The inherent cleansing power of soap renders it invaluable
in combating disease, while it also has distinct germicidal properties, a 2 per
cent. solution proving fatal to B. coli communis in less than six hours, and
even a 1 per cent. solution having a marked action on germs in fifteen
Many makers, however, seek more or less successfully to
still further increase the value of soap in this direction by the incorporation
of various drugs and chemicals; and the number of medicated soaps on the market
is now very large. Such soaps may consist of either hard or soft soaps to which
certain medicaments have been added, and can be roughly divided into two
classes, (a) those which contain a specific for
various definite diseases, the intention being that the remedy should be
absorbed by the pores of the skin and thus penetrate the system, and (b)
those impregnated with chemicals intended to act as antiseptics or germicides,
or, generally, as disinfectants.
The preparation of medicinal soaps appears to have been
first taken up in a scientific manner by Unna of Hamburg in 1886, who advocated
the use of soap in preference to plasters as a vehicle for the application of
Theoretically, he considered a soap-stock made entirely
from beef tallow the most suitable for the purpose, but in practice found
that[Pg 87] the best results were obtained by using a
superfatted soap made from a blend of one part of olive oil with eight parts of
beef tallow, saponified with a mixture of two parts of soda to one part of
potash, sufficient fat being employed to leave an excess of 3 or 4 per cent.
unsaponified. Recent researches have shown, however, that even if a superfatted
soap-base is beneficial for the preparation of toilet soaps (a point which is
open to doubt), it is quite inadmissible for the manufacture of germicidal and
disinfectant soaps, the bactericidal efficiency of which is much restricted by
the presence of free fat.
Many of the medicaments added to soaps require special
methods of incorporation therein, as they otherwise react with the soap and
decompose it, forming comparatively inert compounds. This applies particularly
to salts of mercury, such as corrosive sublimate or
mercuric chloride, and biniodide of mercury, both of
which have very considerable germicidal power, and are consequently frequently
added to soaps. If simply mixed with the soap in the mill, reaction very quickly
takes place between the mercury salt and the soap, with formation of the
insoluble mercury compounds of the fatty acids, a change which can be readily
seen to occur in such a soap by the rapid development on keeping, of a dull
slaty-green appearance. Numerous processes have been suggested, and in some
cases patented, to overcome this difficulty. In the case of corrosive sublimate,
Geissler suggested that the soap to which this reagent is to be added should
contain an excess of fatty acids, and would thereby be rendered stable. This
salt has also been incorporated with milled soap in a dry state in conjunction
with ammonio-mercuric chloride, β-naphthol, methyl salicylate, and eucalyptol.
It is claimed that these bodies are present in an unchanged condition, and
become active when the soap is added to water as in washing. Ehrhardt (Eng. Pat.
2,407, 1898) patented a method of making antiseptic mercury soap by using
mercury albuminate—a combination of mercuric chloride and casein, which is
soluble in alkali, and added to the soap in an alkaline solution.
With biniodide of mercury the interaction can be readily
obviated by adding to the biniodide of mercury an equal weight of potassium
iodide. This process, devised and patented by J. Thomson in 1886, has been
worked since that time with extremely satisfactory results. Strengths of 1/2, 1,
and 3 per cent. biniodide are sold, but owing to the readiness with which it is
absorbed by the skin a soap containing more than 1/2 per cent. should only be
used under medical advice.
A similar combination of bromide of
mercury with potassium, sodium, or ammonium bromide has recently been
patented by Cooke for admixture with liquid, hard, or soft soaps.
Zinc and other Metallic
Salts.—At various times salts of metals other than mercury have been added
to soap, but, owing to their insolubility in water, their efficiency as
medicaments is very trifling or nil. Compounds have been formed of metallic
oxides and other salts with oleic said, and mixtures made with vaseline and
lanoline,[Pg 88] and incorporated with soap, but they have
not met with much success.
Another chemical commonly added to soap is Borax.
In view of its alkaline reaction to litmus, turning red litmus blue, this salt
is no doubt generally regarded as alkaline, and, as such, without action on
soap. On the contrary, however, it is an acid salt containing an excess of boric
acid over the soda present, hence when it is added to soap, fatty acids are
necessarily liberated, causing the soap to quickly become rancid. As a remedy
for this it has been proposed to add sufficient alkali to convert the borax into
neutral mono-borate of soda which is then added to the soap. This process is
patented and the name "Kastilis" has been given to the neutral salt. The
incorporation of borax with the addition of gum tragasol forms the subject of
two patents (Eng. Pats. 4,415, 1904; and 25,425, 1905); increased detergent and
lasting properties are claimed for the soap. Another patented process (Eng. Pat.
