CHAPTER XXIX 
POLISHING, COLOURING AND LACQUERING. 
Polishing—Burnishing—Matt surface—The colouring of gold—Silver--Copper—Brass—Japanese metal colouring—Lacquering. 

There is a wide choice open to you as to the manner in which a piece of work may be finished. It may be plated, coloured, oxidised, left with a dull (matt) surface or polished brightly. Let us take the polishing of a plain silverboxasan example. After the final soldering the work is boiled out in pickle to remove the borax. All superfluous solder and other unevennesses are now removed with a file, and the work is gone over with a scraper. Particular attention is given to scratches. To remove a scratch. Scrapeinthe directionofits length,ordiagonally, untilthe metal round about is scraped down to the level of the bottom of the hollow. Do not scrape across and into a scratch. Work in this direction will only deepen and widen the groove. The box may now be gone over with water-of-Ayr stone, pumice and water, or glass paper. The water­of-Ayr stone is a grey slaty stone. When wetted it should look fight in colour with dark spots,—not even coloured. It is always used wet. The pumice may be in lump form or in powder. In the latter case it is applied with a flat stick or brush. Glass paper is used rather than emery,for if any emery gets into the lemel it is difficult to remove,' while the glass comes away with the flux when the lemel is melted. 

Curved rather than straight strokes are the rule in polish ing : the work goes truer so. The various materials for polishing are used in the order of their fineness, the finest ones coming last. Care must be taken to remove every 
particle of the coarser polishing materials before the finer kinds are applied. The sticks or mops used for each material are therefore kept in separate boxes or tins. 

After one of the three materials mentioned above, a softer cutting material is used. This is blue stone; it is followed by either tripoli or crocus. The whole work is gone over each time, to get it to an equal smoothness. After this, jewellers' rouge is applied. It is mixed with water and just a little grease. The latter keeps the rouge from working into the surface and producing a reddish film which is difficult to get rid of. This discolouration is known as "foxing." A final polish'is given with the hand alone. The work is then washed in warm water and dried in hot 
sawdust. 

When using the polishing lathe take oil and crocus or tripoli on a brush, after the blue stone. Then with the same polishing material use a swansdown mop, centred truly. This will remove the brushmarks. Finish with rouge on another swansdown mop. 

Copper or brass work is filed or scraped smooth. It is then gone over with coarse emery cloth, No. 2, and afterwards with fine, F.F. Or the work may be done on the polishing lathe with suitable bobs and mops. 

The colour of silver when it has undergone the treatment indicated above will be very much that of an ordinary mirror, and its surface will be quite smooth,—all hammer marks, scratches and other traces of its manufacture having been removed. Now this high (" black ") polish is only possible if the whole of the original surface of the silver has been removed in the process of polishing. For when silver " goes through the fire " in annealing or soldering, and is afterwards boiled out in pickle (see Chapter III), the colour of its surface is changed. It becomes rather greyer. This is owing to the burning out, " oxidisation," of the copper alloy near the surface. The copper oxides are dissolved during the pickling, and leave a thin film of pure silver at the surface. But this change of colour, technically known as " fire," extends to an extremely minute depth; and is easily removed by scraping or grinding. Now it was found, long ago, that if you wished to obtain a high (" black ") polish on silver, any parts of the work which had been scraped or filed after it had been made red hot for the last time, and boiled out, looked a different colour from the others. The scraped parts would have a yellowish tinge. So the custom of grinding or scraping the whole surface arose. Many people feel, however, that the " black " polish is obtained at too great a cost, for in the grinding and scraping necessary for its production all those touches which give a human interest to the work tend to disappear, and the work becomes machine-like— " icily perfect." 

