• The characteristic of a liquid is its ability to flow. Liquids vary in the ease with which they flow, and the degree of resistance they offer to flew is known as their 'viscosity‘. It is a property directly related to their inner structure. All liquids possess viscosity, but to the layman to call a liquid viscous usually means that it flows sluggishly compared with water.

    Oils and paints normally flow less easily than water. Viscosity is one of the important criteria of the application properties of paint, the paint specification generally contains a clause indicating what the viscosity range should be.

    The term 'consistency' is also used in referring to the flow characteristics of paint and similar complex heterogeneous liquids. Rheologists distinguish between consistency and viscosity, but the term is often used indiscriminately in the paint industry. Essentially it can he said that ‘consistency’s a qualitative term whereas ‘viscosity’ is quantitative (can be measured].

    ‘Body’ is another term often used instead of consistency and paint may be heavy or light bodied or show false body.

    Viscosity is measured in various ways, the apparatus being used is known as a viscometer. In all methods measurement at a standard, stated temperature is essential (25°C is common). The unit of viscosity normally employed is the poise’, named after one of the great pioneers of viscometry, Poiseuille. The viscosity of water at room temperature is about 0.01poise or one centipoise. Viscometers that involve the flow of liquids under gravity give a direct measure not of the viscosity, but of the viscosity/density ratio. This is termed the ‘Kinematic' viscosity and its units is the stokes.

    The old terminology of poise and stokes are being replaced with SI units:

    1 poise = 10-1 Kg M-1 S-1

    1 stokes = 10-4 M-2 S-1

    But like feet and inches, and miles per hour being changed to meters and kilometers per hour, both are likely to remain depending on which you learn first.

    The bubble-tube method is a simple one employed more particularly for controlling the viscosity of oils and varnishes. The apparatus requires a short, narrow tube of standard dimensions closed with a cork and containing liquid sufficient to leave a small bubble of air above the liquid. By inverting the tube, the bubble travels up the column of liquid, the rate of travel is related to the viscosity of the liquid, and is compared with that of bubbles in standard tubes containing liquids of known viscosity. This is not very accurate, but is a simple method and used regularly in resin manufacture.

    Another simple method uses the rate of flow of paint, by timing the running of a known volume of paint from a cup, in the base of which there is an accurately machined hole of definite bore. This is known as the flow cup method and the viscosity is referred to as so many seconds in a standard cup (e.g. BS4, Ford 4 or Din cup). The results are empirical and cannot be related accurately to standard units.

    In the laboratory a torsion viscometer is sometimes employed. This instrument consists of a cylinder suspended at the end of a wire and immersed in the paint contained in a cylinder of slightly larger diameter than that of the immersed cylinder. The inner cylinder is twisted against the torsional resistance of the wire through 360° and released. The amount of overswing is indicated by the motion of a pointer over a graduated scale. This measures the viscous drag exerted by the paint.

    Another well-known method uses the falling sphere viscometer in which observations are made on the time taken for a ball of steel to fall through a definite distance in a column of liquid contained in a glass cylinder.

    Improvements in apparatus design have led to (a) Rotothinner, in which the rotor end is disc shaped and placed into a £L can for a quick accurate determination of viscosity, (b) Cone and Plate viscometer, where a small amount of paint is placed between a small, slightly conical disc and a temperature controlled base plate. This measures at high rates of shear comparable to those occurring with brush painting.

    Non-drip paints have a gel structure and viscometers to measure gel strength generally rotate very slowly with a small simple paddle which gives minimum disturbance to the gel structure.