Page 105 - QRedition5.0 062021

Quality Requirements Edition 5.0 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-value

Purpose and scope 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 with

2. pH-value in aqueous suspension 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

Analysis instruction Sodium aluminate 0.15
Potassium carbonate 0.10

1. Determination of the methylene blue adsorption Water 47.00
Dry the clay at 105 C. Weigh precisely a sample of 2.2 g blue clay or 2.5 g white clay and boil it
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for 5 minutes in 25 ml of distilled water. Add 3 ml 6n H SO and mix intensively for at least 30
2 4 The specific gravity of the slip should be at least 1.7 g/ml.
seconds. After being cooled, add 25 ml methylene blue solution (I) and stir for Apply the enamel slip onto normal sheet steel plates over ground coat and fire them ac-
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5 minutes (magnetic stirrer). Place one drop of this suspension onto a filter paper. When the cording to the type of the standard frit (e.g. 820 C; 3 minutes).
ending edge is achromatic, titrate the suspension with methylene blue solution (II) till the
ending fluid edge gleams blueish; that is to say until the clay does not absorp any more In order to determine the colour co-ordinates use a Hunterlab or similar colour measuring
methylene blue. In order to guarantee a complete adsorption, the supply of methylene blue apparatus.

during titration has to be slow. Stir about 2 minutes between supply and checkup.
Consumption = a ml. 4. Control of cleanliness
100 g clay are to be suspended in water, intensively stirred and screened through a 3600

(25 x 3.2) + (a x 1.6) = mg methylene blue per 1 g clay mesh sieve (=100 μm mesh aperture). The residue is to be mingled into 100 g white enamel
g weight of sample slip. A test enamelling on ground coated sample plates must result in a faultless white
enamelling.

As a measure of a classification of enamelling clays, the following figures are valid:
Blue clay 40-46 mg/g
White clay 33-38 mg/g

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