SHELLS. - On reference to vol. 1, pages 118 - 120, a few remarks on the descrip tive characters of the porcelanous and nacreous shells will be found. Some of these shells are cut through to show their internal sections or structures whilst others are simply polished exteriorly in their entire states, as speci mens of natural history, or for their intrinsic beauty, some few of the shells are cut up in the manufacture of various useful and ornamental works They are usually treated as follows: - 1. - Porcelanous Shells, which are generally univalve or single shells, such as the whelks, limpets and cowries, so far resemble porcelain or enamel as not to admit of being otherwise cut than with the apparatus employed by the lapidary; and accordingly, when porcelanous shells are divided to exhibit their sections, it is effected by the Slicer, with Diamond Powder.

The porcelanous shells do not in general require the coarser or grinding tools, as few of them present the rough coat or epidermis of the nacreous shells, and it is therefore only commonly needful to restore or increase their natural polish with the list or brush wheel of the lapidary. Putty powder may be used, but rottenstone, from its greater hardness, is more effective on porcelanous shells: of course, similar wheels running in a vertical plane, such as those of the cutler and workers in horn and ivory, may be also used with equally good effect. 2. - Nacreous Shells, which are generally bivalve shells, such as those of the various oysters, muscles, etc, are thus named from nacre, the French for mother-of-pearl, the covering of the ostrea margaritifera of the Indian seas. The nacreous shells are much softer than the porcelanous, and may be sawn, filed and turned with moderate facility, but from the quantity of lime they contain they feel harsh and scratchy under the tools.

The pearl shell is much employed in the ornamental art, and the usual course for its preparation into square, angular and circular plates, and cylindrical pieces, is first, with saws of different and ordinary kinds; the pieces are then roughly shaped on the edge of a grindstone turned into grooves, and afterwards smoothed on the flat side of the stone: many use soap and water with the stone, which lessens its liability to become clogged. See also vol. 1, pages 119, 120. 3. - Pearl Shell in Detached Pieces, such as counters, silk winders, etc, immediately after having been ground, and when shaped on their edges, are smoothed with Trent sand or pumice-stone and water, on a buff wheel or hand polisher, and are finished with rottenstone.

The latter powder, although sometimes used with oil or water, is more frequently moistened with a little sulphuric acid, nearly or quite undiluted, this produces a far more brilliant polish, which may possibly arise from the partial destruction of the surface, thus developing in a more decided manner the striated formation of the pearl shell, and to which peculiarity of structure its variegated lustre is ascribed. 4. - Pearl Works Combined as in Boxes are most generally reduced to a flat surface by filing and scraping. Mr. Vanham says that first pumice-stone and then putty powder are used on buff sticks with water, and the final polish is given with a buff stick and rottenstone moistened with sulphuric acid, this mode is available for inlaid works with gold or silver, but not for those having tortoiseshell or other substances that would be attacked by the acid. The buff stick is expeditious, but for very flat surfaces, a flat deal stick covered with one layer of linen rag is preferable although slower. 5. - Turned Works in general only require fine emery paper, and then rottenstone on woollen rag with sulphuric acid, but oil may be used instead of the latter.

6. - Pearl Handles for Razors. - The Sheffield manufacturers slightly rivet the handles together in pairs, after which they are 1st scraped, 2ndly "sand buffed " on the wheel with Trent sand and water, 3dly, "gloss buffed " on the wheel with rottenstone and oil, or sometimes with dry chalk rubbed on the same wheel, and 4thly they are "handed up" or polished with dry rottenstone and the naked hand.

7. - Pearl Shell, when polished by the lapidary, is treated in the mode followed with Alabaster. See article 3.

8. - Shell Cameos. - A very suitable material for cameos is found in the various conch shells or Strombs, the substance of which consists of two distinct layers of different colours, textures and hardness, and which may be considered respectively to partake of the nature of nacreous and porcelanous shells, the chemical compositions of which were noticed in vol. 1, page 118. The outer coat or layer in the most suitable specimens of conch shells is nearly colourless, of uniform texture, and like that on the nacreous shells admits of being readily operated upon by steel cutting tools, and which may be made to produce a smooth and well-finished surface, this outer layer is therefore suited for the carved parts of cameos, the ground being formed of the under layer of the shell, which in the most suitable kinds is of a dark colour, and allied to the porcelanous shells, being somewhat brittle and so hard and compact as not to admit of being readily cut with steel tools.

