Below is a piece of 'mortar' I picked up from inside the wall of the Duncan House while the interior of the wall was exposed. I think it would be called mortar or stucco, or maybe 'chinking'.
I asked and found out that the custom was to add hair to the mortar to make it stronger and enable it to bond and bind better. Wikipedia says this:
"Hair is used in plaster as a binding medium, and gives tenacity to the material. Traditionally horsehair was the most commonly used binder, as it was easily available before the development of the motor car."
This was new to me and I thought it interesting.