Soldering, Brazing and Welding :: CHAPTER VIII Hard-soldering with Silver Solder

CHAPTER VIII Hard-soldering with Silver Solder

Hard-soldering is chiefly of two kinds, brazing and silver-soldering, the former being employed for iron and steel, the solder used being known as “spelter,” a brass alloy which can be obtained in various degrees of fineness. For copper, brass, and nickel silver, alloys containing silver are the best solders. In both forms of hard-soldering, the flux is borax.

The methods of silver-soldering vary with the size of the work. A jeweller may hold the work in his hand, on the end of a piece of binding wire or on a square of charcoal, the heat being applied by a mouth blowlamp from a horizontal gas jet, as already described. Larger work demands a flame of greater intensity, and sufficient air can be supplied only by a footblower or similar device, or, as an alternative, from the flame of a suitable blowlamp.

Silver Solder.

—This can be purchased at prices up to 3s. 6d. or so per ounce in sheet form, about ¹/₃₂ in. thick. Where the solder is melted down by the amateur, a good way to obtain the sheet form is to turn the globule of molten metal on to the bench and to place a flat iron on it; but the result will not be equal to a rolled ingot.

An old shop method of making silver-solder is to melt up old silver (using current silver coinage is an expensive method of obtaining the silver, and is said to be illegal) with some brass pins, not the iron ones so common now.

According to W. H. Jubb, two solders compulsory for silver articles that have to be sent to assay for hallmarking are: (1) 12 parts standard silver and 1 part brass; (2) 6 parts standard silver and 1 part brass. No. 1 has a low melting point, and is termed “quick,” and No. 2, which requires a higher temperature, is called “stark” (in some parts of the country, “fine”), but the use of this term here is misleading. No. 2 should be used at the first heating and No. 1 at the second. These should make ideal solders for beginner’s use, as the chances of burning the work, even thin brass, are almost nil.

(3) 2 parts brass, 1 part standard silver; (4) 5 parts brass, 2 parts standard silver. Whereas Nos. 1 and 2 are “silver solders,” Nos. 3 and 4 are termed “German silver solders,” as they are not so white and are used on German or nickel silver (an alloy of copper and nickel). Both Nos. 3 and 4 are good, have a comparatively low melting point, are much less expensive than Nos. 1 and 2, and, if plenty of wet borax is used, will “strike up” well. No. 3 is recommended.

“Standard silver” is about 95 per cent. pure silver. Old “sterling silver” is 92·5 per cent. pure silver.

In making any of the above solders the brass and silver should be melted together, and care should be taken to see that the metal is clean beforehand. Where large quantities of solder are made, the metals are scoured with emery cloth before they are put into the crucible.

Requisites for Silver-soldering.

—For light jobs in silver-soldering the special tools and materials required are as follow: Suitable gas jet or other flame, mouth blowpipe, scraper, jar containing a sulphuric acid “pickle,” piece of slate, camel-hair brush, pieces of lump borax, two grades of silver solder, charcoal block, iron binding wire.

The pickle is made by pouring 1 part of common sulphuric acid into 20 parts of water, and its function is to remove all dirt and borax from the metal. Silver-soldered articles should not be thrown into the pickle until they are nearly cold, as otherwise the joints may crack, but in the preparatory annealing (that is, softening) of plain metal, wire, rod or tubing, the article may be put into the pickle when hot—but take care of the splashes! Plunging hot copper or brass into cold water does not have the effect of hardening it. Before placing a job in the pickle, remove any iron binding, as this is immediately attacked by the acid.

Often it is advisable to heat the metal and put it “through the pickle” before working on it, especially in the case of tubing that has been lying by for some time and has become dirty.

For heavier work, a foot-bellows and gas blowpipe, or else a blowlamp, are essential. An “Ætna” paraffin blowlamp with horizontal burner will be found quite satisfactory, although, of course, if a gas supply is available in the workshop the user will find that a foot-bellows and blowpipe are more convenient. A blowlamp or blowpipe requires a suitable tray or “forge” of sheet-iron in which coke and odd pieces of brick or tile may be laid and used to pack round the object whilst the flame is being played on the part to be soldered.

Figs. 56 and 57.—Clip for Holding and
Applying Silver Solder

Fig. 58.—Pricker and Spatula for Consolidating Joints and Applying Solder

Among the smaller additional tools that will be required will be a clip to hold the solder (Figs. 56 and 57) and a brass pricker (Fig. 58).

Preparing the Borax Flux.

