Here is one of the examples of making yeast in the 18th century where they prepare a batch of flour with some boiled hops, then a small batch of barm is added which turns into a large batch of leaven. And note the saving of a piece of “old dough” for the next baking.
The process of making yeast as practised at Edinburgh is as follows: Take two ounces of hops boil them for an hour in two gallons of water and boiling hot scald eight or ten pounds of flour and stir it very well into a paste. Do this about eleven in the forenoon. Let it stand till fix o clock in the evening then add about a quart of yeast to forward the fermentation and mix it well together. Next morning add about as much more flour and water sufficient to make it into dough and in the afternoon it will be sit for setting spunge and baking. Reserve always a piece of the old dough to mix with the new batch instead of the yeast which is necessary only the first time to hasten the process The above quantity of hops will suffice for an hundred and twenty quartern loaves.
From the “The Seaman’s Guide” John Cochrane 1797