4b. Froth develops with whisking
Froth forming takes 15 mins to cover top of vessel. At first it seems as if it will not form, but vigorous whisking proves it does. Alternatively liquid can be exchanged between buckets to create air bubbles, and if arms get tired of whisking.
Be prepared to spend some time doing this. I whisked until I was tired of it, and when bubbles covered surface of vessel.