Parah, Chapter 4, Mishnah 4. Chapter four of Parah works through the service of the parah adumah and the deviations that either ruin it or leave it intact. Our mishnah gathers a set of rules that accompany that service from the slaughtering until the ashes are mixed with the water: who becomes tamei, when the parah is subject to me'ilah, and how long the danger of melachah hangs over the process.
Everyone involved becomes tamei
"Kol ha'oskim baparah mitechilah ve'ad sof, metam'in begadim, u'poslin osah bimelachah." Whoever busies himself with this cow, at any point in its service and on through to the finish, causes the clothes he is wearing to contract tumah, and each one of them also has the power to invalidate the cow by melachah.
Melachah here means engaging in some other labor in the midst of the service. If the kohein, while occupied with the cow, turns aside to do other work, the parah is thereby rendered unfit. That is the melachah the mishnah is warning about throughout.
When the parah is disqualified midway
"Eira bah pesul bishechitasah, einah metam'ah begadim." If a disqualification arose already at the slaughtering, the parah does not render garments tamei at all. Since it was never a valid parah from the outset, the special rule that those who handle it become impure never took effect.
"Eira bah behaza'asah, kol ha'oseik bah lifnei pesulah metam'ah begadim, le'achar pesulah einah metam'ah begadim." But if the disqualification occurred later, during the sprinkling of the blood, then the process is divided in two. Everyone who dealt with the cow before the moment it became unfit renders his garments tamei; everyone who dealt with it afterward does not.
"Nimtza chumrah kulah kalah." The mishnah observes the striking symmetry in this: the very stringency of the parah is at the same time its leniency. The same rule that makes those who serve it become impure is the rule that spares them once it has been disqualified. It cuts both ways.
Me'ilah, wood, day, and kohein
The mishnah now lists three rules. "Le'olam mo'alin bah": the cow never leaves the category of consecrated property with regard to me'ilah. "Umosifin lah eitzim": further wood may be piled on for its burning. "Uma'aseha beyom uvechohein": its service is valid only in the daytime, and only when a kohein is the one who carries it out.
Take me'ilah first. Anyone who takes for himself the value of a perutah from this cow, whether the benefit was taken while it was still whole or once the burning had begun, so long as it has not yet turned into ash, is guilty of me'ilah and owes the guilt offering brought for such misuse of Hekdesh, the asham me'ilos. The reason is that Scripture labels the parah a chatas, a sin offering, and no one may enjoy personal use of what belongs to the Sanctuary.
That same source, however, sets the limit. Since it is the description of the cow as a chatas that brings it under the laws of me'ilah, the implication is that the cow itself is included and not its ashes. Personal use of the ashes therefore does not carry the sacrilege of me'ilah in the same way.
How long melachah remains a threat
"Hamelachah poseles bah ad shetei'aseh eifer." Outside labor can disqualify the parah right up until the moment it has become ash. Once the cow has been reduced to ashes, melachah no longer has the power to render it unfit.
"Vehamelachah poseles bamayim": the water of the chatas is exposed to the very same problem. "Ad sheyitlu es ha'eifer": that exposure lasts as long as the ash has not yet been put in. Once ash and water have been joined together, the mixture is finished and outside labor no longer has the power to spoil it.