Soft Solders
A solder should melt at a slightly lower temperature than the metals
which it unites, and should possess the quality of alloying with the two
surfaces, thus effecting a sound and true metallic joint. Ordinary soft
solders are lead-tin alloys, and the larger the proportion of lead the
commoner is the solder said to be. At an extreme is plumber’s solder,
consisting of 2 parts of lead to 1 part of tin, and, at the other, the
best blowpipe soft solder, which contains 2 parts of tin to only 1 part
of lead. In the ordinary way, a “coarse” or “common” solder is 2 parts
of lead to 1 part of tin; a “fine” or “medium” solder, 1 part of lead to
1 part of tin; and a “very fine” or “best” solder, 1 part of lead to 2
parts of tin.
=Eutectic Alloys.=--Lead-tin solders are eutectic alloys--that is, they
are examples of the phenomenon of a combination of two metals melting at
a temperature lower than one of them would if melted separately. Thus,
lead melts at about 328° C., and tin at about 232° C., yet reference to
the following table, given by Mr. A. H. Hiorns, will show that the
“commonest” solder mentioned fuses at 303° C., and the “best” at 175°
C.
_Melting points of lead-tin alloys_
_Tin %_ _Lead %_ _Melting_
_point (C.)_
10 90 303°
20 80 278°
30 70 255°
40 60 230°
50 50 205°
60 40 187°
63 37 175°
70 30 185°
80 20 198°
90 10 215°
=Hardness of Solders.=--According to the before-mentioned authority,
Saposhniko, in 1908, determined the hardness of various lead-tin alloys
by Brineli’s method, by which a steel cone is forced into the metal. The
results he obtained are as follow:
Lead 100 90 80 70 60 50 40
Tin 0 10 20 30 40 50 60
Hardness 3·9 10·1 12·16 14·5 15·8 15·0 14·6
Lead 34 33 32 30 20 10 0
Tin 66 67 68 70 80 90 100
Hardness 16·7 15·4 14·6 15·8 15·2 13·3 4·1
These results, says Mr. Hiorns, show that the hardest alloy is the one
with 66% (about 2 parts) of tin and 34% (about 1 part) of lead, which
also is the one having the lowest melting point of all the lead-tin
alloys. The results also show that tin is slightly harder than lead.
=Compositions of Soft Solders.=--As already shown, solders vary in
fusibility according to their composition, and the choice should be
determined by the nature of the work and the properties of the metal to
be soldered. Should a solder be used of too high a melting-point, the
metal will itself be fused before the solder begins to flow.
A point to be particularly observed is that the introduction of a
foreign substance into the solder--for example, the addition of a little
zinc to a pot of “very fine” solder--will utterly spoil it and render it
unworkable. To remove zinc from solder, melt the solder in a pot, take
it off the fire and stir in powdered sulphur or brimstone until the
whole is of the consistency of wet sand. Replace the pot on the fire and
melt, but do not stir the contents. The sulphur and zinc will rise to
the surface and form into a cake. Now take the pot off the fire and
carefully remove the cake without breaking by employing two pieces of
hoop iron with bent ends.
It is false economy to use a rough solder for fine work on the score of
cheapness, since more solder is required for a given job on account of
the rough particles of solder clinging to the work; moreover, the rough
appearance of the soldering may completely spoil the job.
The table on the opposite page gives the fluxes and the compositions of
soft solders suited to a number of different metals.
=Making Solder Strips, Wire, Tears, etc.=--Only clean, pure tin and pure
lead should be employed. The lead is first melted and then the tin
added. When all is melted, place a piece of resin on the molten metal
to act as a flux, and after well stirring, the solder is made into
strips by pouring from a ladle. Solder should not be poured into sand.
It may be poured into strips on an oiled sheet of black iron, preferably
corrugated to accommodate the strips. In the absence of a corrugated
iron sheet, some workers use a ladle resembling a large spoon with a
hole about 1/16 in. in diameter near the end. To form the strips, get a
ladle full of solder, place it on a flat iron sheet; then, tilting the
ladle to allow the solder to flow over the hole, quickly draw the ladle
across the sheet. A thin strip of solder should thus be formed, and the
thickness of the strip may be varied by increasing or decreasing the
diameter of the hole in the ladle. A button of solder usually forms at
one or both ends of the strip, and this excess should be melted off the
strips by just dipping the ends into the molten solder in the pot.
