Chalking acc. to Kempf - Senze-Instruments Benelux

The first sign of a paint coating  being affected by the weather is  an initial disintegration of the  surface film where some of the  pigment becomes loose when  originally firmly held in the paint  by means of a bonding agent.

This stage of deterioration can  be shown by simply wiping the  surface, loose pigment particles  will be removed.

This is called  „chalking“.

The sooner the paint  commences to chalk, the sooner  the decomposition of the film has  begun; the life expectancy of the  finish will therefore be  accordingly shorter.

Self-cleansing paints On the other hand, paints are  being manufactured nowadays  which are accurately adjusted  for a predetermined chalking  effect.

Such self-cleansing  paints renew their surface after  every rainfall by the washing off  of the conta-minated pigment.

Their life is practically the same  and they show a high degree of  colour durability and cleanliness.

Purpose and application

The quantitative determination of  the chalking rate is therefore of  great importance for

The pigment particles, lying  loose on the film surface  because of binder decay, are  pressed under a specified  pressure (250 N) into the  steeped gelatine of photographic  paper.

See  Fig. 1: The upper pigment layer has lost   cohesion with the rest of the film

See Fig. 2: Pressed into the gelatine layer of   the photographic paper, the   loose pigment particles are   lifted out.

The Kempf Chalking Tester,  Model 241, consists of a light  metal cylinder equipped with a  40 mm dia. rubber stamp  (Shore-hardness A 60 ± 5).

A  strong integral spring allows the  loading of the punch with a force  of between 50 to 250 N.

Ring  markings engraved at 50 N  intervals into the stamp shaft  permit the application of lower  pressures than the standard  load of 250 N.

Guiding the loading device by  hand is often not accurate  enough for guaranteeing an  even pressure over the whole  area of the stamp. It is therefore  recommended that the special  stand (available as an  accessory) be used which  provides an accurate vertical  guide during application of  pressure.  

Evaluation of the test

Several methods are known in  practice, namely:

This approved method is  specially suitable for white or  light-colour pigmented paints.  For comparison with the  chalking marks obtained, it  uses standard photographic  diagrams which are supplied  as Kempf Chalking Scale with  every instrument.  The comparison (Fig. 4) is  made visually with the 6  chalking stages, whereby 0  represents the weakest and 5  the strongest chalking value.  Fig. 4 Kempf Chalking Scale

(now: Kronos Titan GmbH) Used only for paints pigmented  with titanium dioxide (TiO2).

For this method a standard scale  is divided into 10 stages of  chalking intensity of brightness.

The distances between the  stages of brightness are of an  exactly defined photometric  value and these measurements  are reproducible.

See chalking  scale.

The brightness limit values of  10 and 1 represent absolute  freedom from chalking and  vice-versa a strong chalking  effect.

Chalking value 10  (highest calking resistance)  corresponds with the brightness value of 10 (lowest  brightness value) for an  unaltered black photographic  paper without any imprint.

Chalking value 1  (lowest chalking resistance)  corresponds with the  brightness value of 64,2  (highest brightness) which has  been measured on an imprint  obtained from most intensive  chalking.

This is a method which should  be given preference whenever  coloured paints have to be  tested; testing of white and  very light colours is, of course,  also possible.  The evaluation here is not  carried out by using a scale but  by comparison with paints  which had been produced and  weathered under similar  conditions.  By visually comparing the  impressions in the gelatine  layer of the test specimen with  the comparison paint, the  differences in the chalking  between the test samples and  the comparison paints are  established.

Basically the instrument does  not require any maintenance.

Signs of wear on the rubber  stamp and tensioning spring  fatigue may occur after several  years or after very frequent use.

These parts are available as  spares.  Exchange of stamp:

The rubber stamp is removed by  unscrewing the knurled supporting ring.

Exchange of tensioning spring:

The tensioning spring is  replaced by unscrewing the  domed cap on top of the loading  device.