A caudle (or caudel) was a hot drink that recurred in various guises throughout British cuisine from the Middle Ages into Victorian times. It was thick and sweet, and seen as particularly suitable and sustaining for invalids and new mothers. At some periods of history, caudle recipes were based on milk and eggs, like eggnog. Later variants were more similar to a gruel, a sort of drinkable oatmeal porridge. Like the original forms of posset (a drink of wine and milk, rather than a set dessert), a caudle was usually alcoholic.
The word caudle came into Middle English via the Old North French word caudel, ultimately derived from Latin caldus, "warm". The 1911 Encyclopædia Britannica states the word derived from Medieval Latin caldellum, a diminutive of caldum, a warm drink, from calidus, hot. The Oxford English Dictionary cites the use of the word to 1297. The word's etymological connection to heat makes it cognate with "cauldron".
A related recipe for skyr, a cultured dairy product, appears in the early 13th century.  The earliest surviving recipe for caudle, from 1300–1325, is simply a list of ingredients: wine, wheat starch, raisins, and sugar to "abate the strength of the wine".
Another recipe from the late 14th century has more ingredients and more details on the cooking procedure: mix breadcrumbs, wine, sugar or honey, and saffron, bring to a boil, then thicken with egg yolks, and sprinkle with salt, sugar, and ginger. A 15th-century English cookbook includes three caudle recipes: ale or wine is heated and thickened with egg yolks and/or ground almonds, then optionally spiced with sugar, honey, saffron, and/or ginger (one recipe specifically says "no salt"). In a description of an initiation ceremony at Merton College, Oxford in 1647, caudle is described as a "syrupy gruel with spices and wine or ale added". William Carew Hazlitt provides a number of recipes for caudles and possets in his 1886 book, Old Cookery Books and Ancient Cuisine.
The 1911 Encyclopædia Britannica describes it as "a drink of warm gruel, mixed with spice and wine, formerly given to women in childbed", i.e. as a restorative food during her postpartum confinement.
Emily Post's 1922 Etiquette in Society, in Business, in Politics, and at Home, the classic guide to American manners, states that "although according to cook-books caudle is a gruel, the actual "caudle" invariably served at christenings is a hot eggnog, drunk out of little punch cups" (see Punch bowl).
"For the sick and lying-in"
Aside from the initiation ceremony mentioned above, caudle was often served to people who were seen to need strengthening, especially invalids and new mothers. A historian of Georgian England says that maternity hospitals always served a "traditional postlabor fortified caudle" to women who had just given birth. The British Lying-In Hospital had "Laws, Orders, and Regulations" printed to be displayed on the wards, detailing among other things, the menu. Mothers on the "low diet" had caudle; when they graduated to the "common diet" it was beer caudle; and the "full diet" had no need of the invalid liquid anymore.
Maria Rundell included a "caudle for the sick and lying-in" in her best-selling A New System of Domestic Cookery (1806). ("Lying-in" is an obsolete term for childbirth, referring to the extended period of bed rest that marked the traditional recuperation time.) Judith Montefiore likewise included caudle with the "recipes for invalids" in her The Jewish Manual (1846), the first exposition of Jewish cuisine in English. Five years later, The English Housekeeper by Anne Cobbett (daughter of William Cobbett) gives variants of caudles, of either gruel (oatmeal) or rice, with different types of alcohol, and seasonings, including capillaire. She devotes a chapter to invalid food, making the point that "Often when the Doctor's skill has saved the life of his patient [...] it remains for the diligent nurse to prepare the cooling drinks and restorative foods [...]. Everything which is prepared for a sick person should be delicately clean, served quickly, in the nicest order; and in a small quantity at a time."
As a metonym
As caudel was served to new mothers to build up their strength, so it was offered to their visitors, to share in the happy occasion. "Cake and caudel" or "taking caudle" became the accepted phrases for a "lying-in visit", when women went to see their friends' new babies. These were all-female occasions, as more than one man noted. The American playwright Royall Tyler has one of the female characters in the comedy of manners The Contrast (1787) decline the offer of a man's escorting her by claiming that "half [her] visits are cake and caudel" and therefore unsuitable for him. A generation later in 1821, Thomas Gaspey wrote of these visits (with the italics in the original):
'Twas then Eliza, though now Mrs. T.
We ought to call her, gave her lord an heir,
And all her female friends, the babe to see
And praise its beauty, failed not to repair;
But half what they to utter there thought meet,
While taking caudle, I must not repeat.
Offering caudel, or cake and caudel, to all lying-in visitors is referred to as an old British custom. Queen Charlotte, consort of George III, bore him 15 princes and princesses. After the christening of the youngest, Princess Amelia in 1783, "the greater part of the company then paid a visit to the nursery, where they were entertained (as usual on such occasions) with cake and caudel." This continued into Queen Victoria's reign: the day after she gave birth to the Prince of Wales, "many of the female Nobility called at Buckingham Palace, and were received by Lady Charlemont, the Lady in waiting, and after taking caudle were taken to the north wing to see the infant Prince." 
But it was not just nobles who came. The Court Magazine and Belle Assemblée reported that the aftermath of a royal birth was "a usual reception of the public to cake and caudel". The London Chronicle reported in 1765 that "The resort of different ranks of people at St. James's to receive the Queen's Caudle is now very great."
There was a vessel particular to the drink, a caudle cup, a traditional gift for a pregnant woman, "which used to be handed round the young mother's chamber", according to Dr Oliver Wendell Holmes Sr..
After the birth of Princess Augusta Sophia, the sixth child of George III and Queen Charlotte:
Connected with the event of her Royal Highness's birth, during the usual reception of the public to cake and caudle, on Sunday 13th of November, 1768, a curious incident occurred at the Palace: - two young ladies, after having drunk plentifully of caudle, were detected in carrying off a large portion of the cake, and some of the cups in which the caudle had been served; they were allowed, however, to escape with a severe reprimand, after begging pardon on their knees for so disgraceful an act.
A caudle formed part of the Beltane (May Day) fire festival celebrations collated by James Frazier in The Golden Bough. He quotes at length Thomas Pennant, "who traveled in Perthshire in the year 1769":
on the first of May, the herdsmen of every village hold their Bel-tien, a rural sacrifice. They cut a square trench on the ground, leaving the turf in the middle; on that they make a fire of wood, on which they dress a large caudle of eggs, butter, oatmeal and milk; and bring besides the ingredients of the caudle, plenty of beer and whisky; for each of the company must contribute something. The rites begin with spilling some of the caudle on the ground, by way of libation: on that everyone takes a cake of oatmeal, upon which are raised nine square knobs, each dedicated to some particular being, the supposed preserver of their flocks and herds, or to some particular animal, the real destroyer of them: each person then turns his face to the fire, breaks off a knob, and flinging it over his shoulders, says, 'This I give to thee, preserve thou my horses; this to thee, preserve thou my sheep; and so on.' After that, they use the same ceremony to the noxious animals: 'This I give to thee, O fox! spare thou my lambs; this to thee, O hooded crow! this to thee, O eagle!' When the ceremony is over, they dine on the caudle; and after the feast is finished, what is left is hid by two persons deputed for that purpose; but on the next Sunday they reassemble, and finish the reliques of the first entertainment.
Frazier notes other Scottish May Day celebrations with similar dishes, "a repast of eggs and milk in the consistence of a custard".
Apparently it was "a custom in France to bring the bridegroom a caudle in the middle of the night on his wedding-night", according to an explanatory note in an 1877 edition of The Essays of Montaigne, presumably inserted by the English editor, William Carew Hazlitt.
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