A class of printing stencil is made by the mechanical perforation of suitable paper or tissue. Stencils perforated by a rapidly rising and falling needle-point, actuated by a treadle, have long been used for the printing of embroidery patterns. In such a case, powdered colour, mixed with resin, is dusted through the stencil, after which the device is fixed by the application of sufficient heat to soften the resin. Edison proposes to use such perforated stencils for ordinary autographic printing purposes, and replaces the complex treadle perforating machine by a kind of pen, in which a needle-point is made to move rapidly up and down by means of a small electric motor attached to the instrument. When Edison's electric pen is connected with a battery of two elements, the needle rapidly passes in and out of the perforated point of the instrument. If written with on a piece of blank paper, the paper becomes perforated. The sheet of ink-proof paper having been written on with the electric pen, can be used as a printing stencil by merely laying it down on a sheet of white paper and passing an inking-roller over its back. The operation of printing is very rapid, so that many copies can be produced in a short time.
Other perforating pens have followed in the wake of Edison's electric instrument, among which may be mentioned the "horogiapn," a very convenient and portable clockwork pen, manufactured by Newton, Wilson & Co., of Cheapside. A pneumatic pen, in which the motive power is a stream of air supplied from a foot-bellows, has also been introduced into the market. A still more complex and expensive arrangement than either of the preceding, for producing perforated stencils, consists of an induction coil capable of giving a sufficiently powerful spark to perforate the stencil-paper; and this spark is made to continually pass between a partially insulated metallic pen and a metallic plate on which the stencil paper is laid.
(2) All these perforating arrangements have the disadvantages of being expensive, complex in construction, and liable to get out of order when used by unskilled persons, while the perpendicular position in which the mechanical perforating pens must be held, necessarily hampers the freedom of the writer. In a new perforating method recently introduced by Zuccato, the impervious stencil-paper is laid on a hardened steel plate, cut on the face like a fine file, and the writing is executed by means of a point or style of hardened steel. Under these circumstances, the teeth of the filelike plate perforate the paper wherever the point of the style exerts pressure, and a stencil eminently adapted for printing from is the result. This kind of printing is called "trypograph." A sheet of the prepared paper is laid on the file-like plate and written upon with the hardened steel pencil, the operation of writing being as easy as if a pencil were employed. By fixing the stencil on the frame of a desk-like press, placing a sheet of white paper underneath, and then pushing over the upper surface of the stencil an indiarubber scraper or squeegee charged with printing-ink, the ink passing through the perforation produces a copy of the original writing.
As many as 6000 copies can be obtained from one stencil. Thin metallic plates are readily perforated by Zuccato's method, and calico receives the typographic impression admirably.
A correspondent of the Moniteur Industriel refers to the difficulties encountered in tracing upon cloth or calico, especially the difficulty of making it take the ink. In the first place, the tracing should be made in a warm room, or the cloth will expand and become flabby. The excess of glaze may be removed by rubbing the surface with a chamois leather, on which a little powdered chalk has been strewn; but this practice possesses the. disadvantage of thickening the ink, besides, it might be added, of making scratches which detract from the effect of the tracing. The use of ox-gall, which makes the ink " take," has also the disadvantage of frequently making it "run," while it also changes the tint of the colours. The following is the process recommended: Ox - gall is filtered through a filter paper arranged over a funnel, boiled, and strained through fine linen, which arrests the scum and other impurities. It is then placed again on the fire, and powdered chalk is added. When the effervescence ceases, the mixture is again filtered, affording a bright colourless liquid, if the operation has been carefully performed. A drop or two must be mixed with the Indian ink; and it also has the property of effacing lead-pencil marks.
When the cloth tracings have to be heliographed, raw sienna is also added to the ink, as this colour unites with it most intimately, besides intercepting the greatest amount of light.
(4) Tracing Cloth
Varnish the cloth with Canada balsam dissolved in turpentine, to which may be added a few drops of castor oil, but do not add too much, or it will not dry. Try a little piece first with a small quantity of varnish. The kind of cloth to use is fine linen; don't let the varnish be too thick.
(5) Letterpress or illustrations printed in printers' ink may be copied by simply wetting a piece of stiff paper or card and rubbing it over with an agate burnisher or old toothbrush. If the ink has got dry through age or being kept in a hot room, moisten with spirits of wine or toilet vinegar. Have a soft blotting-pad beneath.
