There are some technological developments that seem initially so modest, and that grow so quietly, that even in our own day no one would wonder about their impact until long after the effects were irreversible. [...]
For heating, the Romans used braziers and hypocausts (radiant heating). Braziers burned charcoal and were thus costly to operate; moreover, in a room tightly closed against the weather they were dangerous: the Emperor Julian, wintering in Paris, was once nearly killed by carbon monoxide from a brazier. Hypocausts were inflexible and wasteful of fuel because they heated the entire mass of the masonry of floors or walls. In the variable climate of Northern Europe, and especially in winter, they would not do. During the early Middle Ages, whether in hovel or royal hall, people centered their lives around a fireplace in the middle of a room with a high, louvered roof to carry out the smoke. Unfortunately it carried out much of the heat as well.
By the ninth century the central fireplace was occasionally moved to a corner of the room and was covered by a hood or mantle to catch the smoke and take it out through a hole, or even up a chimney. Experimentation with the design of chimney stacks led to the discovery that a draft of air can be brought down the flue to reverse itself and draw off the smoke while leaving much of the heat to be radiated into the room. Moreover, replacement of louvers by chimneys meant that fireplaces could be located on any level of a multistoried building and not merely under a roof directly beneath the sky. In the eleventh century chimneys and mantled fireplaces became common in the dwellings of the great. By the end of the twelfth, even the poor were enjoying them.
Much more than simple comfort was involved. In the days of the old central fireplace, to keep warm in Northern Europe everyone from lord and lady to humblest servant lived and ate together in the great hall, and slept there, too, normally in curtained compartments. Society was hierarchical, but the strata knew each other intimately. With the new flexibility of heating made possible by chimney and mantled fireplace, privacy could be implemented. Lord and lady increasingly ate, lived, and slept in withdrawing rooms. As affluence increased, noble residences were redesigned so that rank after rank of the social structure could enjoy the new sense of individuation in its life style. To Dresbeck’s remark that the chimney may have affected the art of love more than the troubadors did, one may add that it may likewise have fostered the individualism of the later Middle Ages more than all the humanists.
Yet a high social price was paid for the new ideal of the idiosyncratic person. As communication between classes decreased, class consciousness and snobbery grew. By the 1370s William Langland was assessing the chimney bitterly:
Woe is in the hall each day in the week.
There the lord and lady like not to sit.
Now every rich man eats by himself
In a private parlor to be rid of poor men,
Or in a chamber with a chimney, And leaves the great hall.
The chimney is as important as any other single factor in the shift from medieval to modern Occidental attitudes, and not all of this process was good. I doubt, however, whether anyone much earlier than Langland could have assessed properly its less desirable effects, and by that time the process could not be turned back.
—From: Lynn White, Jr., “Technology Assessment from the Stance of a Medieval Historian” in The American Historical Review, 79 (1974): 1-13.