In the previous post Benjamin Franklin promised MARY “POLLY” STEVENSON (later HEWSON), the daughter of his London landlady whose education he had taken in hand, another letter on the subject of tides and rivers. He was true to his word. His next letter to her discussed the subject at great length and makes interesting reading, but I have chosen the second part of that letter to present here. It is about fabric colors and how they react to the sun. Several experiments are mentioned; the last was performed by Franklin himself. The results, Franklin argues, have all sorts of applications in daily life. It’s amazing that Franklin discusses this topic with a young girl and believes the information is important for her to know.

. . . . As to our other Subject, the different Degrees of Heat imbibed from the Sun’s Rays by Cloths of different Colours, since I cannot find the Notes of my Experiment to send you, I must give it as well as I can from Memory.

But first let me mention an Experiment you may easily make your self. Walk but a quarter of an Hour in your Garden when the Sun shines, with a Part of your Dress white, and a Part black; then apply your Hand to them alternately, and you will find a very great Difference in their Warmth. The Black will be quite hot to the Touch, the White still cool.

Another. Try to fire Paper with a burning Glass [magnifying glass]. If it is White, you will not easily burn it; but if you bring the Focus to a black Spot or upon Letters written or printed, the Paper will immediately be on fire under the Letters.

Thus Fullers and Dyers find black Cloths, of equal Thickness with white ones, and hung out equally wet, dry in the Sun much sooner than the white, being more readily heated by the Sun’s Rays. It is the same before a Fire; the Heat of which sooner penetrates black Stockings than white ones, and so is apt sooner to burn a Man’s Shins. Also Beer much sooner warms in a black Mug set before the Fire, than in a white one, or in a bright Silver Tankard.

My Experiment was this. I took a number of little Square Pieces of Broad Cloth from a Taylor’s Pattern Card, of various Colours. There were Black, deep Blue, lighter Blue, Green, Purple, Red, Yellow, White, and other Colours or Shades of Colours. I laid them all out upon the Snow in a bright Sunshiny Morning. In a few Hours (I cannot now be exact as to the Time) the Black being warm’d most by the Sun was sunk so low as to be below the Stroke of the Sun’s Rays; the dark Blue almost as low, the lighter Blue not quite so much as the dark, the other Colours less as they were lighter; and the quite White remain’d on the Surface of the Snow, not having entred it at all. What signifies Philosophy that does not apply to some Use? May we not learn from hence, that black Cloaths are not so fit to wear in a hot Sunny Climate or Season as white ones; because in such Cloaths the Body is more heated by the Sun when we walk abroad and are at the same time heated by the Exercise, which double Heat is apt to bring on putrid dangerous Fevers? That Soldiers and Seamen who must march and labour in the Sun, should in the East or West Indies have an Uniform of white? That Summer Hats for Men or Women, should be white, as repelling that Heat which gives the Headachs to many, and to some the fatal Stroke that the French call the Coup de Soleil? . . . That Fruit Walls being black’d may receive so much Heat from the Sun in the Daytime, as to continue warm in some degree thro’ the Night, and thereby preserve the Fruit from Frosts, or forward its Growth?— with sundry other particulars of less or greater Importance, that will occur from time to time to attentive Minds? I am, Yours affectionately,
B. Franklin

The fullers and dyers that Franklin refers to were people who worked with cloth to make it usable. The meaning of “dyers” is obvious. Not so for “fullers” (or “walkers” or “tuckers”). These were workers who stood in vessels of stale urine called wash, which contains ammonium salts that aid in cleansing and whitening, stamping on the fabric (both cotton and wool) for hours at a time. The treated fabric was then laid out on bleaching fields to allow the action of sun and water to whiten it. Afterwards the fabric was washed and dried on stretchers to maintain its shape. Later fuller’s earth, a clay-like substance containing hydrous aluminum silicate, was used with the wash. For more on this subject, including a video of the process, check this website. For information about the sun and its interaction with fabrics of different colors see this article.

“From Benjamin Franklin to Mary Stevenson, [November 1760?],” Founders Online, National Archives, version of January 18, 2019, https://founders.archives.gov/documents/Franklin/01-09-02-0079. [Original source: The Papers of Benjamin Franklin, vol. 9, January 1, 1760, through December 31, 1761, ed. Leonard W. Labaree. New Haven: Yale University Press, 1966, pp. 247–252.]