(1) Pencils made to produce marks from which copies can be obtained in an ordinary copying-press, have usually the disadvantage that, consisting mainly of aniline, the colour of the copy fades very soon. Gustav Schwanhauser overcomes this difficulty by doing away with aniline altogether. He prepares the pencils as follows: - 10 lb. of the best logwood are boiled repeatedly with 100 lb. of water, and the decoction so obtained is evaporated down to 100 lb. The liquid is heated to the boiling point, and small quantities of the nitrate of oxide of chromium added, till the bronze-coloured precipitate formed at first is redissolved in a deep dark-blue colour. The liquid is now evaporated to the consistency of a syrup, and the finest levigated fat clay is added in the proportion of 1 part of clay for every 3 or 3 1/2 parts of the extract. To form a good mass to manipulate, a little mucilage of gum traga-canth may be used. The' quantity of nitrate of chromium must be in the right proportion to the extract, as a surplus prevents an easy writing, and a deficiency prevents the easy solubility of the mass for copying purposes. No other salt of chromium will answer the purpose, as they all crysta lize, and the crystals formed in the mass will cause the pencil to be rough and brittle.
Nitrate of chromium does not crystallize; its combination with the extract of logwood is the most easily soluble and the blackest ink. The nitrate is prepared as follows: - 20 lb. of chrome-alum are dissolved in 200 lb. of boiling water. To the solution is gradually added a solution of carbonate of sodium of the same strength, till all the hydrated oxide of chromium has been precipitated. After subsidence of the precipitate, the supernatant liquid is decanted, and the precipitate is washed with distilled water, till the filtrate does not contain any more traces of sulphate of potassium and sodium, as may be shown by the addition of a little solution of chloride of barium. To the precipitate collected on the filter are successively added small portions of heated pure nitric acid, previously diluted by its own volume of distilled water, in such quantity that on boiling, a small quantity of the hydrated oxide remains undissolved. In this way a perfectly saturated solution of nitrated oxide of chromium is obtained, containing no excess of nitric acid. This is a great advantage, since an addition of nitric acid to the ink changes its colour to a muddy red.
Another advantage is that no basic nitrate is formed, and no excess of hydrated oxide is contained in the produced salt, as is the case in most other salts of chromium. Such basic salts form an insoluble compound with the extract of logwood, instead of entering into solution. The writing furnished by these pencils is easily transferable; it is of a penetrating jet-black colour. Alkalies and acids have no effect on the ink. (Schweizerisches Gewerbeblatt.)
(2) Faber's pencil for copying writing or designs id made of different degrees of hardness, and is stated by the inventor to combine all the advantages of the very best lead pencils. Four kinds are manufactured. No. 1, very soft; composed of 50 parts of aniline, 37.5 graphite, and 12.5 kaolin. No. 2, soft; 46 parts aniline, 34 graphite, 24 kaolin. No. 3, hard; 30 parts aniline, 30 graphite, 40 kaolin. No. 4, very hard; 25 parts aniline, 25 graphite, 50 kaolin. These materials are pounded and mixed with the greatest care, and afterwards made into a paste with cold water. After the paste has been well worked and rendered perfectly homogeneous, it is passed through a wire screen, which divides it into strips of suitable dimensions. These are dried in an ordinary room, and afterwards fitted and glued into wooden cases like common lead pencils. The new pencils may be used like ordinary copying pencils for the reproduction of writing or designs. A sheet of thin paper wetted is laid over the sheet to be copied, and the details are gone over with the copying pencil.
The action of the moisture on the aniline in the pencil gives a deep tint to the tracing, resembling that of ordinary writing-ink.
(3) Jacobson of Berlin has a copying pencil which is said to be composed of graphite and some aniline colour, mixed by a peculiar process. On any ordinary paper, dry, the pencil gives a sharp, well defined mark, which cannot be obliterated with indiarubber without damaging the paper, and when wetted or left to absorb sufficient moisture from the air, assumes the appearance of ink. To well-damped sheets laid over, a very slight pressure will transfer a succession of clearly defined impressions, which never blur, and have all the appearance of ink. If the original has been wetted some days before, the transfer is not made so easily. It is then found best to soak the transfer sheets in vinegar in place of water. The pencil may be used on oiled paper.