Work can be made quite bright by means of the burnisher alone. This tool is made in a number of different shapes, two of which are shown in Figs. 44, 45. The point is of steel, haematite (iron) or agate, and the handle about 9 inches long. In making a burnisher use the best tool steel. Shape it and get it as much polished as possible when soft. Then harden, and temper to a pale straw colour, see Chapter XXX. Polish it again with crocus powder. Finish with Sheffield lime on a buff. Keep the burnisher in order by giving it an occasional rub on a buff with putty powder and rouge. In using the burnisher, hold the blade towards you—at the little finger side of the hand. Lubricate the work with soft soap and water, mixed rather thin and cloudy; or with saliva instead. Turn the work about, alter the direction of the strokes till the surface is burnished quite smoothly. Then go over it with cotton wool on which is a drop of oil and rouge,—finally with clean cotton wool and dry rouge. Wash in warm water with soap, and dry out in hot boxwood sawdust. 

A prong from a tortoise-shell comb is very useful for getting the polishing materials into narrow spaces,—the tortoise-shell being of just the right hardness to hold them efficiently. Quills or sticks of boxwood are also used for this purpose. To polish small openings—pierced right through the work, a jeweller has a bundle of threads fastened under one of the arms of the bench. One or more of these is put through the hole and the work slid up and down the thread till the polishing material—crocus and oil, with which it is coated, has done its work. On other threads is rouge for the final brightening. 

When all the scratches have been removed and the surface of the work is quite true, say, after the water-of-Ayr stoning, or better, after the tripoli, a dull ("matt" or "frosted,") surface can be obtained on silverwork as follows. Make the work red hot—to oxidise the copper alloy at the surface,—then boil the work in dilute sulphuric acid. This solution removes the copper oxides from the sur face and leaves a thin film of pure silver all over. The colour of the silver is now a dead white. A rougher unpolished surface can be obtained by sand-blasting. In this process fine sand is blown by compressed air against the work. It makes quite a dull surface, like that on ground glass. Yet another very fine dull surface is obtained by the use of a wire brush on the lathe. In all these methods of finishing work the scratches must be removed first, otherwise they will show. 

The colouring of different metals will now be discussed —gold first. The colour of alloyed gold, and of the solder employed in goldwork, differs somewhat from the fine yellow colour of pure gold. The surface of all alloyed goldwork is therefore treated in such a way, after it has been polished, that no alloy is left at the surface. The work then, after colouring, looks as though it were composed of pure, unalloyed gold. There are three principal methods of tinting gold—by dry and wet colouring, and by dipping or flushing. In the last-named process, the work after polishing, is dipped for a few seconds into a gilding solution and the electric current passed through it. A thin film of pure gold is thus deposited or flushed all over the work. This process has almost entirely superseded that of dry 
colouring. The latter process cannot safely be applied to gold of a lower standard than 18 carat. 

Wet colouring is the process employed for alloys of 14 carat or better quality. By this method the alloy near the surface of the work is dissolved out, leaving pure gold only. For poorer qualities of gold it can hardly be employed, for in alloys of less than 12 carat more than half the mass of the work is composed of alloy. The solution would there fore attack a large proportion of the surface of the work and leave it rather rough. For the better alloys of gold the roughening of the surface due to the dissolution of the alloy there is very slight, and may be ignored. The 
solution employed is—  

Potassium nitrate =  KN03  (known also  as  saltpetre or nitrate of potash),   8 oz.  
Sodium chloride =  NaCl (common salt)  .  .  4 ,,  
Alum  4  „  

Mix the ingredients in an ordinary melting-pot. The latter should stand on an iron plate on a gas ring. The heat should be applied very slowly at first, but the solution should be used when boiling. Sufficient hot water is to be added to the mixture to make a thick paste, which must be well stirred with a stick. The work should be annealed and boiled out in nitric acid pickle to remove any grease or other foreign matter. It is then suspended on a silver wire or on a horsehair and dipped into the hot colouring solution. It may stay there for five minutes at first. Lift it out and dip it into boiling water. Add one or two tea­spoonfuls of hot water to the colouring solution and replace the work for four minutes. Remove it again and well rinse in boiling water. Its colour should be improving. Add two more teaspoonfuls of hot water to the solution and dip the work again for three minutes. You go on weakening the solution and shortening the time of immersion until the colour of fine gold has been attained. The work should not remain in the solution for more than twenty minutes altogether. 

