Liese Sherwood-Fabre writer Sherlock Holmes Making an Impression

In The Sign of the Four, Sherlock Holmes mentions his monograph on the use of plaster of paris to preserve footprint impressions. The gypsum compound, however, has many uses, some of which are mentioned in the Canon: for casting molds - such as busts of Napoleon - and plastering walls. Perhaps the most common reference to plaster in the cases involved none of the compound at all.

Plaster of paris is calcium sulfate that, when heated and ground to a fine powder, will set up again when water is added. This represents only one of three types of plaster but is the most common. The others are lime plaster, using calcium hydroxide and sand; and cement plaster, combining plaster, sand, Portland cement, and water. (1) If glue is added to the plaster, it creates a surface called gesso that can be used in tempera or oil painting. (2)

Plaster of paris has long been used in construction—from finishing interiors to flourishes on columns or cornices. (3) Until the 1930s, most homes involved lath-and-plaster walls and ceilings (as mentioned in two cases in the Canon). Strips of one-inch-wide wood were nailed onto studs and then covered with about three coats of plaster. The practice declined after drywall became popular. (4)

These white walls gave the name to the compound in the thirteenth century. According to several accounts, King Henry III coined the name after visiting Paris in 1254 and importing the process to England. (5) By the 18th century, most of the gypsum was mined in Montmartre, outside Paris, (6) but other deposits were found in East Sussex in England in 1873. (7)

Plaster of paris has the specific property of not shrinking or cracking when casting molds—such as statues of Napoleon. Because of this feature, not only has it been used for decorative trim, but also for hand and foot castings. While casting babies’ feet has been a common practice since ancient Egypt, (8) it wasn’t until 1786 that a plaster cast of a footprint was used to solve a crime. A local constable noticed a boot print near the home of a murdered girl. He used a cast of the print to identify the culprit by comparing it with the boots of those who attended her funeral. (9)

A shoe or footprint is a “plastic” print when it is left in mud, snow, or other substance retaining a three-dimensional track. Such prints can be traced to a particular individual because of several traits unique to each person. At its most basic, the print provides the size and make of a particular shoe, narrowing the number of possible suspects and eliminating others. This preliminary characteristic is important enough for the FBI to maintain a database of sole patterns. To link a shoe print to a particular individual, the wear pattern is used. Each person has his/her own manner of walking (more weight on the heel, more on the ball, etc.) and wears out shoe soles differently. In addition, cuts or nicks on the sole will leave marks in the print. An investigator can compare the plaster cast to a suspect’s shoe to determine if they match or not. (10)

Plaster is also mentioned in A Study in Scarlet, The Valley of Fear, “The Man with the Twisted Lip,” and “The Adventure of the Greek Interpreter” in reference to a very different item. Plasters are also medicinal compounds applied to the skin (for example “mustard plaster”). (11) In 1880, a pharmacist spread a rubber-like substance over gauze to cover the skin and hold a salve in place and termed it “Guttaplaste.” (12) Sticking plaster soon entered the market and appears in the Canon as an adhesive tape used to cover cuts, as well as to disfigure Neville St. Clair’s and Paul Kratides’ face and cover Kratides’ mouth to keep him from talking.

Whether distorting a person’s features or forming a cast of Napoleon or a suspect’s footprint, plaster has quite an “impressive” history.

Liese Sherwood-Fabre is proud to share the cover of case four in “The Early Case Files of Sherlock Holmes.” “The Adventure of the Purloined Portrait” will be available shortly for pre-order and will be released in 2022. You can learn more about this series and other books at www.liesesherwoodfabre.com. Signing up for her newsletter will ensure you keep up with all the latest news about her books and appearances.

MAKING AN IMPRESSION