SOFT SOLDERS FOR VARIOUS METALS
----------------+--------------------------+--------------------------
| | _Soft Solder_
| +------+------+------------
_Metal to be | _Flux_ | | | _Other
soldered_ | |_Tin_ |_Lead_| constit-
| | | | uents_
----------------+--------------------------+------+------+------------
Aluminium | stearin | _see table on p. 59_
Brass { |[1]zinc chloride, resin { | 66 | 34 |
Gunmetal { | or ammonium { | 63 | 37 |
Copper { | chloride { | 60 | 40 |
Lead | tallow or resin | 33 | 67 |
Block tin | zinc chloride | 99 | 1 |
Tinplate | zinc chloride or resin | 64 | 36 |
Galvanised steel| hydrochloric acid | 58 | 42 |
Zinc | hydrochloric acid | 55 | 45 |
Pewter | gallipoli oil | 25 | 25 | bismuth, 50
Iron and steel | ammonium chloride | 50 | 50 |
Britannia metal | tallow or resin | 25 | 25 | bismuth, 50
Gold | zinc chloride | 67 | 33 |
Silver | zinc chloride | 67 | 33 |
Bismuth | zinc chloride | 33 | 33 | bismuth, 34
----------------+--------------------------+------+------+------------
[Footnote 1: Zinc chloride is the ordinary “killed spirits.”]
Solder wire is very handy for small work, and can be made in the
following way: Roll a sheet of stiff writing or drawing paper into a
conical form, rather broad in comparison with its length; make a ring of
stiff wire to hold it in, attaching a suitable handle to the ring. The
point of the cone should first of all be cut off to leave an orifice of
the size required. It should then be filled with molten solder, and held
above a pail of cold water, and the stream of solder flowing from the
cone will solidify as it runs and form the wire. If held a little
higher, so that the stream of solder breaks into drops before striking
the water, it will form handy elongated “tears” of metal; when it is
held still higher, each drop forms a thin concave cup or shell, and each
of these forms will be found to have its own peculiar uses in blowpipe
work.
The method adopted for granulating tinman’s solder, which is very rarely
called for, is as follows: Place a piece of wood, well greased, over a
tub containing water, and by gently pouring the molten alloy from a
distance in a small stream on to the greased board, the metal is broken
up into a large number of very fine shots, which run off the board into
the water and are immediately cooled. The fine shots are then taken
from the water and gently dried.
=Making Solder from Pewter.=--This alloy is composed of variable
proportions of tin and lead, the average composition being about 4 parts
of lead to 1 part of tin. If old pewter is to be utilised for making
solder, tin will have to be added to the molten pewter. Thus, to convert
5 lb. of average pewter to “coarse” or “common” solder, add 1 lb. of
tin; to “fine” or “medium,” add 3 lb. of tin; and to “very fine” or
“best,” add 7 lb. of tin. The respective proportions of lead and tin
will then be 2 and 1; 1 and 1; and 1 and 2. After the proper quantity of
tin has been added, mix some powdered sal-ammoniac with the molten
metals, and well stir the alloy; it is then ready for pouring into the
moulds.
=Making Coarse Solder from Composition Piping.=--Good composition piping
is made of nearly all tin, or an alloy of tin and lead, in which the
former metal is in excess, and formerly was much used by plumbers in the
making of coarse solder, as the material consisted of odd pieces of
small value. As, however, a great deal of composition tubing is made out
of old metals of which lead, tin, antimony, arsenic, and zinc form the
alloy, it is not advisable to introduce it into solder. Should it be
done, the melting point of the solder would be raised, and in applying
it to the lead to be joined together, would probably partly melt it.
Neither do the metals named alloy in a thorough manner, but partake more
of the nature of a mixture in which the constituents partly separate
when making the joints, and some, especially zinc, show as small bright
lumps on the surface. Joints wiped with what is usually called “poisoned
metal” are difficult to make, almost invariably leak when on water
service pipes, and are dirty grey, instead of bright and clean. The zinc
could be removed from the mixture by the method already given.
=Combined Solder and Flux.=--This consisted of equal parts of lead and
tin made into fine tubing and afterwards filled with flux having resin
as a base. “Tinol” is a paste made of finely powdered solder and a
special flux, and there is also “Tinol wire” having a core of flux.
A “magic” solder, sold by hawkers, consists of the above tubular
flux-filled solder of such low melting point that it can be fused in the
flame of a lighted match.
=Soft Solders that Melt in Boiling Water.=--The following soft solders
melt at a temperature lower than that of boiling water: 1 part tin, 1
part lead, and 2 parts bismuth, melting point about 200° F.; 8 parts
lead, 4 parts tin, 15 parts bismuth, and 3 parts cadmium, melting point
140° to 150° F.; 6 parts lead, 7 parts bismuth, and 1 part cadmium,
melting point about 180° F. To ensure the alloys melting at the
temperatures stated, the metals of which they are formed should be free
from impurities, and care should be taken to prevent oxidation while
making the alloys. When melting the metals, that having the highest
melting point should be melted first, with a layer of resin over it, the
other metals being added in the order of their melting points. The alloy
should then be well stirred with a wooden stick, and poured quickly into
moulds.
=Re-melting and Overheating Solder.=--After solder has been re-melted a
number of times or has been overheated, its content of tin will be
reduced, and the solder will become poorer and coarser. The tin melts
earlier than the lead and, being the lighter of the two, floats over it,
and is thus fully exposed to the air, the oxidising effect of which on
heated, molten metal is extremely active. The oxidised tin forms a
dross, from which most of the tin may, however, be recovered by melting
it with powdered charcoal, which combines with the oxygen and frees the
tin. The addition of a little fresh tin is desirable.