Finings

Another important process was fining. And another one which has a surprisingly long history. The use of isinglass going back until at least to the 17th century. Though, even after its introduction, beer which underwent a long secondary fermentation was still expected to drop bright without finings.

"In coming to the question of fining, we are face to face with a matter requiring thorough investigation, for the use of an artificial fining material has simply revolutionised brewing operations; for whereas some few years ago public taste was in favour of aged beer, and as very lengthy storage of the unfined beer in bulk was necessary to ensure brightness, a vast amount of capital was consequently locked up during such period, while, at the present time, I should say two-thirds of the beer produced in England is consumed within a month of its production, forced into a condition of brightness by artificial means."
"The Theory and Practice of Modern Brewing" by Frank Faulkner, 1888, page 229.

It's a sign of the decline in the popularity of aged beer that two-thirds of beer was sold withing a couple of weeks. At the start of the 19th century, the situation was very different. Especially with the popularity of Porter.

Isinglass, it seems, was superior to other fining agents. Which I guess is why it's still being used.

 "So far as I remember, the introduction of isinglass as a fining material was the primary cause of the complete change in modus operandi spoken of, for all previous artificial methods, such as the use of silver sand, alum, and so forth, were singularly ineffectual by comparison, and the beer so clarified was not calculated to impress people by its degree of brilliancy and early palate character; for I know of nothing that interferes so much with the delicate flavour of beer, especially of that of only moderate gravity, as the semi-cloudiness which means a very intimate kind of connection existing between the amorphous matter and yeast in suspension in the weak alcoholic fluid."
"The Theory and Practice of Modern Brewing" by Frank Faulkner, 1888, page 229.

It seems that the use of finings was important for the development of Mild Ale.

". . . it was only after the introduction of gelatine as a combined chemical and mechanical fining agent that brilliancy was attainable, and a mild beer produced fit for consumption almost directly after fermentation was complete."
"The Theory and Practice of Modern Brewing" by Frank Faulkner, 1888, page 230.

Which pretty much describes the way 20th-century Mild was handled.