Some colourers add \ to 1 oz. hydrochloric acid (spirits of salt or muriatic acid) to the ingredients mentioned above after they have been well mixed. Others add the acid after the work has been dipped once. Others, again, add it only when the solution is getting weak. If hydrochloric acid is used at all the time the work spends in the solution should not exceed fifteen minutes altogether, for the action of the acid is to hasten the dissolution of the alloy If the work remains in the solution too long or the gold is too poor in quality its surface will be eaten away.  It is well to avoid the fumes given off from the hot solution, for they are poisonous. After colouring, the work should be scratch-brushed to make it quite bright; or you may frost it with the scratch-brush. In this process the ends of the wires of which the brush is made are allowed to spring on to the work from a stick held against them for that purpose. 


We now come to the colouring, or oxidising, of silver, copper and brass. To do this successfully it is absolutely essential that the metal to be coloured shall be perfectly clean. Any dirt, grease or oxides on its surface will prevent the solutions from acting evenly. When the work has been cleaned it should not be touched by the hand, or even exposed to the air for longer than can be helped. All borax should have been removed and all filing and polishing of the surface completed before the colouring is undertaken. Acopper wire may be fastened to the article for suspension, to save it from contact with the hands. The work is now boiled in a solution of potash or soda to get rid of any grease. One pound of potash to one gallon of water is a good propor tion. The work is then well rinsed in clean water before putting it into the colouringsolution. Smallwork,a broochor necklace which has been in use, for example, may be cleaned with benzine before colouring. Larger work may be cleaned with fine sand and water, applied with a nailbrush. Bath brick or pumice powder may be used instead. This will not roughen the surface of the work so much. 

Care must be taken not to allow any of the colouring solutions to make their way into the settings of translucent stones, for the solution would dull the foil or other backing. 

It is a good plan to remove the work from the solution as soon as it has begun to take effect, and to give it a good scratch-brushing. Then after a rinse in clean water it may be replaced in the solution. A more even colouring is generally attained in this way than if the scratch-brushing is omitted. 

The action of the solutions given below may be stopped at once by removing the work and rinsing it well in clean water. The work will come a little lighter in colour during the subsequent brightening up, so the colouring should be allowed to proceed till the shade produced is rather darker than that finally required. 

The Colouring of Silver. 

Blue. This colour may be obtained by exposing the work to the fumes of sulphur (S) heated in a closed box. 

Grey. Platinum chloride (PtCLJ in alcohol, painted on with a brush. When dry it may be lightened in colour by scratch-brushing or by brushing with pumice powder. 

Blue-black. One of the following solutions used hot :— 
Potassium sulphide (K2S) 10 grains 
Water 10 ounces 

or— 

Barium sulphide (BaS) 10 grains 
Water 5 ounces 

Greyish-brown. Ammonium sulphide ((NH4)2S) painted on with a brush. 
Gold Colour. A cold solution of 

Barium sulphide 10 grains 
Water 10 ounces 

or


a hot solution of 
Ammonium sulphide 10 grains 
Water 5 ounces 
If the work is allowed to remain long in either of these solutions its colour will gradually change to crimson and brown. 

Brown. If the work is dipped in a hot solution of Barium sulphide 10 grains 
Water 10 ounces 

or— 

Ammonium sulphide 10 grains 
Water 5 ounces 

its colour will change rapidly from gold, through various shades of crimson, to brown. If it remains after this colour has been attained it will turn rather greyer. 

The Colouring of Copper. 

Most beautiful golden-brown colours may be produced on copper by heat. The work should be suspended a foot above an ordinary gas ring and moved about so that it is evenly heated. After the golden colours the work turns purple and finally grey. If the heating is continued it will turn black. None of the colours just described are perma nent, though they may last for some months. 

