Page 108 - 20250602 EEA QM Edition 6.0

Quality Requirements Edition 6.0 European Enamel Association

8.5 Examination of enamelling clays 2. pH-value of the clay-water-suspension

Stir 50 g clay into 100 ml distilled water. Store it for 30 minutes and measure the pH-
Purpose and scope value of the unfiltered sludge. pH-values between 4.3 and 5.5 are valid.

The rheological effects of enamelling clays in enamel suspensions can be
classified by a methylene blue titration. The following should be controlled: 3. Determination of colour co-ordinates

The colour of the fired clay is to be checked in a white enamelling as described.
1. Adsorption of methylene blue
A white enamelling with a standard clay is to be compared to such a white enamelling
2. pH-value in aqueous suspension
with a test clay. For milling, a Rosetti-mill is used with the following formula:
3. Colour co-ordinates
4. Cleanliness Standard frit 100.00
Clay 3.50
Sodium aluminate 0.15
Analysis instruction Potassium carbonate 0.10
Water 47.00
1. Determination of the methylene blue adsorption

Dry the clay at 105 C. Weigh precisely a sample of 2.2 g blue clay or 2.5 g white The specific gravity of the slip should be at least 1.7 g/ml.
0
clay and boil it for 5 minutes in 25 ml of distilled water. Add 3 ml 6n H2SO4 and mix
Apply the enamel slip onto normal sheet steel plates over ground coat and fire them
intensively for at least 30 seconds. After being cooled, add 25 ml methylene blue
0
ac- cording to the type of the standard frit (e.g. 820 C; 3 minutes).
solution (I) and stir for 5 minutes (magnetic stirrer). Place one drop of this suspension
onto a filter paper. When the ending edge is achromatic, titrate the suspension with
In order to determine the colour co-ordinates, use a spectrometer or similar colour
methylene blue solution (II) till the ending fluid edge gleams blueish; that is to say
measuring apparatus.
until the clay does not absorb any more methylene blue. In order to guarantee a
complete adsorption, the supply of methylene blue during titration has to be slow. Stir
about 2 minutes between supply and checkup. Consumption = a ml.
4. Control of cleanliness

(25 x 3.2)+(a x 1.6) = mg methylene blue per 1 g clay 100 g clay are to be suspended in water, intensively stirred and screened through a
g weight of sample
3600-mesh sieve (=100μm mesh aperture). The residue is to be mingled into 100 g
white enamel slip. A test enamelling on ground coated sample plates must result in a
As a measure of a classification of enamelling clays, the following figures are valid:
faultless white enamelling.
Blue clay 40-46 mg/g
White clay 33-38 mg/g

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