494 Compofition and Fabrication of the 



brittle as this compofition without the brafs would prove. A fmall portion of filver will 

 make the metal much whiter, though if too much is added, it is apt to be porous. Having 

 faid fo much relative to the compofition of the metal, which indeed is a capital article, I 

 pafs on to 



The Manner of cajiing the Metal. 



The fand moft proper for cafling this, and indeed any other metal, is a fine fand, with 

 no more loam or clay mixed with it naturally than is abfolutely necefiary to make it tena- 

 cious enough to adhere together when properly moiftened. If too much clay is mixed by 

 nature with the fand, it will always hlciu the metal in different direftions, fometimes, indeed, 

 to the great danger of the operator. On the contrary, if the fand docs not contain a fuffi- 

 eient quantity of clay, it will not remain in the flafks, or take a proper impreflion from the 

 pattern or model. The beft fand I could ever meet with for the purpofe of calling fpecula 

 is the common Hlghgate fand, (near London) generally ufed by the London founders. 

 It fhould be as little wet as may be, and well beaten, but not too hard. The flaflcs fhould be 

 at lead two inches wider than the metal intended to be caft. If the fand is not of a fuffi- 

 cient thicknefs round the metal, it will inftantly become dry when the hot fluid metal is 

 poured into it, and, confequently, will contraft, and, of courfe, the fluid metal will run out 

 of the flalks. A proper thicknefs of fand will, however, prevent this accident. The metal 

 or pattern flionld be made of brafs, or hard pewter, and muft be a little larger and thicker 

 than the fpeculum intended to be caft from it, as the thing caft is always a little lefs than 

 the pattern, owing to its contradling a fmall degree in cooling. A wooden pattern will not 

 quit the fand near fo well as one made of metal ; befides wood will always warp by the 

 moifture of the fand, and, confequently, will give a falfe figure or form to the intended fpe- 

 culum. As the compofition I have given for the fpeculum is the hardejl, and, confequently, the 

 moft brittle of any metal yet known, fo it is the moft difficult to caft. The common method 

 of cafting other fpecula will not avail in the leaft degree here ; and it was a very confiderable 

 time before I found out a certain and infallible way to caft them free from faults or flaws in 

 the face. In general, they cracked in the cooling, from the moifture of the fand. The only 

 method poffible to caft them well (for indeed I have tried many methods) is to caft them with 

 the face downwards. The ingate or git fliould be at the back of the metal, and at the very 

 edge of it ; its breadth, where it joins the metal, fhould be at leaft half the diameter of the 

 metal, and its thicknefs muft be half the thicknefs of the metal at the edge ; the upper part 

 of the git fhould contain as much metal at leaft, or even more, than the fpeculum itfelf : I 

 could give my reader fufficient reafons for every part of the procefs above direfted, but I 

 might be thought too tedious: fuffice it then that I inform him that he will find thefe diredions 

 will anfwer in pradlice ; and I believe I can fay, that no one whatever can caft fpecula of 

 this brilliant and brittle compofition by any other means than what I have now pointed out. 

 When the pattern, with its Ingate or git, is taken out of the fand, ten or a dozen fmall holes 

 fliould be made through the fand at the back of the mould with a fmall wire, or common 



knitting 