Antique Bronze Patina. 
Bury the copper articles in a box of mould which has been previously well moistened with strong vinegar. Or, dip the work in 
Copper nitrate crystals (Cu(N03)2) 10 grains Ammonium chloride (NH4C1) 10 ,, Calcium chloride (CaCl2) 10 ,, Water 1/2 ounce. 

Dark Brown. A cold solution of dilute ammonium sul 
phide, or a similar solution of potassium sulphide. 
Black. A strong hot solution of barium sulphide. If this is used cold the action is much slower. The colour changes from light to dark brown before turning black. 

The Colouring of Brass. 

Black. A hot solution of 
Copper sulphate (CuS04-5H20) 2 ounces Water just sufficient to dissolve the copper sulphate Ammonia (NH4OH) just sufficient to redissolve 
the precipitate which first forms, 
or-
Arsenic 50 parts Hydrochloric acid 250 „ 
then add-
Chloride of antimony 35 
Finely pulverised hammer scale 35 

This solution is used cold. Rinse the work before and after 
immersion in a warm soda solution. Brown. A strong hot solution of barium sulphide. The following notes were given to the author by the late 
Sir W. C. Roberts-Austen, Master of the Royal Mint, 
London :— 
" The method adopted at the Imperial Mint, Asaka, Japan, for colouring copper medals— 
Copper sulphate 2-25 grammes Rokusho (a kind of verdigris) T 5„ Distilled water 1-8 litres. 
The solution, without nitration, is placed in a copper pan and briskly boiled. The medal is first boiled in a weak lye made by lixiviating wood ashes. It is next carefully polished with charcoal powder, and then dipped in plum vinegar containing a little common salt. After removal from the vinegar solution it is passed through a weak alkaline lye, and then placed in a tub of water and well washed. The medal is now placed in the boiling colouring solution and kept immersed for several minutes, in a wire cage. Onceortwiceitisliftedout quicklyandimmediately placed in cold water to ascertain the shade of colour which has been produced. If too light it is replaced in the boiling solution. If it is the proper colour it is washed thoroughly with cold water and wiped dry with a cotton cloth. When the solution has been in use for some hours a pinch of verdigris or copper sulphate should be added if its action appears to have been weakened. Otherwise, after prolonged boiling, loss by evaporation should be remedied by the addition of pure water. 

" In preparing the medals for the colouring solution they should not be allowed to remain for very long in the water before boiling them, or the colouring will be irregular. 

"Japanese Pickling Solutions. 

1  II  Ill  IV  
Verdigris Copper sulphate (CuS04)  175 gr. 1161 „  18 gr. 87| „  90 gr. 214 „  333 gr.  
Sulphur  ....  48  „  
Nitre (KNO,) . . Salt(NaCl) . . . Vinegar ....  18 15  „ „  foz.  o33  ,,  
Water  ....  70 oz.  35 oz.  70 oz.  35 oz.'!  


These four formulae differ somewhat from those already published by Roberts-Austen, but they represent the solutions which he had found most successful in actual practice; see Japanese alloys, page 177 of this book. 
A valuable work on Metal Colouring has been written by Mr. A. H. Hiorns of the Municipal Technical School, Birming ham. To this book the reader is referred for any further information on the subject. 
To preserve the colours produced by chemical solutions on copper and brass the work is sometimes lacquered. This process consists in coating it with a thin layer of some transparent lacquer or varnish. The work should be done with judgment or the metal may lose its true metallic sheen and appear as though painted. The work is either painted with or dipped into the liquid lacquer. It is then put into an oven and thoroughly baked. A lacquer, however, which can be used oold is known as Zapon. It is a preparation of celluloid in acetate of amyl. The work afte being painted with this liquid should be stood on a warm plate till the semi-opaque streaks, which have appeared during the apphcation of the lacquer, have disappeared, and the lacquer has become quite transparent again everywhere.