The greatest objection to gypsum hard wall plaster for Gypsum Hard interior plastering is due to its making such a brittle Wall Plaster and hard wall that it reflects and transmits sound very much more readily than a wall that is made with lime plaster.
Buildings plastered with lime plaster do not reflect and transmit sound the same as buildings plastered with gypsum hard wall plaster. The question of sound-proofing buildings never had to be studied as is done at present until gypsum hard wall plaster became extensively used. This was because the lime plaster in drying hardened in a porous condition, containing innumerable minute dead air cells, which proved an eflPective and permanent retarder of sound.
Even though deadening felt and the most costly class of construction be used, there will continue to be reflection and transmission of sound if the plastered walls are constructed of gypsum hard wall plaster. The gypsum manufacturing companies of the United States have devoted great amounts of money and much time trying to overcome this serious objection to gypsum hard wall plaster. Their experiments have consisted of adding percentages of asbestos fibre, wood fibre, lime, etc.
This has, however, been of no avail, because with gypsum hard wall plaster this characteristic is inherent. A hard, brittle wall, s uch as that made with gypsum hard wall plaster, is not necessary for plastering purposes lime plaster, which was universally used for plaster, was sufficiently strong to with- stand all normal conditions of wear imposed.
This has been demonstrated an indefinite number of times by examination of the plaster in many of the buildings erected even over a century ago, which buildings are still standing with the plaster in an excellent state of preservation.
When extreme hardness is a desirable feature in an interior plaster, it is now customary to add Portland cement to the lime mixture which makes a plaster even harder and more brittle than gypsum hard wall plaster. In using hydrated lime plaster in places where sound reflec- tion and sound transmission are not important features, such as warehouse or store construction,
Portland cement is very frequently added to the plaster, but for all other classes of construction, such as residences, apartment houses, office buildings, schools, churches, hospitals, etc., extreme hardness of the walls and the resulting resonance or sound carrying capacity, is to be avoided in every way possible.
Another serious objection to a finished wall made with gypsum hard wall plaster is the fact that this material begins to set and harden so quickly after the material is mixed with water and ready for applica- tion, that a plasterer finds it exceedingly difficult to straighten and true the walls, and as a result they are often left with a wavy and uneven surface.