CHAPTER X.

Enriching the Surfaces of Silver.

By the application of the processes about to be described, the finishing touches in their relation to articles or wares of silver manufacture are effected. These processes, as adopted by the trade, are various, almost every firm having a specially prepared mixture and mode of employing it. We shall refer only to those which, from their practical utility, are likely to be of service to those workmen who have to do with this particular metal. The branch of the art of which we are now treating comes only into operation when every other process of workmanship has been completed; and some of these processes must be executed in a perfect manner in order to arrive at the highest possible results in this one. The best and richest surface is produced when the metal to be operated upon is good in quality, and the workmanship of a fair order, so far as regards smoothness, and freedom from surplus solder-marks.

One of the oldest methods for producing a pure snowy whiteness upon articles of silver was as follows:--Take an iron or copper annealing pan (the latter is much to be preferred), place the work upon it in proper order, so that it may be heated all over alike. It should, previous to this, be immersed in a thick solution of borax, or otherwise brushed over with it. After the work has been properly arranged upon the pan for annealing, it must be sprinkled over with fine charcoal dust; the pan is then placed in the muffle upon a bright clear fire without blaze, and when the work has assumed a degree of heat approaching to cherry redness, it is withdrawn and allowed to cool. When this has taken place, it is removed and boiled out in a very weak solution of sulphuric acid, commonly called oil of vitriol. If the right colour was not then produced, the process was repeated as many times as circumstances permitted, though usually two or three times was found to be amply sufficient. The annealing process required great attention, for the work being in contact with borax, if slightly overheated, it was liable to become melted, therefore the operation was a delicate one to perform, and was only intrusted to such workmen as were experienced in the art. Small delicate articles were commonly treated with the mouth blow-pipe and gas-jet, and were placed upon a pumice-stone, or some other suitable substance capable of withstanding the power of burning. According to the inferiority of the silver alloy, is the difficulty of producing a good white surface on wares of such standards. Fine silver requires very little whitening, generally one process suffices to effect a good colour; but inferior standards require half a dozen or more to bring up the proper degree of whiteness; and those ranging below 9 oz. 12 dwts. to the lb. Troy cannot be whitened at all by the means we have described, but require the application of the modern chemical process, known as electro-plating.

The East Indian silversmiths never touch their manufactures with any kind of abrasive substance, from the most delicate, to the more strongly made article. But then it should be remembered, that in India the natives work from the pure material, a point which they rigidly adhere to; whereas we are compelled in this country to manipulate in all sorts of qualities; and some of these require no little trouble and difficulty to bring back to the surface the snow-white appearance of the pure metal. In the former case it is effected without any difficulty whatever--in fact, the metal scarcely undergoes any change throughout the whole of the manipulations to which it is subjected in the various processes of manufacture. The Indian mode of procedure is as follows:--Some juicy lemons are cut into slices; the silver articles are briskly rubbed with these for a short time, and subsequently covered with them, being placed in a suitable vessel for a few hours for the completion of the process. For very delicate articles of jewellery the natives cut a large lime nearly in two halves, into which they insert the work; the halves are then tightly closed up again, and placed aside for a few hours; when the article is removed, it is well rinsed in clean water, and consigned to a vessel of nearly boiling soap-suds, where it is well brushed, again rinsed in fresh hot water, and finally dried on a metal plate placed over hot water; the process is rendered complete by a little gentle rubbing with wash-leather if the work be of a plain nature. Green tamarind pods are also employed by them for the purpose of whitening silver, in the same manner as just described; they are great detergents both of gold and silver manufactures, and are largely employed by artisans in the East for the removal of oxides and fire-marks.

Another process for the whitening of silver goods is performed in the following manner:--Take the work, which must be cleanly prepared, and give it a coating of the following mixture:--Finely powdered vegetable charcoal four parts, saltpetre one part; the ingredients should be well mixed with a little water, and may be applied to the surface of the metal either by brushing over with a soft brush, or by dipping the work into it. The work is then placed upon the annealing pan and submitted to the heat of the muffle, until the wet powder has become perfectly dry and ceased to fly about; it is then withdrawn from the muffle, allowed to cool, and afterwards boiled out in a solution of potash prepared for the purpose, in the proportion of about one ounce of bi-sulphate of potash to twenty ounces of water. The boiling out is done in a copper pan (Fig. 39), and if the work be put through the above process two or three times a beautiful dead-white colour is the result. It is then washed in a hot solution of soda, soap, and water, or if preferred bright, scratched, or burnished, and the process is finally completed by drying it in fine boxwood sawdust, which should be made hot, but not allowed to char or burn in any way, as it would produce a stain upon the work very difficult to remove, and thus the finish would be considerably impaired.

