax based.
https://www.derwentart.us/inspiratio...e-your-pencil/