Numbers 19 discusses administering of the Red Heifer's ashes by way of sprinkling water. In verse 14, we see that vessels as well as humans can be impure from the dead. In verse 18, we see that vessels must be sprinkled by the water as well, but how? A natural reading of the verses seems to indicate that they were only sprinkled once. On the other hand, we see that humans were sprinkled twice, on the third seventh days, so perhaps vessels were given the same treatment? What was done, practically?
Rambam Parah Adummah 11:1 (English)
כיצד מטהרין טמא מת במי נידה: לוקח אדם טהור שלושה קלחין של איזוב, ואוגדן אגודה אחת, ובכל בד ובד גבעול אחד. וטובל ראשי גבעולין במי נידה שבכלי, ומתכוון ומזה על האדם או על הכלים, ביום השלישי וביום השביעי, אחר שתנץ החמה; ואם הזה משעלה עמוד השחר, כשר. ואחר שיזה עליו ביום השביעי, טובל ביום; ומעריב שמשו, והרי הוא טהור לערב.
How is a person who is impure because of contact with a human corpse purified by the water of sprinkling? A person who is pure takes three stalks of hyssop and binds them with one bond. Each stalk should have at least one bud. He should dip the top of the buds in the sprinkling water while it is in a container, focus his intent, and sprinkle it on the impure person or utensil on the third and seventh days after sunrise. If one sprinkled after dawn, it is acceptable. After the water has been sprinkled on a person on the seventh day, he should immerse in a mikveh during the day, wait until nightfall, and then, in the evening, he is pure.