From: TERRY KING, 101522,2625
TO: Risa S. Horowitz, INTERNET:babbleon@terraport.net
DATE: 15/05/96 11:21
RE: Copy of: Re: Young'uns Q? for the day
Risa
EDTA is ethylenediaminetetraacetic acid, known to its friends as EDTA. It is
what is known as a chelating agent. That means that it will gather to itself any
unwanted metal atoms from a print so that they can be washed away. This is very
important in platinum printing where, if iron, which is there as part of the
light sensitive salt, is left in the print, it goes rusty leaving red stains
like the foxing on old maps and prints. The difficulty is that when it is used
in conjunction with some sensitisers it does not work as effectively as one
would wish. Mike Ware has developed a process using a different sensitiser so
that the platinum print can be printed out and then 'cleared' with EDTA and
other chemicals. Even though platinum and palladium prints have been around
commercially for 120 years, there seems to be as much argument today as to the
'best' way as there was in the 1880s.
Precipitation can best be explained by a familiar example. When you mix up your
three solutions for making a brown print, you mix the ferric ammonium citrate
first as 'A' and then the tartaric acid as 'B' and the silver nitrate as 'C'.
You mix 'A' and 'B' together and slowly add the silver nitrate. If you just
slosh it in, the slow reaction cannot take place and instead of making a nice
brown print sensitiser the silver in the silver solution turns straight to
silver and falls to the bottom as little black bits of silver. That is
'precipitation'. In my experience it is a little like getting lumps by adding
the flour too quickly when making a roux or making the egg curdle by adding it
too quickly to mixtures in cooking. In fact there is more than one contributor
to this list who uses cooking analogies when explaining old processes. I refer
to the formulae as recipes. To make both work you need the same kind of know-how
and feel for the process.
Thanks for the print. It is very graphic and almost Etruscan in its impact. I
liked the three different colours. Purely as a matter of seeking information,
do you do a lot of continuous tone work in gum as well ?
Terry King