“I first discovered it in the parish of Germo, in a hill called Tregonnin Hill; the whole country in depth is of this stone. It reaches, east and west, from Breag to Germo, and north and south, from Tregonnin Hill to the sea. From the cliffs some of this stone hath been brought to Plymouth, where it was used in the casemates of the garrison; but I think the best quarries are in Tregonnin Hill. The stone is compounded of small pellucid gravel, and a whitish matter, which, indeed, is Caulin petrified; and as the Caulin of Tregonnin Hill hath abundance of mica in it, this stone hath them also. If the stone is taken a fathom or two from the surface, where the rock is quite solid, it is stained with abundance of greenish spots, which are very apparent when it is wetted. This is a circumstance noted by the Jesuits, who observe that the stones which have the most of this quality are the most proper for the preparation of the glaze; and I believe this remark is just, as I know that they are the most easily vitrifiable, and that a vein of this kind in Tregonnin Hill is so much so that it makes an excellent glaze without the addition of vitrescent ingredients. If a small crucible is filled up with this stone, or a piece of it put in it, and exposed to the most violent fire of a good wind furnace for an hour, the stone will be melted into a beautiful mass; all its impurities will be discharged, one part of it will be almost of a limpid transparency, and the other appear in spots as white as snow. The former is the gravel, the other the Caulin, reduced by fire to purity. If the fire is not continued long enough to effect this, the upper part and middle of the mass will be of a dirty colour, and the bottom and parts of the sides fine.
“CAULIN.
“This material, in the Chinese way of speaking, constitutes the bones, as the Petunse does the flesh, of china ware. It is a white talcy earth, found in our granite countries, both in the counties of Devon and Cornwall. It lies in different depths beneath the surface. Sometimes there shall be a fathom or more of earth above it, and at other times two or three feet. It is found on the sides of hills, and in valleys; in the sides, where, following the course of the hills, the surface sinks, or is concave, and seldom, I believe, or never where it swells, or is convex. By what I have observed, it is by no means a regular stratum, but is rather in bunches or heaps, the regular continuance of which is frequently interrupted by gravel and other matters. At times there are veins of it among the solid rocks, when it is constantly very pure from gravel. I have a piece by me of this kind, very fine.
“There are inexhaustible stores of this Caulin in the two western counties. The use it’s commonly put to is in mending the tin furnaces and the fire-places of the fire-engines, for which ’tis very proper. The sort I have chiefly tried is what is got from the side of Tregonnin Hill, where there are several pits of it. As the stone hath a pretty large quantity of Caulin in it, so the Caulin hath a large mixture of the same sort of gravel as enters into the composition of the stone. It contains, besides, mica in abundance.
“In order to prepare the Caulin for porcelain, nothing more is necessary but pouring a large quantity of water on it, so that it may not, when dissolved, be of so thick a consistence as to suspend the mica. Let it settle about ten minutes, and pour off the dissolved clay into another vessel. Let it settle, pour off the water, and dry it. I will observe here, that care ought to be taken about the water used in washing off both the Petunse and Caulin. It ought to be pure, without any metallic or calcareous mixture. Our rivers in the west afford excellent water for this purpose, as they arise, the most of them, and run through a granite country. The Caulin of Tregonnin Hill is very unvitrifiable, and exceedingly apt to take stains from the fire. I know no way to burn it clean but the following:—Form it into cakes of the thickness of two or three crown pieces, and beat some of the stone to a very coarse powder; cover the bottom of the crucible with this powder; then put in a cake of the Caulin; cover this the thickness of one-third of an inch with the powder of stone; fill the crucible in this way, ending with a layer of the stone; cover the crucible, and treat it as in the process for melting the stone before described. If the stone is burned to purity, the Caulin will be as white as snow; if but partially calcined, so far as the stone is pure, the Caulin will be so; and when that is of a dirty colour, the Caulin will be of the same hue.
“I have lately discovered that, in the neighbourhood of the parish of St. Stephen’s, in Cornwall, there are immense quantities both of the Petunse stone and the Caulin, and which, I believe, may be more commodiously and advantageously wrought than those of Tregonnin Hill, as, by the experiments I have made on them, they produce a much whiter body, and do not shrink so much, by far, in baking, nor take stains so readily from the fire. Tregonnin Hill is about a mile from Godolphin House, between Helston and Penzance. St. Stephen’s lies between Truro, St. Austel, and St. Columb; and the parish of Dennis, the next to St. Stephen’s, I believe, hath both the ingredients in plenty in it. I know of two quarries of the stone—one is just above St. Stephen’s, the other is called Caluggus, somewhat more than a mile from it, and appears to be the finer stone.
