My Mother always reckoned that Nana was a better artist than she was. Sadly I've never seen any works of art that Nana did to be able to take a judgement on the issue. Nana did however, at the age of 20, receive this certificate from the grandly titled Society of Science, Letters and Art of London.
Regrettably, upon investigation I find that the Society whilst sounding grand seems to have been fairly transitory and created primarily for the aggrandisment of its members. It seems that it hoped to have itself confused with the Society of Arts and similar august bodies so that anyone describing themsles as a Fellow of the Society of Science, Letters and Art might be mistaken for a member of one of the more genuine bodies.
I cannot imagine that Nana would have sent off an oil painting to such a body unless she had thought it genuine so it is quite possible that it presented itself to the general public as a valid judge of paintings - possibly even charging a fee for such judgement. It seems quite ironic that Nana kept this certificate all her life and yet not a single painting of hers survives.