In large manufactories the process of whitening silver goods is repeatedly required to be performed, and where such is the case, the above methods are found not only tedious and expensive, but occupy much unnecessary time and labour; to dispense with a portion of which, the custom of covering the work with a chemical preparation was accordingly departed from, and yet it was made to show its former brilliancy. To effect this object the liquid for boiling it in was differently prepared, which only required to be made of a proper strength to do all that the surface mixture had done before. The following is the method adopted in preparing the cleansing liquid. Boiling-out mixture:--To one pound of smoking salts, add two ounces of cream of tartar; well shake the ingredients together, so that they may be thoroughly incorporated. The smoking salts employed for this purpose are not the ordinary _spirits of salts_ of commerce, but a preparation of the common oil of vitriol; therefore the one should not be taken for the other; the spirits of salts would turn the work black, whereas, if the proper ingredient were procured, a fine dead matte or blanched surface would be the result of its application. The mixture employed for boiling the work in consists of the proportion of one ounce of the above preparation to about thirty ounces of water. The silver articles simply require to be annealed, allowed to cool, and then boiled for a minute or so in this solution, when the desired result will be attained; if, however, the exact colour be not obtained the first time, the process should be repeated a second, and if necessary, a third time; the right colour will then be produced, if the articles are not made of a too inferior standard.

The mode we have ourselves adopted for the colouring or whitening of silver goods is somewhat different, and still more simple than even the above. We will proceed to give the details of the process. A mixture of very dilute sulphuric acid is first provided, in the proportion of one ounce to forty ounces of water, and well mixed together; the work, after being heated to a good red heat, is boiled in this, which soon removes the oxide from the surface, and shows the fine white colour of the pure silver. For fine silver work, such as Indian filigree, one process will generally suffice, for English standard quality two, and for low qualities three, but these latter must not by any means be too low; if so, no colouring can take place by the method just described. Objects of delicate workmanship are usually annealed by the gas; being placed on a pumice-stone of light material, the flame of the gas is blown with the mouth blow-pipe, in such a manner that the object gradually becomes heated all over alike; and the work should be well heated, as this facilitates the process of oxidation, and subsequently that of whitening. The oxidation takes place at the expense of the copper in the silver alloy, and this is only effected by raising the articles to a very high temperature, which produces the oxidation of the copper coming in contact with the air, and which necessarily exists upon the surface of the alloyed goods. Whitening silver goods then is nothing more than the removal of the base alloy from the surface, leaving the pure metal behind with its full rich colour. Therefore to be clear, the process of annealing in contact with cold air oxidizes the copper upon the surface, and the pickling mixture so dissolves and removes it, that it gradually undergoes a process of refining, and is ultimately made to represent the finest material in all its purity.

Sometimes silver work is to be seen having a brown colour upon it; this is produced when the acid employed for cleansing has been too strong; it can only be remedied by another annealing and boiling-out in a much more diluted mixture. There are various other methods employed in the trade for the purpose of whitening silver work of the best quality; and although annealing is always a part of the process, other ingredients, such as salt and tartar, permanganate of potash, cyanide of potassium, alum, &c., have been severally used for the cleansing or whitening mixture. They may be useful in their application to plated work (articles that have received a coating of pure metal by means of the electro-metallurgical process), for cleansing purposes only, but for all practical purposes the process to which we have called special attention is to be much preferred.

Common articles of silver cannot be whitened by annealing and boiling-out in a diluted acid; a thin film of pure silver must be deposited upon their surface by the process of electro-deposition, or by the action of some chemical preparation in which fine silver forms the principal ingredient. Such preparation, however, as the latter can be used only to plain surfaces, therefore they are not applicable to all kinds of work. They are composed of the following chemical ingredients:--1st, chloride of silver 1 part, cream of tartar 1 part; 2nd, chloride of silver 1 part, common salt 1-1/2 parts; 3rd, chloride of silver 1 part, prepared chalk 1 part, pearl-ash 1 part; 4th, chloride of silver 1 part, alum 1 part, common salt 2 parts. The chloride of silver is easily prepared by precipitating it from the nitrate with a solution of common salt or hydrochloric acid. The various mixtures should be worked up with water into a thin paste, and applied to the work by rubbing with a soft cork or piece of wash-leather, or by thoroughly stirring it about in the mixture until it has acquired the requisite degree of whiteness. For the purpose of silvering watch and clock faces, &c. these mixtures may be used with advantage and entire success.