“Having given this sketch of the natural history of the materials, ’tis needless to say much about the composition. Pottery being at present in great perfection in England, our potters’-mills prepare the Petunse much better than stamping mills, and excuse one from the trouble of washing it off, it being fit to be used as it comes from the mill. I would further observe that the mills should be made of the Petunse granite, it being obvious that, in grinding, some of the mill-stones must wear off and mix with the Petunse. If those stones should be of a nature disagreeable to the body, this mixture must, in some degree, be hurtful to it; whereas, whatever wears off from mill-stones of the same stone, cannot be so in the least degree. I have generally mixed about equal parts of the washed Caulin and Petunse for the composition of the body, which, when burnt, is very white, and sufficiently transparent. The Caulin of St. Stephen’s burns to a degree of transparency without the addition of Petunse. The materials from this place make a body much whiter than the Asiatic, and, I think, full as white as the ancient chinaware, or that of Dresden.
“The stones I have hitherto used for glazing are those with the green spots of Tregonnin Hill. These, barely ground fine, make a good glaze. If ’tis wanted softer, vitrescent materials must be added. The best I have tried are those said to be used by the Chinese, viz., lime and fern-ashes, prepared as follows:—The lime is to be slacked by water, and sifted. One part of this, by measure, is to be mixed with twice its quantity of fern-ashes, and calcined together in an iron pot, the fire to be raised till the matter is red hot. It should not melt, and for that reason should be kept continually stirred. When it sinks in the pot, and grows of a light ash colour, ’tis done. It then must be levigated in the potter’s mill to perfect smoothness. It may be used in proportion of one part to ten, and so on to fifteen or twenty of the stone, as shall be found necessary. We found one to fifteen of the stone a very suitable proportion. Our manner of mixing was to dilute both the stones and the ashes to a proper degree for dipping, and then to mix them as above. On mixing, the whole grows thicker. If ’tis too thick for dipping, more water must be added. Our method of dipping was just the same as is used by the delft-ware people. We first baked our ware to a soft biscuit, which would suck, then painted it with blue, and dipped them with the same ease; and the glazing grows hard and dry, as soon as it does in the delft-ware. Large vessels may be dipped raw, as the Chinese are said to do it. But the proper thickness of the glaze is not so easily distinguished this way, as when the ware is biscuited; for, the raw body being of the same colour and consistence with the glaze, when the latter is dry, ’tis hardly possible to determine the limits of either; a thing very easy to be done when the body is hardened by biscuiting. Our chinaware makers in general deny it to be possible to glaze on a raw body or soft biscuit. And so it is with their glaze; which, abounding in lead and other fluxing materials, melts soon and runs thin, and, melting before the body closes, penetrates it, and is lost in the body, whereas our stone is almost as hard to melt as the body is to close; and, not melting thin, neither runs nor penetrates the body. I insist on the truth of this observation, and ’tis necessary to be insisted on, as scarcely any of our potters, misled by too slavish dependence on their own too partial experience, will allow it. I have said above that the Jesuits observe that the Chinese paint and glaze their ware on the raw body. I know this can be done, for I have done it; and so may anyone else who pleases to try it. I have now by me the bottom of a Chinese punch-bowl, which was plainly glazed, when it was raw, or a soft biscuit; for the ware wants a great deal of being burnt, it being of the colour of coarse whited-brown paper. But the same body, when exposed to a proper degree of fire, turns to a chinaware of a very good colour—a demonstration that it had not, as our ware in England hath, the great fire before the glaze was laid on. I don’t point out the advantages of painting and glazing on a soft biscuit, as they are very obvious to anyone, ever so little used to pottery.
“In regard to burning, I have to remark, that by all the experiments we have made, the north of England kilns, where the fire is applied in mouths on the outside of the kilns, and the fuel is coal, will not do for our body, at least when it is composed of the materials of Tregonnin Hill.
“In those kilns especially, when bags are used, there is no passage of air through the middle of the kiln; and a vapour, in spite of all the care that can be taken, will either transpire through the bags, or be reflected from the crown, which will smoke and spoil our ware, though it doth not appear to affect other compositions. How true this remark may be, with regard to the St. Stephen’s materials I cannot determine, as they have not yet been tried in a kiln. The only furnace or kiln which we have tried with any degree of success, is the kiln used by the potters who make brown stone. It is called the 36-hole kiln. Wood is the fuel used in it. They burn billets before and under it, where there is an oven or arch pierced by 36-holes, through which the flame ascends into the chamber which contains the ware, and goes out at as many holes of the same dimensions in the crown of the furnace. The safeguards at bottom stand on knobs of clay, which won’t melt, about two inches square, and two inches and a half or three inches high; by which means more of the holes are stopped by the bottoms of the safeguard, but the air and flame freely ascend, and play round every safeguard; by which means those tingeing vapours, which have given us so much trouble, are kept in continual motion upward, and hindered from penetrating and staining the ware.