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The weaving of wire is no different, in principle, from the weaving of any other yarn: warp wires are set up along the loom and weft wires across it. The product is a roll of woven mesh, which then is processed in a variety of ways, to produce the filter medium (or for many other purposes). The terms wire cloth and gauze are frequently used to refer to meshes woven from finer grades of wire, while the term bolting cloth refers to light weight versions of square mesh cloths, comprising
those based on the finest wires (and originally used for the sieving of flour).

A wide variety of wire meshes is produced by weaving individual wires of either ferrous or non-ferrous metals on looms up to 2 m wide. Two main categories of mesh can be distinguished, in terms basically of the shape of the apertures. One
category utilizes plain weave with single wires of the same diameter for the warp and weft, to form rectangular apertures (the great majority being square); many of these are the screens typically used for sieving and sizing operations. The other category is known as zero aperture filter cloths, with the wires pressed closely together to leave as little space as possible between them, and made in a number of more complex weaves, such as Dutch twills, which are commonly used in pressure and vacuum process filters.

In a woven wire mesh, each warp or weft wire bends where it passes over or under a wire of the other kind. This crimping of the wires occurs as part of the weaving process for fine wires, but the crimp has to be imposed on the wire above a certain thickness, before it is fed into position. Although this adds a stage to the weaving process, it has the valuable benefit of holding the crossing wires in place should the wire mesh move or vibrate.
Woven wire cloth has been widely used for filtration for well over 100 years and is available in an extremely wide range of materials and mesh sizes. It can be woven from virtually any metal ductile enough to be drawn into wire form, preferred metals being phosphor bronze, nickel-chrome stainless steels, and monel. Other materials widely used include aluminium alloys (combining good strength with light weight and good corrosion resistance), copper (cheaper than bronze but
not suitable for corrosive conditions), brass (stronger than copper but more subject to corrosion) and mild steel or coated mild steel (such as galvanized or tinned). Nickel, nickel-chrome alloys and titanium may be used for high temperature duties.
Special alloys such as Inconel, Hastelloy and Incoloy are also used.

The minimum practical size of wire that can be used depends on the alloy from which it is made, the strength required in the mesh, the operating temperature, and the degrees of corrosion and abrasion likely to be experienced in service. Thus,
finer wire diameters in aluminium, brass, bronze or copper are not normally used for other than light duties. Stainless steel wire on the other hand is available and is used down to 15 um.

The key dimensions of wire mesh are illustrated in Figure 2.8, for plain weave and square mesh (the usual form for plain woven wire meshes). Aperture width, w, is the distance across the aperture, between the bounding wires, measured in
the projected plane at the mid positions. The wire diameter, d, is the diameter of the wire forming the mesh. The pitch, p, is the distance between the middle points of two adjacent wires and therefore is the sum of the aperture width and the wire
diameter:

The number of apertures per unit length, n, is the number of apertures that are counted in a row one behind the other for a given unit length (which may be 1 cm, 1in or any other convenient unit). The term ‘ Mesh ’ is a traditional one referring to the  number of apertures per linear inch, so that, with the aperture dimensions measured in millimetres:

and

The open screening area, A0, is the percentage of the total screening surface represented by all the apertures in that surface, or the ratio of the square of the aperture width to the square of the pitch:

Mesh numbers may range from 2 (two apertures per inch) up to 400. Meshes are usually classified as coarse (w  1–12 mm), medium (w  0.1–0.95 mm) and fine (w  0.02– 0.16 mm or 20–160m). A typical set of wire mesh dimensions is given in Table 2.7.

Sets of test sieves are available, matching the various standard ranges of mesh size. These are supplied in standard-sized holders, which fit one into the other, and which can be held in the frame of a shaker machine. The design of the mesh holder
is important, since it is essential that the mesh be held fast and taut. There should be a bevelled entry to the mesh area to ensure that all of the material to be sieved is transferred to the mesh area when hand-sieving at an angle of 30°. Table 2.8
shows the comparative mesh sizes for some international and trade standard series of meshes.

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