17,218, 1904) consists of coating the borax with a protective layer of fat or
wax before adding to the soap with the idea that reaction will not take place
until required. Boric acid possesses the defects of
borax in a greater degree, and would, of course, simply form sodium borate with
liberation of fatty acids, so should never be added to a neutral soap.
Salicylic Acid is often
recommended for certain skin diseases, and here again the addition of the acid
to soap under ordinary conditions results in the formation of sodium salicylate
and free fatty acids.
To overcome this a process has recently been patented for
rubbing the acid up with vaseline before addition to soap, but the simplest way
appears to be to add the soda salt of the acid to soap.
Amongst the more common milled medicated toilet soaps may
be mentioned, in addition to the above:--
Birch Tar Soap, containing 5 or
10 per cent. birch tar, which has a characteristic pungent odour and is
recommended as a remedy for eczema and psoriasis.
Carbolic Soap.—A toilet soap
should not contain more than 3 per cent. of pure phenol, for with larger
quantities irritation is likely to be experienced by susceptible skins.
Coal Tar.—These soaps contain,
in addition to carbolic acid and its homologues, naphthalene and other
hydrocarbons derived from coal, naphthol, bases, etc. Various blends of
different fractions of coal tar are used, but the most valuable constituents
from a disinfectant point of view are undoubtedly the phenols, or tar acids,
though in this case as with carbolic and cresylic soaps, the amount of phenols
should not exceed 3 per cent. in a toilet soap. An excess of naphthalene should
also be avoided, since, on account of its strong odour, soaps containing much of
it are unpopular. The odour of coal tar is considerably modified by and blends
well with a perfume containing oils of cassia, lavender, spike, and red
Formaldehyde.—This substance is
one of the most powerful disinfectants known, and it may be readily introduced
into soap without[Pg 89] undergoing any decomposition, by milling
in 2-3 per cent. of formalin, a 40 per cent. aqueous solution of formaldehyde,
which is a gas. White soaps containing this chemical retain their whiteness
New combinations of formaldehyde with other bodies are
constantly being brought forward as disinfectants. Among others the compound
resulting from heating lanoline with formaldehyde has been patented (Eng. Pat.
7,169, 1898), and is recommended as an antiseptic medicament for incorporation
Glycerine.—Nearly all soaps
contain a small quantity of this body which is not separated in the lyes. In
some cases, however, a much larger quantity is desired, up to some 6 or 8 per
cent. To mill this in requires great care, otherwise the soap tends to blister
during compression. The best way is to dry the soap somewhat further than usual,
till it contains say only 9 or 10 per cent. moisture and then mill in the
Ichthyol or Ammonium-Ichthyol-Sulphonate
is prepared by treating with sulphuric acid, and afterwards with ammonia, the
hydrocarbon oil containing sulphur obtained by the dry distillation of the
fossil remains of fish and sea-animals, which form a bituminous mineral deposit
in Germany. This product has been admixed with soap for many years, the quantity
generally used being about 5 per cent.; the resultant soap is possessed of a
characteristic empyreumatic smell, very dark colour, and is recommended for
rosacea and various skin diseases, and also as an anti-rheumatic. Ichthyol has
somewhat changed its character during recent years, being now almost completely
soluble in water, and stronger in odour than formerly.
Iodine.—A soap containing iodine
is sometimes used in scrofulous skin diseases. It should contain some 3 per
cent. iodine, while potassium iodide should also be added to render the iodine
Lysol.—This name is applied to a
soap solution of cresol, "Lysol Soap" being simply another form of coal-tar
soap. The usual strength is 10 per cent. lysol, and constitutes a patented
article (Fr. Pat. 359,061, 1905).
Naphthol.—β-Naphthol, also a
coal-tar derivative, is a good germicide, and, incorporated in soap to the
extent of 3 per cent. together with sulphur, is recommended for scabies, eczema
and many other cutaneous affections.
Sulphur.—Since sulphur is
insoluble in water, its action when used in conjunction with soap can be but
very slow and slight. Sulphur soaps are, however, very commonly sold, and 10 per
cent. is the strength usually advocated, though many so-called sulphur soaps
actually contain very little sulphur. They are said to be efficacious for acne
Sulphur soaps, when dissolved in water, gradually generate
sulphuretted hydrogen, which, although characteristic, makes their use
disagreeable and lessens their popular estimation.[Pg 90]
Terebene.—The addition of this
substance to soap, though imparting a very refreshing and pleasant odour, does
not materially increase the disinfectant value of the soap. A suitable strength
is 5 per cent.
