I don't know if it is the best - but at least
for here -- lemon juice works very well on rag,
cotton and wood pulp papers -- use a candle
or stove to 'develop' the writing.
good choice is one of the ultra-violet dyes --
e.g. Ultra Violet Products A800 is a common
choice... Colorless, but blue emission
under long-wave UV.
Finally, any colorless
acid will effect a common indicator, so you
can write with Sodium Bisulfate and develop with
methelene blue, or even red cabbage dye (a nice
red/green indicator of pH).
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