Other solutions are sometimes employed for similar purposes and are very useful; being simple in their preparation and easy of management. We have selected the following as being the most practical:--Take one ounce of the _nitrate_ of silver and dissolve it in one quart of pure distilled water, or if this cannot be procured, water which has been boiled, by which it loses some of its impurity. When the nitrate of silver has become thoroughly dissolved, throw into the mixture a little powdered hyposulphite of soda, this will precipitate the silver, and when it has taken place, a further addition of hyposulphite of soda should be made, which will eventually re-dissolve the precipitate, and the solution is then ready for use. To produce a good mixture, the salt of soda should be added slightly in excess. The solution is used by simply dipping a sponge in it and rubbing it over the surface of the articles to be coated, and this is continued until they have assumed the desired colour.

For improving the colour of silver and electro-plated wares, the following mixture has been strongly recommended:--Nitrate of silver 4 pennyweights, cyanide of potassium 5 ounces, and water 1 quart; the ingredients should be well mixed together, and applied by means of a soft brush or sponge to the surface of the work. In using this cyanide solution, the operator should be careful to guard against a too frequent contact with it, as it is decidedly injurious to the hands, especially if there be any abrasion of the skin; it being one of the deadliest poisons known. Sufficient details of the process of silver whitening and cleansing having now been given to assist the workman who manipulates in this particular metal, and to enable him to select a form of recipe in every way adapted to the kind of work in hand, we shall now proceed to the modern process of _electro-plating_, and give a practical description of it in its applicability to the trade of the silversmith.

This art is decidedly of modern origin, as far as concerns its employment for commercial purposes. The invention is supposed to be due to the electrical and chemical researches of Mr. Spencer, of this country, and Professor Jacobi, of Russia, both of whom claim to have found out the art of depositing one metal upon another, somewhere about the same period. Of course it was left to others to apply the invention to the industrial arts, and it was not until after the discovery of the _Constant Battery_, by Professor Daniell, about half a century ago, that the art began gradually to extend in the direction of commercial pursuits. The Messrs. Elkington, of Birmingham, were the first to employ it in their manufactures, with a success which their enterprise thoroughly merited. This took place about the year 1840, and since that time the art of electro-plating and gilding has wonderfully developed, in its application to the various manufactures of the country. Its progress would be a subject highly interesting, were we to trace the general details of it, but the part of it we are considering being the practical mode of its employment in manufactures, we shall at once direct our attention to it, by giving a complete description of the process; so that the ordinary silversmith may be enabled to employ it in his business with safety and advantage.

The first thing to be considered in electro-plating is what _Battery_ to employ, which will be the most simple, inexpensive, and effective one. When the battery is only occasionally required for use we prefer the Smee before any other. It is a small portable apparatus, and consists of a high, but narrow, glass or stoneware jar, in the form of a cylinder, capable of holding about two quarts; inside this jar is fitted a thin plate of platinized silver fitted to a frame with two zinc plates, one on each side of it, the zinc plates being held to the frame by means of a binding screw. Strong copper wires are firmly secured to these screws, which serve as the positive and negative poles of the battery. Those parts of the plates which are not exposed to the action of the acid solution may be advantageously coated with sealing-wax varnish or melted paraffin wax, to protect them from the destructive influences of the battery acid, and to prevent it from creeping upwards, which destroys the connections. The jar is filled with water acidulated with sulphuric acid, in the proportion of 1 of acid to 20 of water; the frame containing the plates is then lowered into the solution, and the battery is ready for use. In the above form of battery for occasional use we have one simple in construction, easy of management, of fair constancy, and when once prepared very inexpensive, merely requiring a little free acid at times to keep up the strength of the current. Two cells of this form of battery, each holding two quarts of mixture, will be found sufficient for all ordinary purposes. The zincs should be well amalgamated, and not touch the bottom of the cells. The connections should be regularly examined, and kept perfectly free from corrosion, which would stop the passage of the current. For plating small delicate articles of jewellery one cell of the above form will be found powerful enough for the purpose.

