Wash. Rev. Code § 86.09.505
The county treasurer, before delivering any certificate must file the same and enter in the assessment book opposite the description of the land sold, the date of sale, the purchaser's name and the amount paid therefor, and must regularly number the description on the margin of the assessment book and put a corresponding number on each certificate. Such book must be open to public inspection without fee during office hours, when not in actual use.
On filing the certificate of sale as provided in the preceding paragraph, the lien of the assessment vests in the purchaser and is only divested by the payment to the county treasurer making the sale of the purchase money and interest at the rate of ten percent per annum, from the day of sale until redemption for the use of the purchaser.
RCW 86.09.505