An Acid free glue which can be dispensed through a fine metal nib. When wet use as an ordinary permanent glue. Leave to dry after applying and it then becomes a removable adhesive. This glue goes on blue and dries clear do you can see where you are applying it.
To increase the flow replace the cap several times to pump the nib. Reduce the glue flow by holding the pen upwards and pressing onto a firm surface to release the pressure.
Clean the nib and replace the cap after every use.