The only descriptions I have seen seem to show by implication how experimental (and perhaps accidental) the subject was. Newton was a long way from being the first to investigate speculum metal, but for the following reasons his descriptions can form a useful reference-point: (a) Some of the difficulties of accounting for early developments in 'chymistry' or any other proto-chemistry arise because for many materials, techniques for achieving and assessing chemical purity were in short supply, and for end-products gained by use of such materials, the effects of procedure and impurities are specially hard to trace; (b) Newton's descriptions had a strong practical content and focus on desired end-product qualities in relation to his experiences of methods for achieving them.
For example, Newton's view on the purpose and effect of including small proportions of arsenic in the formulations was clearly given. He described the danger of getting a product "full of small pores onely discoverable by a Microscope", resulting in irregularity of the (seemingly) polished reflecting surface. His remedy and explanation was that "white Arsenick both blanches the Metall and leaves it solid without any such pores, especially if the fusion hath not been too violent" (18 January 1671|2, letter to Oldenburg, in 'Correspondence of Isaac Newton', ( vol.1, Cambridge UP 1959, ed. H W Turnbull) p.82). (His descriptions give many more practical details than that (ibid., esp. p.82-88), including alternative compositions that did not include arsenic at all.) At the same time the matter is not simple and caution is needed before accepting his ingredient-lists as an indication of the final compositions, for example, as a later writer put it (in Journal of the Franklin Institute, vol.5 (1828), 39-46 at p.41): "Sir Isaac Newton melted the copper first, then added the arsenic, and lastly the tin; as, without doubt, he knew that the tin should remain in a fluid state the shortest time possible. It is true that he added the arsenic to the melted copper, but as he well knew that a great part of it would be rendered volatile, he therefore added a very large quantity of it, viz. arsenic one, to copper six." So according to that account, much of the arsenic added in Newton's processes was volatilized away. In these and other ways the subject for a long time remained highly experimental and practical, systematization was hard to achieve and seems to remain so in historical retrospect. An interesting view of speculum metals in the hands of Newton and some contemporaries is given by A M Roos, "A speculum of chymical practice", Notes & Records of the Royal Society (2009) [http://rsnr.royalsocietypublishing.org/content/roynotesrec/early/2009/12/15/rsnr.2009.0067.full.pdf].