Generally, several layers of enamel are applied. Fabergé, who was perhaps the best enameller of all time, used up to twenty-five. Between applications, the layers are ground with a carborundum or emery stone to make sure they are absolutely smooth.

All metals may be enamelled except phosphor-bronze and platinum, as can ceramics. Enamel can be translucent or opaque, colourless or multi-coloured. The terminology associated with enamelling is French.

Enamelling is described as either à jour (day) - transparent, or à nuit (night) - applied onto a thin plate of metal. À nuit is the most widespread and covers quite a number of categories - those used in the making of hat pins were mainly cloisonné and champlevé.

Cloisonné - is where enamel is poured into compartments (cloison is the French for a compartment or partition) which are formed by thin metal strips that are shaped by pincers or tweezers in order to follow the lines of a design which has been traced on the metal plate. The strips are then soldered onto the plate so as to form the partitions. Braided cloisonné is produced when the metal strips are twisted into a braid.

Champlevé - means raised field. The name is derived from the method of forming hollows in the metal, either by carving out by hand or making a diecasting and leaving raised dividing strips. The hollows are then filled with enamel. Today, champlevé is also produced using acid etching techniques. Cloisonné and champlevé produce basically the same result, although cloisonné is much more refined.

À jour enamel is where, instead of laying enamel into metal cells, a liquid mixture of enamel is painted onto a metal, or ceramic, surface. The most frequently used for hat pins is guilloché. Plique-á-jour hat pins are rare and exquisite.

Guilloché - is when transparent or translucent enamel is applied over a surface that has been hand-engraved or engine-turned on a lathe to form regular, repeated patterns. Guilloché actually refers to an ornamental border formed by two or more interlaced bands round a series of interlocking circles. The origin of the word is not really known but it may be from the French for William - Guillaume. The surface of guilloché enamel is smooth, unlike cloisonné or champlevé where the pattern can be felt.

Plique-à-jour is a backless enamel which imitates stained glass. The design is formed in cloisonné wire on a thin sheet of metal or mica and transparent enamel is fired into the cells. The backing is peeled away after firing and the enamel is left suspended in a supporting frame of cloisonné wire. The effect can be stunning.

High quality enamelling is an extremely skilled process. With the number of firings involved, the craftsman has to watch the piece very closely to ensure that on each occasion it is fired for exactly the right time and with exactly the right degree of heat to prevent the previous layers from melting and the colours running. Over-firing also removes the pigment, and hence the colour, of the enamel.

Enamel, being glass, is fragile and easily cracked or chipped. It is not difficult to see why a beautifully enamelled period hat pin in undamaged condition is a real treasure.

I have had the exceptional experience of being shown the entire process by a craftsman enameller in London and also, some time ago, of discussing the production of Charles Horner enamelled jewellery with a lady, then ninety, who worked in the firm's enamelling department for many years, making, in particular, the stunning butterfly brooches in the 1920s.