_Of Plastering_.
To make Plaster that it may continue a long time, and not crack; you
must take Care to lay it on Walls that are very Dry; for if the Walls be
Moist, the Plastering being expos'd to the Air, and drying faster than
the Walls, will crack.
To do this Methodically, it must be laid, Bed after Bed, or Lay after
Lay, having a great Care not to lay one Bed till the other be almost
dry. The Ancients put six Lays, three of Mortar made of Lime and Sand,
and three of Stuck. The first Lays or Beds were always thicker than the
last, and they were very careful to make use of no Mortar made either of
Sand or Stuck in their Plastring, that had not been a long time beaten
and mix'd together; especially the Stuck, which must be beaten and mix'd
till it will not stick to the Trowel.
They took likewise a great deal of Pains to run several times over and
beat the Plaster, which gave it a Hardness, a Whiteness, and Polish'd it
so well, that it shin'd like a Mirror.
These Plasterings so made, serve to Paint in _Fresco_ upon; for the
Colours being laid upon the Mortar before it was dry, pierced it, and
Embodied with it; so that the Painting could not be defaced though it
were wash'd; which would easily be wash'd off if the Mortar were dry.
They likewise laid these Plasterings upon Partitions of Wood filled with
fat Earth, nailing Reeds to them, as we do Laths, and daubing it over
with Clay, and then putting on another row of Reeds across upon the
former, and another Bed of fat Earth or Clay, upon which they laid Beds
of Mortar and Stuck, as we have said before.
[Sidenote: _Lib. 7. Chap. 4._]
For the Plastering of low and moist places, they had a great many other
Precautions, especially within the House; for as what belonged to the
Out-part of the House, they contented themselves to Plaster from the
Bottom of the Wall to the height of three Feet, with Cement.
But as to the Inward-parts of the House, when the Ground without was
higher than the lowermost Flooring; they run up a little narrow Wall
against the great one, leaving betwixt the two Walls only the distance
of a Channel or Sewer, which they made lower than the Flooring, to
receive the Water which might be gather'd against the Walls, and let it
run out; and to the End they might hinder the gathering of much Water,
by the Vapours which might be enclosed between these two Walls, they
made towards the top of the little Wall Vents to let it out, and this
little Wall was Plastered on the Out-side with Mortar and Stuck, as we
have said before.
When the Place was too narrow to permit those Counter-Walls to be made
within, they put hollow Tiles one upon another against the Wall, and
placed and plaster'd them over with Mortar and Stuck. These Tiles which
were Pitch'd over within, and were Demi-Channels, let the Water fall
down into the Sewer, which sweat from the great Wall, and so let all the
Vapours, which were engendred by Humidity, go out at the Vents.