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Hollow Carat Gold Jewellery from Strip and Tube
By Peter Raw - Consultant, Bookham, Surrey, U.K.
Gold Technology 35, Summer 2002
One approach is the use of mass production techniques to lower the unit manufacturing cost of items. Such techniques include the well-established chain making, stamping and investment casting, the more recent electroforming and the very recent powder metallurgy processes. These techniques are continually developing as equipment manufacturers make more advances and market their products more widely.
A second route is to make the jewellery out of thinner material, for example, in stamping, carat gold as thin as 0.1mm can now be utilised with individual items weighing significantly less than 1g. However, as thickness decreases so the fragility of the items increases, although it may be possible to counteract this to an extent by means of a final heat treatment if the carat gold alloy is hardenable.
Yet another approach is to reduce the caratage of the gold alloy but this is hardly a realistic proposition as consumers buy preferred caratages and there are only certain defined national standards of fineness allowed. The low carat gold alloys can suffer from tarnishing and stress corrosion problems. In fact, in recent years there has been a significant focus on the very high carat golds and on ways in which they can be strengthened. In the UK, for example, where 80% of the gold jewellery sold is 9 carat, the four legally-accepted standards for gold jewellery (9, 14, 18 and 22ct) were increased in 1999 by the addition of two 24ct standards, 990 and 999 parts per thousand of gold.
A further option that can be considered is hollow jewellery; this gives the opportunity to produce sizeable items in high caratages but at a relatively low weight. Electroforming, of course, is one means of producing hollow jewellery. Stamping can also be used to produce matching halves of components such as earrings and pendants which then require soldering together to produce the finished article. However, there is a large category of hollow jewellery items made directly from formed strip and tube which includes chain, earrings, bracelets and bangles and it is the technology and equipment used to make these products that is the subject of this paper. Some examples of these products are depicted in Figures 1 and 2. The complexity of these hollow jewellery items is increasing all the time and depends on the extent to which the formed strip or tube can be deformed. This, in turn, is largely determined by whether the tube is deformed directly (when only the simplest of shapes can be made) or whether the tube has a core to support it during deformation (when more complex shapes can be made). If the latter, it depends on whether the core is bonded to the tube, in which case the most deformation can be tolerated.
Perhaps the most demanding of hollow products is hollow chain and this is the subject of the first part of the paper; the second part concentrates on techniques for making other hollow products from formed strip.
There are some major technical problems to be overcome. These include selecting an appropriate core, aligning the composite wire such that the folded edges of the strip are always on the inside of the chain links and out of view, making good soldered joints in the links as the joints are ultimately between very thin sections of carat gold and, finally, ensuring that the base metal core is fully dissolved out after making the chain. This latter is important in order to meet national hallmarking/marking requirements.
Selection of core material
Aluminium is the easiest to work with, being the softest metal. However, aluminium finds extremely limited use as, with its low melting point of 660°C, annealing of the composite wire is not possible during processing nor is it possible to solder the chain links. Dissolution of the core is by the use of caustic soda (sodium hydroxide). Copper and Tombac overcome the problems of using aluminium as annealing of the cored wire and soldering of the chain links can be readily achieved. The working characteristics of these core materials are very similar to those of many carat golds and this makes drawing and link forming relatively easy. In addition, a bond can be formed between gold and core, especially with the Tombac where the presence of zinc aids bonding, and this bond helps to avoid wrinkling of the core during link forming. These core materials tend to be used for the higher caratage gold alloy chains, say 18ct and above, although they are used on occasion for 14ct golds. This is because nitric acid is used to attack and dissolve the core and, at the lower caratages, the acid will also attack copper and silver in the carat gold alloy itself. Finally, soft iron can be used universally as a core, although usually without the formation of a bond between core and carat gold. Also the difference in mechanical properties between iron and carat golds means more care is needed during working and annealing of the composite wire.
Production of cored wire
A more commonly used method involves a square section core rod that contains two closely aligned longitudinal slots that hold the folded gold strip. Having formed the strip around the core, the composite is rolled and drawn on conventional machines, with inter-annealing as necessary, until wire of the requisite size is produced. A variant on the method is to make a core with a third longitudinal slot on the opposite face to the other two, into which a continuous gold solder wire is fed, so the finished composite contains a solder wire held in place by the core. The use of this system makes it easy to maintain alignment of the seam which is ultimately the path for acid to enter at the core removal stage. The system is known as Aggraffato. A different system, Morini, involves a gold solder wire being fed between the folded edges of gold strip in such a configuration that, on subsequent working to wire, the solder finishes in the centre of the composite rather than at the edge. This helps solder to flow between the chain link faces during the soldering operation and is likely to result in a better, more reliable joint. It also means that the solder is better supported during working, allowing the use of less ductile solders and, hence, a wider choice of solder. These systems are shown schematically in Figure 3. In all cases removal of the core eventually takes place by chemical attack through the seam between the folded edges of the gold strip.
