|| JOBS | DRUGS | ANATOMY | DIAMONDS | HEALTH TOPICS | DISEASES | ENERGY | GEOLOGY | AIRPORTS | COUNTRIES | FLAGS ||
PENCIL (Lat. penicillus, brush, literally little tail), a name originally applied to a small fine-pointed brush used in painting, and still employed to denote the finer camel's-hair and sable brushes used by artists, but now commonly signifying solid cones or rods of various materials used for writing and drawing. It has been asserted that a manuscript of Theophilus, attributed to the 13th century, shows signs of having been ruled with a black-lead pencil; but the first distinct allusion occurs in the treatise on fossils by Conrad Gesner of Zurich (1565), who describes an article for writing formed of wood and a piece of lead, or, as he believed, an artificial composition called by some stimmi anglicanum (English antimony). The famous Borrowdale mine in Cumberland having been discovered about that time, it is probable that we have here the first allusion to that great find of graphite. While the supply of the Cumberland mine lasted, the material for English pencils consisted simply of the native graphite as taken from the mine. The pieces were sawn into thin sheets, which again were cut into the slender square rods forming the "lead" of the pencil.
Strenuous efforts were made on the continent of Europe and in England to enable manufacturers to become independent of the product of the Cumberland mine. In Nuremberg, where the great pencil factory of the Faber family (q.v.) was established in 1760, pencils were made from pulverized graphite cemented into solid blocks by means of gums, resins, glue, sulphur and other such substances, but none of these preparations yielded useful pencils. In the year 1795 N. J. Conte, of Paris, devised the process by which now all black-lead pencils, and indeed pencils of all sorts, are manufactured. In 1843 William Brockedon patented a process for compressing pure black-lead powder into solid compact blocks by which he was enabled to use the dust, fragments, and cuttings of fine Cumberland lead. Brockedon's process would have proved successful but the exhaustion of the Borrowdale supplies and the excellence of Conte's process rendered it more of scientific interest than of commercial value.
The pencil leads prepared by the Conte process consist of a mixture of graphite and clay. The graphite, having been pulverized and subjected to any necessary purifying processes, is "floated" through a series of settling tanks, in each of which the comparatively heavy particles sink, and only the still finer particles are carried over. That which sinks in the last of the series is in a condition of extremely fine division, and is used for pencils of the highest quality. The clay, which must be free from sand and iron, is treated in the same manner. Clay and graphite so prepared are mixed together in varying proportions with water to a paste, passed repeatedly through a grinding mill, then placed in bags and squeezed in a hydraulic press till they have the consistency of stiff dough, in which condition they are ready for forming pencil rods. For this purpose the plastic mass is placed in a strong upright cylinder, from which a plunger or piston, moved by a screw, forces it out through a perforated base-plate in a continuous thread. This thread is finally divided into suitable lengths, which are heated in a closed crucible for some hours. The two factors which determine the comparative hardness and blackness of pencils are the proportions of graphite and clay in the leads and the heat to which they are raised in the crucible. According as the proportion of graphite is greater and the heat lower the pencil is softer and of deeper black streak.
The wood in which the leads are cased is pencil cedar from Juniperus virginiana for the best qualities, and pine for the cheaper ones. A board of the selected wood, having a thickness about equal to half the diameter of the finished pencil and as wide as four or six pencils, is passed through a machine which smooths the surface and cuts round or square grooves to receive the leads. The leads being placed in the grooves the board is covered with another similarly grooved board, and the two are fastened together with glue. When dry they are taken to rapidly revolving cutters which remove the wood between the leads. The individual pencils thus formed only need to be finished by being dyed and varnished and stamped with name, grade, &c. Instead of wood, paper has been tried for the casings, rolled on in narrow strips which are torn off to expose fresh lead as the point becomes worn down by use. Black pencils of an inferior quality are made from the dust of graphite melted up with sulphur and run into moulds. Such, with a little tallow added to give them softness, are the pencils commonly used by carpenters. Coloured pencils consist of a mixture of clay, with appropriate mineral colouring matter, wax, and tallow, treated by the Conte method, as in making lead pencils. In indelible and copying pencils the colouring matter is an aniline preparation mixed with clay and gum. The mixture not only makes a streak which adheres to the paper, but, when the writing is moistened with water, it dissolves and assumes the appearance and properties of an ink.
- Please bookmark this page (add it to your favorites).
- If you wish to link to this page, you can do so by referring to the URL address below this line.
Copyright © 1995-2011 ITA all rights reserved.
A * B * C * D * E * F * G *H * I * J * K * L * M * N * O * P * Q * R * S * T * U * V * W * X * Y * Z