I'm quite sure it is gum arabic <
http://en.wikipedia.org/wiki/Gum_arabic>, which is a very common paper adhesive where toxicity might be an issue (stamps, envelopes, ...) Readily dissolves in water; stable; long-lived.
Donald.
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----- Original Message -----
> From: "Stefan Trethan" <stefan_trethan@...>
> To: "Homebrew PCBs" <Homebrew_PCBs@yahoogroups.com>
> Sent: Sunday, July 8, 2012 2:45:38 AM
> Subject: Re: [Homebrew_PCBs] DIY laser transfer paper with water-based "gum" glue
> I would guess your gum my be either gum arabic or water glass (sodium
> silicate).
> Try to get the MSDS (material safety data sheet).
>
> ST
>
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