The black plaster coating is called trim and is the first layer applied to the interior walls of a building, before being coated with other types of finish (normally plaster).

Walls covered with gypsum paste have been found in ancient Egyptian burial chambers dating to around 2000 BC. C.[1]​ Plaster is still used today as the main coating on interior brick walls.

The bricklayer applies this layer to the partition walls and ceilings, as a preparation layer, to be able to later be coated with a finer textured material, which presents a better finished surface, such as plaster.

The plaster used is black plaster, grayish in color due to the greater presence of impurities in its composition, and with a coarser granulometry than white plaster. The material must be mixed with water (in 50-60% mixtures) at the same time of application, since in a few minutes it begins to set and will not have the ability to adhere to the wall. When the plaster has already begun to set, it is called "dead plaster" and needs to be thrown away.

Before applying the plaster, the wall must be moistened to prevent the support from drying the material prematurely, preventing it from setting correctly. The plaster is applied by hand with a trowel, with which it is smoothed. The black plaster trim is usually one to two centimeters thick, with thicknesses much greater than one centimeter not being recommended, as the material can crack more easily.

There are two methods of applying plaster:

It is advisable not to plaster on excessively dry and hot days, as premature drying of the plaster can cause shrinkage cracks.

The trim improves several properties of the enclosures, such as acoustic insulation, thermal insulation, and fire resistance. It also allows the elimination of irregularities no larger than one centimeter on the walls, which is why it is commonly used to cover brick walls. Trims are only used on interior walls protected from the elements, as humidity degrades them.