Thymol.—This furnishes a not
unpleasant, and very useful antiseptic soap, recommended especially for the
cleansing of ulcerated wounds and restoring the skin to a healthy state. The
normal strength is 3 per cent. It is preferable to replace part of the thymol
with red thyme oil, the thymene of which imparts a sweeter odour to the soap
than if produced with thymol alone. A suitable blend is 2-1/2 per cent. of
thymol crystals and 1-1/2 per cent. of a good red thyme oil.
Of the vast number of less known proposed additions to
toilet soaps, mention may be made in passing of:--
Fluorides.—These have been
somewhat popular during recent years for the disinfection of breweries, etc.,
and also used to some extent as food preservatives. Of course only neutral
fluorides are available for use in soap, acid fluorides and soap being obviously
incompatible. In the authors' experience, however, sodium fluoride appears to
have little value as a germicide when added to soap, such soaps being found to
rapidly become rancid and change colour.
Albumen.—The use of albumen—egg,
milk, and vegetable—in soap has been persistently advocated in this country
during the past few years. The claims attributed to albumen are, that it
neutralises free alkali, causes the soap to yield a more copious lather, and
helps to bind it more closely, and a further inducement held out is that it
allows more water to be left in the soap without affecting its firmness.
Experiments made by the authors did not appear to justify any enthusiasm on the
subject, and the use of albumen for soap-making in this country appears to be
very slight, however popular it may be on the Continent. Numerous other
substances have been proposed for addition to soaps, including yeast, tar from
peat (sphagnol), Swedish wood tar, permanganate of potash, perborates and
percarbonates of soda and ammonia, chlorine compounds, but none of these has at
present come much into favour, and some had only ephemeral existence. Of the
many drugs that it has been suggested to admix in soap for use in allaying an
irritable condition of the skin, the majority are obviously better applied in
the form of ointments, and we need not consider them further.
Ether Soap.—Another form of
medicated soap made by a few firms is a liquid ether soap containing mercuric
iodide, and intended for surgeons' use. This, as a rule, consists of a soap made
from olive oil and potash, dissolved in alcohol and mixed with ether, the
mercuric iodide being dissolved in a few drops of water containing an equal
weight of potassium iodide, and this solution added to the alcohol-ether
Floating Soaps.—Attempts have
been made to produce tablets of soap that will float upon the surface of water,
by inserting cork, or floats, or a metallic plate in such a manner that there is
an air space between the metal and the soap. The more usual method is to
incorporate[Pg 91] into hot soap sufficient air, by means of
a specially designed self-contained jacketed crutcher, in which two shafts
carrying small blades or paddles rotate in opposite directions, to reduce the
density of the soap below that of water and so enable the compressed tablet to
float. The difference in weight of a tablet of the same size before and after
aerating amounts to 10 per cent.
Ordinary milling soap is used as a basis for this soap;
the settled soap direct from the copper at 170° F. (77° C.) is carefully
neutralised with bicarbonate of sodium, oleic or stearic acids, or
boro-glyceride, perfumed and aerated.
Floating soap, which is usually white (some are of a cream
tint), cannot be recommended as economical, whilst its deficiency in lathering
properties, owing to occluded air, is a serious drawback to its popularity as a
Shaving Soaps.—The first
essential of a shaving soap, apart from its freedom from caustic alkali or any
substance exerting an irritating effect upon the skin, is the quick production
of a profuse creamy lather which is lasting. Gum tragacanth is used in some
cases to give lasting power or durability, but is not necessary, as this
property is readily attained by the use of a suitable proportion of potash soap.
The best shaving soaps are mixtures of various proportions of neutral soda and
potash soaps, produced by the combination of ordinary milling base with a white
potash soap, either melted or milled together. Glycerine is sometimes added, and
is more satisfactorily milled in.
Every precaution should be taken to ensure thorough
saponification of the soaps intended for blending in shaving soap, otherwise
there will be a tendency to become discoloured and develop rancidity with age.
Shaving soaps are delicately perfumed, and are placed on the market either in
the form of sticks which are cut from the bar of soap as it leaves the
compressor, or stamped in flat cakes.
Shaving creams and pastes are of the same nature as
shaving soaps, but usually contain a larger proportion of superfatting material
and considerably more water.