Barytes, Barium Sulphate (Ba SO₄)

Obtained by crushing naturally occurring heavy spar, barytes makes little contribution to the volume of the paint. It appears to reinforce the paint and can lead to settlement if used in appreciable amounts.

Calcium Carbonate (Ca CO₃ Chalk)

Can be naturally occurring, dug out of the ground, but commonly obtained as a by product from large water softening plants. It is used as a matting agent, in primers, putties and fillers, coated calcium carbonate (coated with stearic acid) makes the particles more bulky and is used as an anti-settling agent, to prevent pigments forming a hard mass at the bottom of the can.

China Clay/Bentonite - Aluminum Silicates (Al₂ O₃ 2SiO₂ 2H₂O)

This material has a fine texture, soft, with high oil absorption. It is used in undercoats as a thickening and flatting agent.

Talc known as French chalk - Magnesium Silicate Mg₃ H₂ (SiO₃)₄ is used like china clay.

ALUMINIUM FLAKES

Aluminium flakes are used to give the silvery Sparkle to metallic paints.

Pure aluminium is actually a very bright silver coloured metal, although we normally perceive it as being a dull whitish colour. This is because aluminium is very reactive, and as soon as it is exposed to the oxygen in the air, the surface oxidises to give a coating of aluminium oxide (a white powder) to the metal. In order to preserve its brightness, we must protect it from oxidation.

The flakes which we use in metallic finishes are made as follows:

A stream of molten aluminium is spray dried into an inert (i.e. oxygen-free) chamber, to give fine aluminium dust. This dust must be made resistant to oxidation before the next stage in processing, in order to maintain its silverness, and prevent the explosive hazard associated with oxidation of aluminium (Aluminium powder is one of the major constituents of thermite, used in incendiary devices).

To achieve this, the dust is made into a paste or 'slurry' with a non-reactive liquid, usually white spirit, and fed into a machine called a ballmill. This consists of a large horizontally mounted jar made of steatite (a very hard form of porcelain) and approximately half-filled with steatite balls of about a  inch in diameter.

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The mill is sealed and then rotated around the horizontal axis. The speed of rotation is very important: enough centrifugal (spinning) force must be generated to carry the balls up the side of the mill to about 5O°-60°, at which point the force of gravity overtakes, thus the balls cascade down over one another.

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The impact of the falling balls hammers the round dust particles out into flat flakes. They thus end up with the same sort of shapes as a piece of pastry which has just been rolled out - smooth flat top and bottom surfaces with a tatty, uneven edge. The shape and size will be somewhat irregular, and since aluminium is quite soft, they may be somewhat bent or curled.

Effect of Flake Size on Tinting

The silver faces of the flakes act as mirrors, and reflect the light. Therefore the larger the unbroken areas of face, the greater the "mirror” effect, i.e. the brighter, more sparkly the paint.

The edges, on the other hand, will interfere with the light and have a greying, dirtying effect. This is because of their rough, uneven nature. The grade of aluminium used in a paint will therefore affect both the cleanness and brightness of a colour.

If we halve the diameter of a circle, we quarter its area, but only halve its circumference, or edge. If, as in the case of aluminium flakes, the shape is somewhat variable, and the edges irregular, then the amount (and consequently the effect) of the edge will increase accordingly.

(This can be best illustrated by taking equal weights of fine and coarse aluminiums, and gradually adding to each separately a bright organic yellow tinter such as Permanent Yellow. Only a fairly small amount will have to be added to the Coarse Aluminium before a bright gold colour is developed. A very much greater amount will be needed to achieve the same depth of colour with the fine aluminium, and the tone will be much dirtier and greener.)

Laydown of Aluminium Flakes

Ideally, aluminium flakes should lay down parallel to the substrate and the surface of the film. This enables the flakes to act as tiny mirrors when viewed from above, to give a bright, clean, silvery "face”.

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When viewed along the film, the colour will normally appear considerably deeper, because the light has to travel through a much greater effective film depth (and consequently more pigment particles). The dirtying, graying effect of the flake edges also plays a much greater part.

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This is known as the ‘flip‘-tone, or the angle. The greatest face to-flip contrast will be achieved when the flakes are laying parallel to the substrate; if lay down is irregular, then the greater the disruption of the flakes, the less contrast will be seen between face and flip.

Alignment of the flakes can be disturbed by excessive wetness during application of the paint, for whatever reason. Damaged or warped flakes will be unable to lay completely flat, and will also lead to a reduction in face/ flip relationship. Laydown of flakes can also be disrupted by the rough edges, becoming enmeshed or engaged with each other.

Some of these problems are now being overcome, but at a cost. The so-called narrow cut” aluminiums use a new grading process which enables a much narrower range of particle sizes to be achieved, thus giving cleaner paints and more predictable laydown. Raw material cost is double that of conventional aluminiums.

“Polished“  aluminiums are another new concept which have received some publicity. During the ballmilling stage, the flakes are not hammered out so thin. This means that the edges are not so  ”tatty” - giving cleaner colours and the flakes are more robust, hence less susceptible to curling. A further kneading” process is also included after the milling, whereby the flakes are rubbed over one another giving flatter flakes with smoother surfaces. Laydown is thus considerably improved-

Now for the bad news: the extra polishing process increases the cost to about 7 or 8 times that of ordinary flakes. Since the flakes are thickener, you get less flakes per kilo, so coverage is bad, and more flakes must be used, thus increasing cost even more. Thicker flakes equals heavier flakes, equals more likelihood of settlement (equals marbles in the gun pot!}. They are currently of more appeal for specialist paints, because of the cost.

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Increasingly novel paints are being demanded for motor cars. The use of mica paints has increased from use in the glamour/cosmetic and trinket areas.

Prismatic mica, transparent mica particles which resemble glass are used. These, when used to replace aluminium in conventional basecoat clear finishes, rely on the addition of pigment for opacity and colour and have the advantage of giving great depth of colour. You do not see the grey ends of the aluminium flakes, but you also do not get the same dramatic change of colour from face to angle (high flip); 2 stage pearl finishes are able to produce cleaner brighter finishes.

Three stage pearl particles rely on coatings to give interference colours, or the thickness of the mica particle to reflect back specific frequencies, using a white groundcoat helps with the reflective processes.

Because no pigments are used for the colour, this type of finish is very dependant upon the amount of light available for the intensity of colour produced and because certain frequencies are reflected back, the colour passing through the pearl will have that colour missing and appear as the opposite colour from the colour circle e.g. blue pearl appears yellow when viewed through or at an acute angle and lilac pearl appears green.

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