There is a video to watch on line that shows a 20 x 24 inch plate being poured. It is on the John Coffer web site. http://www.johncoffer.com/
On the left side it says "videos", go there and choose the longest one, ten minuets.
Now, this is a wet plate video, but it is a huge plate that is hand poured. I am into dry plates and have hand poured 4 x 5 inch plates and am sweating how to do 12 x 15 inch ones. I suppose that if the liquid emulsion is thin, warm, and spritzed with Everclear it'd cover a large surface in red light. Perhaps a piece of marble could be warmed and the glass set on it. If there was a gimble under it, it could be tipped this way and that and thus eliminate hot finger spots. John used his hands, but he used a metal plate. It didn't matter.
Has anyone here poured large plates for dry plate photography? How?