I thought I'd continue with a little more information from The Book of Household Management by Isabella Beeton ©1861
TO PRESERVE AND TO CHOOSE SALT BUTTER. 1633. In large families, where salt butter is purchased a tub at a time, the first thing to be done is to turn the whole of the butter out, and, with a clean knife, to scrape the outside; the tub should then be wiped with a clean cloth, and sprinkled all round with salt, the butter replaced, and the lid kept on to exclude the air. It is necessary to take these precautions, as sometimes a want of proper cleanliness in the dairymaid causes the outside of the butter to become rancid, and if the scraping be neglected, the whole mass would soon become spoiled. To choose salt butter, plunge a knife into it, and if, wh«a drawn out, the blade smells rancid or unpleasant, the butter is bad. The layers in tubs will vary greatly, the butter being made at differes! times; so, to try if the whole tub be good, the cask should be unhooped, and the butter tried between the staves.
It is not necessary to state that butter is extracted from cream, or from unskimmed milk, by the churn. Of course it partakes of the qualities of the milk, and winter butter is said not to be so good c spring butter.
A word of caution is necessary about rancid butter. Nobody eats it on bread, but it is sometimes used in cooking, in forms in what the acidity can be more or less disguised. So much the worse; it is almost poisonous, disguise it as you may. Never, under any exigency whatever, be tempted into allowing butter with even a soupcon of "turning" to enter into the composition of any dish that appears on your table. And, in general, the more you can do without the employment of butter that has been subjected to the influence of heat the better. The woman of modern times is not a "leech;" but she might often keep the "leech" from the door, if she would give herself the trouble to invent innocent sauces.