“I well remember for many years it was not expected, nor was it thought possible [for porter] to be made fine and bright, and four and five months was deemed to be sufficient age for it to be drunk at. The improvement of transparency has since been added to it by means of more and better workmanship, better malt, better hops and the use of isinglass.”

[1] http://tempestinatankard.com/2013/11/21/of-isinglass-and-oth...