The unworkable recipe for John Fords flint glass most curious. It is definitely out of whack compared to any other flint glass recipes I have seen from the period. The norm for lead or litharge being anywhere from 55%-75% of the amount of sand used, Fords recipe is at about 30%. Leighton degrades [lessens] the sand and still can barely get it out of the pot as he says [too thick]. Leighton's stay in Edinburgh was short and possibly a bit of a whirlwind by his own account " it appears like a dream". There is one other ingredient if added to Fords recipe that will in all probability make it work. He does not write it down for Leighton as it is a closely held secret, surely he tells Leighton of the secret ingredient but Leighton can't recall.
Some other aspects of the letter from Nov. 28, 1839 I think may have some relevance and my take on it.
"I received your kind present that you sent to Liverpool" [ Leighton's departure point back to America ]
"According to your directions I bought of Mr. McClenan a small quantity of the white oxide of Besmith [bismuth], the oxide of antimony and the cromit of iron"
I think the gift sent to Liverpool not from Edinburgh, probably sent from London at Fords direction and probably done by Mr. McClenan [Fords agent in London?] as it seems to be timed to Leighton's departure so as to nullify any obligation on Leighton's part to reciprocate. Of the ingredients bought of Mr. McClenan the standout would be the Bismuth, this ingredient is the missing link I think in Fords recipe for his flint glass. A short article on the properties of Bismuth
https://digitalfire.com/oxide/bi2o3 I think the probability that Ford has glassware in London in 1837 could be very possible. After he gains full ownership in 1835 he is determined to grow the business [Holyrood eventually does become the largest glassworks in Scotland } and to do this he must enter the London market. Surely there is a vibrant wholesale/retail market to the trade as it were. Obviously this would be a highly competitive market as well. To standout he sends his best work in the newest fashion and a novel color. Given the limited time frame Davenport has I think they are sourcing for the most part from the London wholesale/retail market. The finger bowls in question most likely engraved to order in London. A plausible case for a Scottish attribution for the finger bowls? As to shipping from Scotland to London the cheapest and probably safest [no breakage] would be by coastal schooner.