[db48x]

A quick search would turn it up, but ok.

In the early 1800s a sir Francis John Davis traveled in China disguised as a Mandarin. He discovered that green teas prepared for export were adulterated with tumeric, prussian blue, and gypsum to give them a richer green color.

    … At each pan stood a workman stirring
    the tea rapidly round with his hand, having previ-
    ously added a small quantity of turmeric in powder,
    which of course gave the leaves a yellowish or
    orange tinge ; but they were still to be made green.
    For this purpose some lumps of a fine blue were
    produced, together with a white substance in pow-
    der, which, from the names given to them by the
    workmen, as well as their appearance, were known
    at once to be prussian blue and gypsum. These were
    triturated finely together witha small pestle, in such
    proportion as reduced the dark colour of the blue
    to a light shade; and a quantity equal to a small
    teaspoonful of the powder being added to the yel-
    lowish leaves, these were stirred as before over the
    fire, until the tea had taken the fine bloom colour of
    Hyson, with very much the same scent.

    …

    If the tea has not highly deleterious
    qualities, it can only be in consequence of the
    colouring matter existing in a small proportion to
    the leaf; …

It is possible that the Chinese didn’t know that prussian blue is a poison. It’s also possible that they were using real tumeric!

See page 421—423 of <https://ia800902.us.archive.org/18/items/chinesegeneral02dav...>.