I hope that historic plaster enthusiasts are visiting and gaining some insight into the technique of constructing an authentic 18th century wall surface.
Don above has eluded to the use of a variety of ingredients that could be used if it became imperative to use a very cheap and plentiful supply of something else.
Don is quite right in our region the ingredients he eluded to were used at times, in outbuildings of lesser importance than the home say the barn, heated sheds, hen houses, pig pens, you name it but normally not the home.
Most period home's walls were finished with slacked lime and sand mixtures, the interior of the wall cavity might be brick or stone filled which could have been laid with a clay mortar mixture, which does not work bad but similar to lime mixtures and will harden well. Clay mixtures though do not weather well unless fire hardened, and its use should be restricted to dry areas.
The lime that was used then was a white lime powder that resulted from the burning of limestone, shells, or anything that was created with lime. The resulting white powder is very volatile and will when exposed to moisture begin to heat rapidly and can burn up any wooden or flammable container near it.
Great care has to be exercised when one begins to slack it with water to obtain slacked lime for the use in plaster mixtures.
I wonder if Ken would like to comment on lime mixtures at this point, or anyone else who is knowledgeable on this subjectt and who might like to add a comment or two
Don also eluded to not covering up hewn timbers, well unfortunately in an historic sense most times the rough hewn frames were covered up and disappeared into the wall cavities, Ceiling timbers were finished differently, at times (not always) The timbers were adzed finished and beaded on the bottom corners and left exposed. In the process you could look right up at the bottom of the second story t&g flooring and made for a lovely look.