It's another one of those topics I find bizarrely fascinating. Maybe because it's a very British tradition, at least when it comes to cask beer. The original purpose seems to have been to quickly bring into condition running beers, mostly Mild Ales. From there the practice spread to pretty much all cask beers. Though, admittedly, after WW I little other than running beers was brewed in Britain. At least in terms of draught beer.
If any brewers are reading this, I'd love to know if they prime their casks or not.
That's enough of my bollocks. Time for a nice juicy quote, fresh off the word grill:
Some brewers prefer rather to assist the yeast in degrading the maltodextrins into maltose, than to add extra fermentable sugar. To do this they add to the beer a small quantity of cold-water extract of malt, which, containing active diastase, will supplement the action of the secondary yeasts in degrading the maltodextrins. Only a small quantity needs to be used ; larger quantities would produce too rapid a removal of the maltodextrins, and this excessively early disappearance would be obviously dangerous in regard to the stability of the beer. Pale ales will stand one pint of concentrated extract per one hundred barrels of beer, added at racking; running ales double that amount.* The condition produced in this way is a steadier and more lasting condition than that produced by priming, and it is in most respects preferable to it. It depends, however, upon having a sufficiency of maltodcxtrin naturally present; priming, on the other hand, is independent of this. When the extract cannot be conveniently added to the beer in racking vessel, it may be added to the individual casks. In that case, it is previously diluted with beer to such an extent as to permit of the quantity due to each cask being sufficiently appreciable to be conveniently measured.
* Take two parts by weight ground pale malt, five parts by weight cold water; soak for six hours, stir periodically, then strain off and filter the solution through flannel. One pint of the solution to be used as above staled for too barrels of pale ale, and one quart to the same amount of running ale. The extract to be made fresh on each occasion. The grains are to be discarded."
"A text-book of the science of brewing" by Edward Ralph Moritz, George Harris Morris, 1891, pages 403 - 405.
Right, glucose, invert sugar and cane sugar are the favourite primings. With invert being the most popular and can sugar only being used for Porter and Stout. Not much very surprising there.
Chadwick recommended feeding the yeast after the first skimming with a combination of wheat flour and salt. For every four barrels of wort, two pounds of flour and half a pound of salt were mixed with a little wort and then added to the gyle-tun. The flour helped the fermentation and the salt clarification. It's worth noting that this preparation would have been illegal in a commercial brewery. (Source: "A Practical Treatise on Brewing" by William Chadwick, 1835, pages 48-49.)
If a fermentation were not vigourous enough and the yeast head discoloured, the solution was to "dress" the wort. The old method was to mix 1 lb of wheat flour and 4 ounces of salt per barrel into the wort and then rouse it thoroughly. When the problem was caused by too many unfermentable carbohydrates in the wort, this "dressing" could be effective. The diastase in the malt acted on the carbohydrates, making them more fermentable. The new method was to use just malt flour, without any salt. (Source: "Principles & Practice of Brewing" by Walter J. Sykes & Arthur R. Ling, 1907, pages 534-535.)
Barclay Perkins logs record priming particularly well. Once I've harvested the numbers I'll put together some lovely tables. Exciting times.