Page 123 - QRedition5.0 062021

Quality Requirements Edition 5.0 European Enamel Association

8.7.3 Valuation of cracks in enamellings The formation of cracks, as a result of a too long and unreasonable influence of high
temperature, (e.g. more than 530 C) often displays a metallic luster as a result of the
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Definition and cause diffusion of metal oxide.
Following the definition of DIN 50903: Cracks are mostly laminar interruptions in enamel-
lings. They often run vertical to the surface, but occur also under different angles. Their Test report
causes are mainly mechanical - or thermal tensions in the compound system. In the test report to be mentioned:
Under thermal shocks, tensile stresses are built up due to different expansion behaviours kind and origin of the piece
of the metal substrate and the enamel to which an enamelling is less resistant as to place and date of testing
compressive stresses. Unreasonable deformations or impact stresses on finished description of defects, number and extent.

enamellings set off cracks.

Consideration and description
The judgement of damage by cracks is performed by naked eye. One differentiates be-
tween:
a. open, piercing cracks
b. open, non-piercing cracks
c. masked cracks

Open, piercing cracks are interruptions in the enamel layer down to the metal substrate.
They can be made visible clearly by physical (e.g. electrostatic) or chemical methods.
Open, non-piercing cracks cease in the enamel layer. They cannot be made visible by such
method, as there is no prerequisite reaction with the metal substrate. Wet or dry pigments

are rubbed into the enamel surface, in order to recognize such crack formation. These Illustration: Formation of cracks in an enamel layer (enlargement 200 : 1)
pigments concentrate on the edges of cracks, by which they clearly indicate the crack
propagation.

Masked (inner) cracks glimmer in the enamel layer like mother-of-pearl, dependent on the
lighting equipment and lighting angle.

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