The best kind of conch shell for carving into cameos is found on the Southern coast of America, and also on the coast of the West India Islands, and commonly known as the "Hack conch; " in these shells the contrast of colour is the most decided, the under layer being very dark or nearly black, especially in the old or full grown shells, which are the hardest and most compact, and also possess the greatest amount of the white or outer layer, the part to be carved. In the pink conch shell the contrast of colour is not so great, and as it does not at all resemble the onyx in which antique cameos were cut, it is but little used for the best works; nevertheless, some very beautiful specimens of carving on the pink conch shell are to be met with, and the delicacy of the colours gives a very pleasing effect.

The most suitable shell having been selected it is cut into pieces of the required forms for the cameos; this process, which must be cautiously performed, is best effected by means of the slitting mill fed with diamond powder, described in the chapter on Lapidary Work, but the cutting may be also effected with a blade of iron or steel, such as a thin table knife blade notched to form teeth, and fed with emery and water, a process similar to that by which the stone mason cuts slabs of freestone and marble with a smooth blade of iron fed with sand and water.

The piece of shell having been cut out is next carefully ground to the general form of the cameo, as square, lozenge, elliptical or other shape, upon an ordinary grindstone, the face and back of the shell being also levelled and reduced to the appropriate thickness. A slip of Turkey oilstone may be used with advantage to give the last finish to the edges of the shell after the upper white layer has been removed from it, for when the shell has lost the support of the white layer, it will be found that the coarse cut of the grindstone will fill it with minute cracks, which frequently spread over the surface after the cameo has been some time finished.

Having prepared a piece of shell of the desired form and thickness it is next cemented on a block of wood about 3 inches diameter, or of a convenient size to be grasped firmly in the hand; care should be taken to place the piece of shell level and near the center of the block, in order that all parts of the cameo may be operated upon with equal facility. Now sketch with a pencil the contour of the subject to be carved, and follow this pencil-mark with a scratch point; having removed the surrounding white substance by means of files and gravers proceed to develope the figure by the use of smaller tools. A very convenient form of carving tool for this purpose may be made of pieces of steel wire about 6 or 8 niches long, flattened at the ends and hardened; they are lastly ground to an angle of about 45 degrees, and carefully sharpened on an oilstone. The largest tools may be made of wire about 1/8 of an inch diameter; smaller wire will serve for tools of a medium size, but for the smallest tools an ordinary darning needle left quite hard, and ground to the same angle, when inserted in a wooden handle, will be found very useful in deepening the finer lines. The advantage of this form of tool consists in the absence of any angles that would be liable to scratch the work, and a tool thus formed admits of being used either as a gouge, or as a chisel, according as the flat or round side is brought to act on the work.

To guide the tool in the act of cutting, the left hand should grasp the block upon which the cameo is cemented, the thumb being placed close to the cameo; the tool held in the right hand should be so rested against the thumb of the left hand as to form a fulcrum, upon which the tool may be moved as a lever in short arcs of the circle, with a scraping action which removes the material as a powder, care being taken that every cut is made obliquely downwards towards the black ground; should any of the cuts be made towards the surface, or even parallel therewith, there would be danger that small pieces would be chipped off, and which would be destructive to the cameo.

As in all other processes of producing form by reduction, the general shape should be first wrought with care to leave every projection rather in excess, to be gradually reduced as the details and finish of the work are approached. To render the high parts more distinct during the process of carving, it will be found convenient to mark them slightly with a black lead pencil. Throughout the cutting great caution should be observed, that in removing the white thickness, the dark ground is not damaged, as the natural surface of the dark layer is far superior to any that can be given artificially; indeed, should the ground be broken up at one part, it would be requisite from its lamellar structure to remove the entire scale or lamina from the whole surface, a process that will be found very tedious, and much more difficult than the separation of the white from the black thickness.

In order that the finished cameo may possess a distinct outline at all points of view, it is desirable to adopt the system followed in antique cameos, namely, to leave all the edges of the figure quite square from the ground, and not gradually rounded down to the dark surface; should this latter method be followed, it will be found that the outline is in many places undefined, owing to the colour of the white raised figure of the cameo gradually merging into that of the dark ground; this evil is entirely avoided by leaving the extreme edge of the figure quite square, for about the thickness of one-fiftieth of an inch.

The surface of the cameo should be finished as nearly as possible with the cutting tools, as all polishing with abrasive powders is liable to remove the sharp angles of the figures, and deteriorate the cameo by leaving the form undefined. When, however, the work has been finished as smooth as possible with the cutting tools, the final polish may be given with a little putty powder used dry, upon a moderately stiff tooth brush, applied with care, and rather to the dark ground than to the carved surface: this is the concluding process; after which the cameo is ready for removal from the block prior to mounting.