The battery that we prefer and have of late years employed, for regular continuous working, is the Bunsen, consisting as before of a cylindrical glass or stoneware jar of the same size and dimensions, fitted with a well amalgamated _cylinder_ of zinc and a copper wire secured to it; a porous cell is placed in the centre, and a bar or rod of carbon is put into this cell with a copper wire also secured to it. The porous cell is filled with a mixture of equal parts of nitric and sulphuric acids, or sulphuric acid alone; we prefer the latter, as it does not give off such fumes as does the other acid; a little of the more powerful acid, however, is sometimes required to be added in order to increase the action, as with this acid alone it sometimes becomes slow. The outer cell is filled with a mixture of 1 part of sulphuric acid to 20 parts of water, and the connections being in proper order the battery is then ready for use. In action this form of battery is regular and continuous, it lasts a long time upon one charge, and is therefore inexpensive in use; if the two cells are coupled for power or intensity, an unusual quantity of work may be got through in a given time. This cell is admirably suited to the work of the manufacturing silversmith, and to those who prefer doing their own plating.

The amalgamation of the zinc is effected as follows:--The cylinders are best treated by putting some mercury into a coarse flannel bag, dipped repeatedly into muriatic acid and applied to the surface of the zinc, both inside and out; and when they present the bright characteristic appearance of mercury they are sufficiently operated upon, and may be rinsed and set aside to drain. The zinc plates may be advantageously amalgamated by placing some mercury in a shallow dish with a little muriatic or sulphuric acid, a hare's foot or a piece of cloth tied to the end of a stick is then dipped into the mercury and acid, and rubbed over the plates until they are sufficiently protected with mercury, when they should be rinsed in clean water and set aside to drain. If possible the process of amalgamation should always be conducted in the open air, as the fumes which are given off, if breathed, are highly injurious. The best possible way to amalgamate rods of zinc is by pouring mercury into the melted metal just before casting it into rods, in the proportion of 1-1/2 oz. of mercury to the pound of zinc. This makes the rods exceedingly brittle, and they should therefore be handled with care. The mercury should not be added to the zinc when the latter is at too high a temperature, and the best manner of testing this is by the application of a piece of paper to the molten metal, when if it takes fire, the temperature is still too high; it should be allowed to cool until the paper refuses to ignite, then and not till then is the proper time for the addition of the mercury.

The copper conducting wires and binding screws must be cleaned when they become much corroded; if not they add resistance to the current, and it will become considerably diminished, or cease altogether. The cleaning may be effected by simply annealing and then plunging them while still hot into dilute sulphuric acid pickle, or dipping them into nitric acid for about an instant.

The zincs should be taken from the battery liquids when not required for use; and the porous cells should be removed every night and their contents poured into a large jug kept for the purpose. The porous cells should be placed in clean water to prevent the salts of the battery liquid from crystallizing in the pores, which would crack and spoil them for future use; the carbons should also be placed in water; and when required for use again these arrangements should be reversed, when the battery will work as well as ever.

The solution for electro-plating articles of jewellery is the next part of the subject we have to consider, of which there are many, containing various proportions of the metal employed in silver depositing. The following is one of the best we have employed: take pure silver and dissolve it in a mixture of nitric acid and water:--

Fine silver 5 dwts. Nitric acid 4 drms. Water 2 drms.

The silver should be put into a small Florence flask, so as to allow the mixture of acid and water to cover it thoroughly; this mixture on being added to the metal soon promotes a chemical action, and the silver becomes gradually dissolved. If the acid employed has been weak, it will necessitate a further addition of it to complete the dissolution of the silver, or the removal of the flask to some other and warmer place, such as a sand-bath, but care should be taken not to apply too much heat. As the chemical action proceeds, red fumes are formed in the flask; and the action should be allowed to go on until these cease to rise, when the silver should by that time have become dissolved. The mixture then consists of a solution of nitrate of silver, and should be carefully poured into a suitable porcelain or Wedgwood capsule; this is then heated upon a sand-bath until a scum or pellicle appears upon the surface, when it should at once be transferred from the sand-bath to a suitable place for cooling. During the last operation the mixture begins to form itself into crystals, and the liquid which appears reluctant to crystallize should be poured away from those already formed, into another capsule, and again heated until it has sufficiently evaporated to crystallize. When the whole of the liquor has finally undergone this process, the crystals of nitrate of silver must be removed to another vessel, and about one pint of cold water added, the whole being then well stirred until they have become thoroughly dissolved. A solution of cyanide of potassium is next prepared in water, in the proportion of about one ounce of cyanide to the pint of water; some of this solution is then added to the one containing the nitrate of silver. It must, however, be added very cautiously, for precipitation soon takes place, and if too much be used, the precipitate becomes again dissolved. For this reason it is advisable to take a little of the solution from the vessel, in a wine-glass or test tube, and to add a few drops of the prepared solution of cyanide, in order to ascertain its exact state. If the application of this solution produces no effect upon the nitrate of silver, the operation of precipitation is complete.

