Matthew Poole and continuators on Numbers 4:13

13 And they shall take away the ashes from the altar , and spread a purple cloth thereon:

Of burnt-offerings. Whereby it may seem probable, though it be denied by most, that they did offer some sacrifices in the Wilderness, though it may be not so frequently nor orderly as afterwards they did. Whence else were these ashes? And there are some undeniable instances of their sacrificing there, as Exod. 18. 12. and 24. 4. Numb. 7. &c. from which it seems rational to conclude that they did offer sacrifices at other times, though not so constantly and diligently, as they did in Canaan. And for the argument against it from Amos 5. 25. that may be easily dissolved, as we shall there see, if the Lord please. Moreover the taking away of the ashes onely doth sufficiently imply that the fire was preserved, which as it came down from Heaven, Levit. 9. So it was by Gods command to be continually fed and kept burning, and therefore doubtless was put into some vessel, which might be either fastened to the altar and put within this covering, or carried by some person appointed thereunto.