How is the impurity of a dog purified, and must it be washed seven times or is one time sufficient?

To purify an impurity from a dog, the vessel must be washed seven times, one of them with earth. It is obligatory to wash it seven times with earth, whether the earth is in the first wash or another, though it is preferable for it to be the first wash. There are several methods for this washing with earth: one can wash it with water and then sprinkle earth on it, or sprinkle earth and then follow it with water, or mix earth with water and wash the vessel with it.