Many people today will tell you that the use of sidewalk chalk is a perfectly acceptable material to rub on gravestones in order to bring out the carving on hard to read stones. You may find upon closer inspection that they are advocating its use on a stone that it's entire face has been covered with newsprint, pellion or some other such rubbing surface. They are not suggesting or recommending the introduction of the chalk directly to the surface of the stone.
However, some will in fact not only recommend this but highly encourage it. It this an acceptable method of highlighting the carving on the stone? Saving Graves researched the question and put it to experts to find out if it should be viewed as an acceptable alternative method.
According to the Crayola website, Molded chalk, such as Crayola Colored chalk, is a softer chalk made of plaster of Paris, which is defined as quick-setting gypsum plaster consisting of a fine, white or gray powder, Calcium Sulfate Hemihydrate (CaSO 4 ½H 2 O), which hardens when moistened and allowed to dry. According to one manufacturer, Calcium Sulfate Hemihydrate has great applications in the manufacture of stucco, tablets for ceilings, division panels, boards and sanitary porcelain. In fact, it is most commonly called stucco but not the same material as used on the outside of buildings. A prime example of its primary use would be drywall sheeting.
Gypsum, as a rule, has a tendency to expand as it sets. Therefore if the material is left on the surface of the stone it is quite possible that particles could work its way into the stone and set causing the potential of damage to the stone in several forms up to and including causing the stone to break.
If it is introduced into the stone by way of moisture, as that liquid evaporates the gypsum will increase by double in strength and hardness. In addition, if a gypsum based element such as chalk is left exposed to open air, once the liquid is added to it, the setting time is greatly shortened.
While sidewalk chalk is somewhat softer than regular chalk we still do not recommend its use. In a test conducted by Saving Graves, it was proven that it regular chalk will actually scratch a typical chalkboard. While being overall softer in nature, the hardness of sidewalk chalk varies greatly from manufacturer to manufacturer. Some of the lesser or off brands that we tested were found to be as hard as regular chalk.
To get an informed opinion on the use of sidewalk chalk as a gravestone rubbing took, we contacted Binney & Smith, the parent company of Crayola located in Easton, Pennsylvania. We ask them if they would as a manufacturer of sidewalk chalk recommend its use for gravestone rubbings. Saving Graves received the following response from Crayola concerning the use of sidewalk chalk:
"Crayola sidewalk chalk contains plaster of paris which has a gritty texture. Plaster of paris is not considered to be biodegradable, nor are most of the pigments contained in Crayola sidewalk chalk. Also, product packaging warns of colorants that may stain. This could be a good factor depending on the exact nature of what you are trying to do. While packaging does warn of colorants that may stain, chalk used outside generally washes away because of extreme weather conditions and excessive rain. Again, this could vary depending on the surface it is applied to."