This is not the world's 14,593rd blog post about the merits or crimes of adding milk to tea, whether it should be added first or last, etc. This is an attempt to assemble a bit of historic fact about the reasons this custom started in the first place. I was piqued by an article in the current edition of Tea Time magazine, a discussion of tea cups and their many charms, which mentioned the following while running down the history of this crucial vessel:
But porcelain had its drawbacks, as well. Mme de La Sabliére, a French hostess of an influential literary salon during the 17th century, is often credited with being among the first to add milk to tea. The practice began by pouring milk into the cup before filling it with the hot tea. While tempering the tea in this manner made handling more comfortable, Mme La Sabliére was actually seeking to prevent cracking or breaking the porcelain.
That reason for adding milk was a new one on me. I've always heard this discussed as a matter of taste — originally reported by a different madame, Mme de Sévigné, who wrote a letter commonly cited as one of (never definitive) the first mentions of adding milk to tea. She frequently wrote about tea, among her gossipy details of the Sun King's court, once citing our other madame's custom: "Madame de la Sablière took her tea with milk, as she told me the other day, because it was to her taste."
Every other mention of adding milk to tea that I've ever read approaches it from that perspective, of taste, which frankly always struck me oddly (even though tea's Asian origins have a long history with dairy products, often from animals other than cows, namely butter). Jane Pettigrew and Bruce Richardson's New Tea Companion still features a page about "Milk in Tea," suggesting in the section's first sentence that the custom "perhaps developed because milk and cream were found to soften the slightly bitter taste of tea." In their earlier version of the book, however, The Tea Companion, published just a year before in 2004, they at least addressed the other possibility by way of questioning what the initial motives might have been, adding, "Or was a little milk poured into the Chinese tea bowls used in the 17th and 18th centuries before the hot tea in order to reduce the risk of shattering the fine porcelain?" For the second edition, they excised this thought. (They also added that Ms. Sabliére was alone in her taste for this combination, that it didn't catch on in France before that country moved squarely into the coffee camp.)
Victor Mair and Erling Hoh's great True History of Tea gives similar credit to our French madame, but states no particular motivation:
While milk tea was drunk by the Manchu officials that the Europeans would have encountered, and the Dutchman Johann Nieuhoff had been offered tea with milk at a banquet in Canton in 1655, the honor of introducing the custom to Europe is traditionally ascribed to Madame de la Sabliére, who in 1680 served tea with milk at her famous Paris salon ...
Their discussion of this, however, comes two paragraphs after exploring the development of porcelain, "with its translucent fragility."
As someone who's had that experience — I once poured boiling water into a large glass infusion jar, to sterilize it, and watched the bottom quickly crack and crash into the sink — I can see how the practical matter would drive the custom rather than the questionable taste involved. Just found this curious. If anyone has other primary sources of information on this, do tell.