Wash. Rev. Code § 61.16.010
Any person to whom any real estate mortgage is given, or the assignee of any such mortgage, may, by an instrument in writing, signed and acknowledged in the manner provided by law entitling mortgages to be recorded, assign the same to the person therein named as assignee, and any person to whom any such mortgage has been so assigned, may, after the assignment has been recorded in the office of the auditor of the county wherein such mortgage is of record, acknowledge satisfaction of the mortgage, and discharge the same of record.
RCW 61.16.010
Validating-1897 c 23: "All satisfactions of mortgages heretofore made by the assignees thereof, where the assignment was in writing, signed by the mortgagee or assignee, and where the same was recorded in the office of the auditor of the county wherein the mortgage was recorded, are hereby validated, and such satisfactions of mortgages so made shall have the same effect as if made by the mortgagees in such mortgages." [ 1897 c 23 § 2.]