HELP.

I have a DRAWING made and signed by BOSBOOM.
That dutch painter made oils and drawings, but my drawing is made on a white
plate of DELFT CROCKERY of 45/55 cm. (approx)

On the back the word "DELFT" is engraved in the middel of the plate
(it seems to have been engraved after the cooking of the crockery.)
 and in a corner there is the lettre "L" and the digit "5" and also a mark
representing 2 dashes (=) under which there is a triangle. (that mark has
been made before the cooking of the crockery.)

On the right side, the drawing represent the interior of a kind of church
and seems to be made with indian ink.

There is also a sort of varnish layer on the plate and I wonder if that
layer is between the plate and the drawing (because the plate was too glossy
to draw on it with indian ink) or if it is on the drawing as a protection
layer. Or perhaps there are 2 layers.

Anyway that varnish is getting really yellow and the drawing looks very
different of what is had to look in the past. (black on yellow instead of
black on white)

QUESTIONS:

1) Does someone know someting about the fact that BOSBOOM has
    made drawings on Delft crockery plates?

2) Is there a possibility to clean the varnish without destroying the
    indian ink?

3) Who is the specialist of that painter?

4) What could be the value of that drawing?


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THANK YOU FOR YOUR KINDNESS AND SORRY FOR MY BAD ENGLISH (I use to speak
french)