Other techniques include the continuous folding of gold strip around the core followed by automatic resistance welding of the strip edges, sometimes with the incorporation of solder wire into the composite. Equipment for producing this composite is shown in Figure 4. In this approach, seam alignment is no longer an issue but, when making chain out of this composite, the chain machine is fitted with a device to make small openings on the inside surfaces of the links through which acid can attack the core during the core dissolution stage. These techniques, several of which are patented, do require the use of special drawing dies to make the shaped cores, forming dies to fold the gold cladding around the cores and drawing dies to draw the composite down to finished wire size. Much of the technology has evolved in Italy where hollow chain is an established product.
A further technique involves welding carat gold sheet to copper or iron sheet at a thickness ratio of between 1:5 and 1:10. The composite is rolled to strip and then drawn through dies which fold the strip round with the copper or iron on the inside acting as a core. The composite can then be drawn to finished size and made into chain. This technique is capable of giving a bonded iron core which helps in the production of some types of chain.
Making the chain itself is essentially carried out on the same chain machines as are used for solid wires except the machines can be fitted with an electronic device to maintain the seam between the folded edges of the gold strip on the inside of the links. For a typical composite of 5mm diameter or section, the gold strip might be of about 0.55mm thickness; this might then be rolled and drawn down to a finishing size of 0.6mm diameter with a wall thickness of 0.04mm. The minimum sizes currently produced are as low as 0.25mm diameter.
One further issue that has to be considered when making the composite wire is the gold make-up of the alloy, not least to ensure meeting hallmarking requirements. When copper or Tombac is used as a core material, the annealing treatments during processing can result in significant diffusion of copper and zinc into the gold and this can affect the gold content of the chain after eventual dissolution of the core. To compensate, the gold make-up has to be higher in the initial alloy, the extent depending on the final wall thickness of the chain. For very thin wall chain, it may be necessary to make up 14ct gold at as much as 60% and 18ct gold at 75.5% gold. Experience will enable the correct level to be found.
Soldering of hollow chain
Because of the thin section of the soldered joint in hollow chain, the requirements that are put on solid chain wire are even more critical for wire used for hollow chain. Hence the cored wire must be annealed sufficiently to ensure the formed links do not spring apart and leave too wide a gap for soldering but the wire must not be over-annealed which will lead to grain growth and an orange-peel effect on the chain link surfaces. The presence of a core, especially iron, complicates the annealing process and it is extremely important to establish the correct annealing parameters. Again it is of great importance to use a sharp link cutting tool when working with wires which contain a solder wire in the core so as to ensure the solder can flow readily when it is melted. Automatic soldering of some styles of chain is now possible on chain soldering machines using a wire filler. Laser welding is also finding application for hollow chains and so it is likely that the future will see significant advances in hollow chain joint quality.
Dissolution of the core
Iron core dissolution is usually by means of hydrochloric acid. However there is evidence of some grain boundary attack of low carat gold alloys by the acid (possibly due to stress corrosion) and more recent developments use sulphuric acid. As mentioned above, complete removal of the core is essential if national marking requirements are to be met. Thus, determination of when core dissolution is complete is an important issue although it is frequently assessed by operator experience. In the case of iron-cored chains, a simple test is to check for any residual magnetism in the chain. The safest test is to assay the chain. If the assay value is below the nominal figure, then dissolution is not complete. If the assay value is significantly above nominal, this suggests dissolution has gone too far and some of the gold alloy itself has been attacked. The equipment used must also be capable of giving a thorough final washing operation so that no risk exists of acid residues remaining in the hollow chain.
Hollow Balls and Ball Chain
Hollow Jewellery made from Formed Strip
A typical sequence of operations to produce components from strip might be as follows:
There are, perhaps, three approaches to setting up to make formed hollow components from strip in this way. Firstly, many equipment manufacturers sell individual machines to cover operations such as strip cutting, tube making, seam welding, drawing, tube patterning (knurling), annealing, tube head hammering, tube twisting, tube cutting and tube winding. The appropriate dies and mandrels for the different shapes and sizes of tube and formed component are also available. The manufacturing jeweller then purchases equipment and accessories as and when required.
Secondly some companies will provide a whole production line and so the manufacturing jeweller will discuss his requirements and the various pieces of equipment will be built and supplied as an integrated unit.
Finally, there is at least one company that makes a single type of machine but with a series of interchangeable attachments. With the supply of matching draw dies and mandrels, it allows the manufacture of a wide range of coiled hollow shaped and twisted tubes and sections, albeit on a fairly small scale, Figure 9. One version of the machine is shown in Figure 14 while a more recent version employs two dies and two feeder strips to make more complex shapes and allow the manufacture of multi-coloured hollow jewellery.
One potentially difficult problem to overcome is that of marking hollow jewellery with the hallmark or caratage and manufacturers marks without damaging what is probably quite a fragile component. Often there will be an attachment or fastener on which the marks can be made but by far the most elegant solution is the use of lasers for applying the marks, a practice which has now been adopted by Assay Offices in the UK.