The liquid above the sediment should next be carefully poured away to avoid any waste of silver; when this is done fresh water should be added, well stirred with the sediment, and allowed to settle; it is then again poured off, and the process repeated until the precipitate has become thoroughly washed. Now add sufficient cyanide of potassium to dissolve the precipitate and a little more, and make up two quarts of solution with fresh clean water. It is better to filter the solution before using it.

The solution may be made by means of the battery, and, if preferred, the above mode of chemically preparing it may be dispensed with. The following is the most simple method by the battery process:--Dissolve in two quarts of water about half an ounce, and no more, of best black cyanide; this should be done in an oval, or still better, oblong stoneware vessel, placed in an iron one of the same shape containing water. The stone jar should not be allowed to touch the bottom or sides of the iron one, a space being left for holding the water. When the cyanide has become dissolved, fill a porous cell with some of the solution, place this cell in the other vessel containing the cyanide; the solution should be about the same height in both vessels. Now put a piece of sheet copper, secured to the end of the wire issuing from the zinc of the battery, into the porous cell, and place in the larger vessel containing the cyanide solution about one ounce of sheet silver, properly secured to the wire issuing from the carbon of the battery. In a short time the solution in the larger vessel will have acquired the right proportion of silver (5 dwts. to the two quarts) for use; when this has been effected, the porous cell should be removed and its contents thrown away. These solutions are both worked hot, at a temperature of not exceeding 120° Fahr. with the battery we have described.

The solutions are heated by means of gas-jets, and the articles are plated by being suspended to the wire proceeding from the zinc of the battery. To the wire proceeding from the carbon is to be attached the piece of sheet silver which dissolves and keeps up the strength of the solution. The piece of silver or anode being lowered into the solution, upon the immersion of the work, an almost instantaneous deposit of fine silver takes place, the thickness of which depends entirely upon the period of immersion.

When a solution begins to plate of an inferior kind through the acquisition of organic matter, it will be better to abandon it altogether and make a new one, rather than to waste valuable time in repeated attempts at improvement, which seldom can be effected in solutions that have been employed for all kinds of work. The silver may be recovered from such solutions by means of the battery, by precipitation, and by evaporation; the first process, however, we have not always found successful, the solution in some cases refusing to give up its silver to the action of the battery. It is put into operation as follows:--The anode which supplies the solution with silver is replaced by one of platinum, on which the cyanide solution has no action whatever; a piece of clean sheet copper should be hung upon the zinc wire of the battery, and the battery kept in action until the whole of the silver held in the solution has become deposited upon it; at which stage it may be removed, and the exhausted solution thrown into the waste water tub. The piece of copper containing the silver may be used again in the place of the silver anode until it has become dissolved, or it may be removed by any other means, if preferred. In the event of the above plan failing, the process of precipitation or evaporation should be resorted to. If the former one be adopted, the solution should be poured into a large open vessel, and considerably diluted with water; sulphuric acid should then be carefully poured in, a little at a time, until it produces no effervescence. The sulphuric acid precipitates the silver, and the fumes which it creates are highly deleterious to health, therefore the process ought to be performed in the open air, and not in ill-ventilated workshops. When the sulphuric acid produces no effect upon the solution, it should be allowed to stand for a while for the precipitate to subside, when the water above (which should be clear) may be drawn off, the precipitate well washed, to free it from the acid, dried, and fused in a crucible with a little potash or soda. If the latter plan be adopted, the solution may be placed in a cast-iron kettle or saucepan, and then heated upon a gas-jet or stove, until evaporation takes place, after which the sediment should be removed and fused in a crucible, as before.

