Types of cracks
A.
Plaster cracks not originating from the plaster base (cracks due to the mortal and plastering technique)
Verification by wetting with water. When wetted with water, the existing cracks appear darker on the surface.
A.1
Plaster surface cracks
A.2
Cracks passing through plaster layers

Thy can occur as hairline and net cracks. Depending on the cause, they are sinter or shrinkage cracks in the surface off the uppermost plaster layer.

If the plaster is dry, it is often difficult to detect them.

They occur as hairline cracks and appear more clearly than plaster surface cracks.
Causes for cracks
Causes for cracks

Too fine sand of same granulation in the last plaster layer.

Too many suspended components of clayey nature in the mortal sand.

Too high proportion of binding agent in the upper plaster layer.

Extreme surface treatment (polishing with felt wheel, smoothening, rubbing), binder enrichments.

Too fast extraction of the mixing water.

Too many suspended components, e.g. in case of loamy mortal sand (shrinkage cracks).

Too high binding agent content in the mortar composition (shrinkage cracks).

Too thick plaster layer (settlement cracks, shrinkage cracks).

Too fast extraction of the mixing water due to heat, sun, wind or extremely sucking substrates.
Source of the pictures and texts: BSF note no. 19, January 1997
Cracks in exterior plasters, coatings and reinforcements, editor:
Bundesausschuss Farbe und Sachwertschutz e.V., Hahnstraße 70, D-60528 Frankfurt am Main