Introduction Numbers 19:7-10 states, "The priest shall wash his garments and bathe his body in water; after that the priest may reenter the camp, but he shall be unclean until evening. 8 He who performed the burning shall also wash his garments in water, bathe his body in water, and be unclean until evening. 9 A man who is clean shall gather up the ashes of the cow and deposit them outside the camp in a clean place, to be kept for water of lustration for the Israelite community. It is for cleansing. 10 He who gathers up the ashes of the cow shall also wash his clothes and be unclean until evening." Most of our mishnah deals with who must wash their clothing.
All who are occupied with the preparation of the [red] cow from the beginning until the end, defile their clothing, and they also render it invalid by [doing other] work. The mishnah rules that anyone who performs any aspect of the red cow service has defiled their clothing. They also render the red cow invalid if they do some other type of work while occupying themselves with the red cow. No multi-tasking while burning the red cow (so put away your cellphones ☺).
If some invalidity occurred while it was being slaughtered, it does not defile clothing. If the cow turns out to be invalid while it is being slaughtered (for instance the slaughtering is not done correctly) then it doesn't count as a red cow ritual and the person's clothes will not be made impure.
If it occurred while the blood was being sprinkled, for all who were occupied with it before the invalidity occurred, it defiles their clothing, but for those who were occupied with it after it had become invalid it does not defile their clothing unclean. Thus it follows that the stringency turns into a leniency. The basic principle here is that if the person is occupied with the red cow ritual after it is already invalid, his clothes are not defiled. But if he was occupied with it after it was slaughtered but before it was invalidated, his clothes are defiled because the cow did count, at least for a time, as a red cow because it had been slaughtered properly. The result is that the stringency (the red cow has been invalidated) turns into a leniency (the clothes are not defiled).
It is always subject to the rules of trespassing. "Trespassing" is the illicit use of a sacred property. Any use of the red cow is considered trespassing, no matter when it is done.
Wood may be added to the fire. It is permitted to add wood to the fire to help it burn. However, this is true only before it has become ash. If one adds wood after the cow has been turned to ash, he invalidates the ash.
The service must be performed by day and by a priest. The entire service must be performed during the day and by a priest.
Work renders it invalid. Work renders it invalid as we learned above in section one.
[All of this is only] until it becomes ashes These words relate to everything that was taught in sections four through seven. Those rules apply only until it has been turned into ash. After it has been turned into ash, the rules of trespassing do not apply: one cannot add wood and the service does not need to be performed during the day or by a priest.
And work causes the water to be invalid until the ashes are put into it. Doing work with the water set aside for the sprinkling ritual renders it invalid, but only until the ash has been added.