What is the ruling on soil used to remove the impurity of a dog: is it pure or impure?

The dust used to purify dog impurity becomes impure if used before the seventh wash, or if it has changed due to the impurity's effect. However, if it is used in the seventh wash and has not changed, then it is pure. Al-Nawawi stated that the seventh wash for purifying dog impurity is pure, while those preceding it are impure. This ruling applies to the view that the washes are for purifying the impurity.