The finishing of silver work requires some little knowledge and skill to perform it properly; and we think that a few observations bearing upon it will be of service to those for whom this manual is written. After either of the processes of whitening or plating, the work has to be scratched, unless required to be left a _dead_ white, then this process does not take place; the scratching removes from the surface the dull white colour produced by the above processes, and effects a characteristic bright and uniform colour to the work of the silversmith. Scratching is done at the lathe (Fig. 40) by the application of a very fine brass-wire brush of circular form running upon the spindle; a solution of weak ale runs from a barrel with a tap to it, placed upon the framework of the lathe so as to enable the beer running from it to fall upon the brush during the whole time of its rotary action, and this assists the brush the more easily to glide over the surface of the work submitted to it. A large quantity of silversmith's work receives no other treatment than this, after the whitening processes have taken place. Silver chains are burnished by means of a polished steel jack chain, and the application of a little soft soap and hot water, or otherwise scratch-brushed. The beautiful frosted surfaces to be seen upon silver lockets, and other work of a similar nature, are all produced by means of the scratch-brush.

Burnishing is another mode of finishing silver work. It produces a polished surface, which reflects like a mirror, and gives the greatest lustre: it removes marks left by the polishing mixtures, and produces a darker surface than the other modes of finishing. The tools employed for this process are extremely variable, and well adapted to the different kinds of work to which they are applied; they are of two kinds, one being formed of hard stone, and the other of polished hardened steel; they vary with regard to shape, some being straight with rounded points, or with curved and blunted edges, others with large rounded surfaces, &c. Stone burnishers are made of blood-stone, which is mounted in a wooden handle with a brass ferrule, which firmly secures the stone to it, in which state it is used. Steel burnishers are likewise fixed in wooden handles, which enable them to be firmly grasped by the operator. Throughout the whole process of burnishing, the tool should be repeatedly moistened with a solution of soap and water; which causes it to glide more easily over the surface of the work, prevents it from becoming too much heated, and generally facilitates its action. In consequence of the great friction which the burnishing tool undergoes, it soon loses its bite, when it slips over the work as if it were greasy; its effectiveness must therefore be restored from time to time by rubbing it upon the leather which the workman has beside him for the purpose. It generally consists of a piece of buff leather, impregnated with a little crocus. In very small articles only steel burnishers are used, as they are finer in make, and by their greater variety of form, are exceedingly well adapted to all kinds of work; in this class of work, if any soap-suds should adhere to the article they may be removed by the application of a little tissue paper. Large pieces of work are rubbed with a piece of old linen, or washed in a warm solution of soap and water, rinsed, and dried in boxwood sawdust, which finally completes the process.

Silver work may be oxidized by any of the following processes:--

I.

Sal-ammoniac 2 parts. Sulphate of Copper 2 " Saltpetre 1 part.

Reduce the above ingredients to a fine powder, and dissolve it in a little acetic acid. If the article is to be entirely oxidized, it may be dipped for a short time into the boiling mixture; if only in parts, it may be applied with a camel-hair pencil, the article and the mixture both being warmed before using.

II.

Platinum 1 part. Hydrochloric acid 2 parts. Nitric acid 1 part.

Dissolve the platinum in the mixture of acid, evaporate to crystallization, and when cold, dissolve again in a little sulphuric ether. Apply the mixture with a camel-hair pencil to the parts required to be blackened.

III.

Saltpetre 2 parts. Common Salt 1 part. Spirits of Salts 1 "

Reduce the salts to powder, and place it in a black-lead crucible along with the acid, boil up, and then dip the articles into the mixture for a short time, or otherwise apply it to the parts required to be oxidized.

These mixtures will give the various tints of oxidation to silver work if properly treated; but if other tints be desired, the following chemical substances may be employed according to taste:--For slate-coloured surface, dip the articles into a boiling solution of sulphuret of potassium. Strong hydrosulphate of ammonia produces a dark tint of oxidation, and if diluted with much water a light tint is produced. Nitric acid produces a light surface. The fumes of sulphur produce a beautiful blue-coloured surface. This operation should be conducted in a closed box, and all parts not to be blackened should be coated with a suitable resist varnish. After any of these processes the articles may either be scratched, or otherwise burnished.