—Make up the flux on the slate by rubbing on it a piece of the lump borax moistened with water. This paste may be applied with the camel-hair brush to the parts to be jointed. The flux prevents the oxidisation of the surfaces, which would resist the amalgamation of the metals and the solder.

The Method of Silver-soldering.

—In hard-soldering with silver solder, first file or scrape the parts bright, and cover them and the solder with the borax paste. Heat gently at first so as to harden the borax; then continue to heat by blowpipe until a red heat is reached, at which the solder will run. The secret is to blow continuously until the solder runs, and not to stop half-way.

For soldering a silver watch case, an ordinary easy-running silver solder, which melts at a lower heat than silver, will do. But to make sure, shred the solder into very thin strips, and apply plenty of borax to them as well as to the joint to be united. Use the blowpipe gently at first so as to bake the borax, then heat the case all over almost to the melting point of the solder, and direct the flame to the part to be soldered until the solder runs and glistens. Cease blowing instantly, and plunge the case into a solution of sulphuric acid 1 part and water 10 parts, to whiten it; then wash in hot water and dry in sawdust. Be careful to remove all steel springs before soldering the case.

Soldering Small Work with Paillons.

—These are small squares, say, ¹/₈ in., of sheet silver solder, made by using the snips as in Fig. 61, and prepared by well covering with the borax paste. Each paillon is placed in position with the tip of the brush, this job requiring a little practice. The solder should be clean, and if not, should be made so by passing through the fire and pickling. The work with the paillons in position should be slowly heated by blowing the gas jet on a part of the job farthest away from the solder; the borax will dry, and should the solder have moved, replace it with a suitable tool or the point of the wet camel-hair brush. The heat must not be applied too suddenly at first, otherwise the borax will boil up and push off the pieces of solder. The heat may be increased when the bubbling has ceased. Do not hold the work too far away, or it will get dirty in the smoke of the flame, or yet too near, else the gas will not be used to advantage. As the work begins to get hot, slowly work the flame towards the joint until the solder melts and runs into the joint. Give it now a little extra heat to get the solder thoroughly down into the crevices, and then let the work cool down. When nearly cold twist off the iron binding wire and put the job into the pickle. Leave the work in the pickle about ten minutes, when all the borax will be dissolved.

Fig. 59.—Section through
Pipes prepared for Silver-soldering

Fig. 60.—Pipes prepared for
Silver-soldering

Fig. 61.—Cutting Up Silver Solder into Paillons

Larger Work.

—An example of silver-soldering larger work is the joining together of two pipes, one smaller than the other. The best course to adopt is to file or scrape the end of the smaller and the inside of the larger (reaming and filing them if necessary) until a good fitting joint is obtained, as in Figs. 59 and 60. A strip of solder is then cut off, and, after the joint is well coated with borax paste, this solder may be wound round the smaller pipe. If the joint is soldered in a vertical position, the larger pipe should be the lower. The heat should be conserved by laying the work in the coke and building the same round, or, if the work is too large or the joint in an awkward part of the pipe, a shield of tin plate or iron should be placed behind the joint so that the flame is thrown back on to the work. Should the pipe be attached in close proximity to the joint to a heavy piece of metal, then warm this metal up first, otherwise all the heat will travel to this part, and the work will take much longer to get to the proper temperature. In all cases where one part of the joint is of heavier substance than the other, that part should receive the greater amount of attention from the flame.

Quenching Silver-soldered Work.

—Many craftsmen object to the cooling of the work by plunging it into water or pickle whilst it is hot; but no damage or cracking of the joint occurs, it is thought, if the work is not plunged when it is red hot or anywhere near red hot. Plunging into a pickle certainly cracks the burnt-in borax, which can be readily removed and the joint examined to better advantage. Many a silver-soldered joint has been passed as quite sound when it has only been the borax that has been stopping the interstices, and only after it has been placed under service for some time does the faulty joint make itself apparent.

Silver-soldering cannot be done on work that has been previously soft-soldered unless the soft-soldered part is first cut away; but, of course, soft-soldering can be readily accomplished after silver-soldering or brazing so long as the work is clean and all burnt borax is first removed.

Removing Soldering Marks.

—Solder will run away from a part of a joint instead of running into it when the edges of the joint have been imperfectly cleaned preparatory to the application of flux and solder; also, if some dirt has got into the flux, or on the paillons of solder employed. Another cause is unequal heating of the joint, or allowing it to expand too much whilst being heated. The resulting fire marks may be removed by warming the articles on a pan over gas, and plunging them whilst warm into the sulphuric acid pickle. Or the marks may be removed in a hot and strong solution of potassium cyanide, and the polish renewed by a light polishing with a revolving swansdown mop and rouge composition.

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