Have you ever noticed the priest wiping the inside of the chalice with the purificator after pouring in the wine and water, and wondered why? I first noticed this being done in the televised Mass on EWTN many years ago. I now do this myself.
In the older form of the Mass (the Extraordinary Form), the priest was to wipe around the inside of the chalice with the purificator while preparing it, for a very practical reason: because when you pour in the wine and water, it inevitably splashes a little onto the interior sides. If those drops were left on the sides they would be consecrated. Then, to purify the chalice after communion, the priest would have had to tilt it and “twirl” it to pick up those drops with the purifying liquid. By wiping the interior sides after pouring in the water and wine, it makes purification of the chalice after communion easier and faster: he can pour in the purifying liquid* and, instead of having to “tilt and twirl”, just drink it from the same spot where he drank the Precious Blood, and all of it is picked up with the purifying liquid. This is why there is traditionally also a cross on the base of the chalice: it shows the priest which side he should drink from so that he could be consistent about it, and so make purification easier. (ATTN sacristans and servers: the cross should always be facing the priest!)
In the Ordinary Form of the liturgy this gesture is not stipulated, but you will nonetheless see some priests wiping the chalice in this way while preparing it (for example, when I first saw it on EWTN). There is not always, however, the same practical benefit. For example, in concelebrated Masses, where multiple priests (and possibly deacons) will be receiving from the same chalice, if they do not all drink from the exact same spot, the “tilt and twirl” method of purification will be necessary in order to pick up all the bits of the Precious Blood that are on the sides and even on the rim from the chalice’s having been handled by multiple ministers. The same is true for chalices that are used for the distribution of the Precious Blood to the faithful: since the ministers usually rotate them in-between communicants the entire interior will have to be purified. In Masses, however, where a single priest only receives from the chalice, this method of preparation can be used to good effect, making it easier and quicker to purify afterwards.
Well, what does any of this matter? I know, first of all, that many people have seen this practice and wondered why it was done. Now you know. Also, I think many priests do it out of a sense of decorum and tradition, without always knowing exactly why it is done. Many older practices like this serve us well even now – even when they are not required (but not forbidden either) – and it is good to know of them and understand them.
* In the Extraordinary Form the purification is done with both water and wine. In the Ordinary Form this method is permitted also, although you will hardly (if ever) see it done – nearly everyone purifies with water only.