Blued steel

Fernando Gilabert Lorenzo <fergilabert [at] terra__es> writes

>I'm working on the conservation treatments of a XV century Persian
>helmet in blued steel with gilded decorations. Does anyone know the
>nature and composition of the bluing layer surface on late medieval
>armours? Is it possible to affect this layer with chemical
>treatments with sodium hydroxide or metal corrosion inhibitors like
>BTA?

The nature and composition of blueing layers from late medieval
origin I do not know, but it is possible that the techniques of
blueing  that are used in these days do not differ that much from
the old ones.

Blueing in our times could/can be achieved in two ways: heating the
iron to a certain temperature (between 290-320 degrees) or
patination. Heating to this specific temperature can be done in an
oven or a sand -bath kept at this temperature, or dipping the object
in a liquid that has exact this temperature. A mixture of 25 parts
lead and 1 part tin seems (according to literature) to have this
temperature. Things that can influence the colour: type of iron,
heatingtime and if the iron is hardened or not.

The patination can be achieved in various ways, one of them placing
the iron in a boiling bath of a mixture of sodium hyposulphite and
lead acetate in water (probably there are much more patination
methods).

The blue layer achieved by heating is a thin oxidation layer (the
colour of this thin oxidation layer depends on the temperature the
iron was heated on:temp. range from 215 degrees up to 320 degrees
can give colours from light yellow to light blue). The patinated
layer is also an induced oxidation/corrosion layer. To be certain
you could try to analyse this layer to see what it is exactly made
of (for example XRD-method).

Your question about affecting this layer with NaOH or BTA:  what
needs to be removed? Is there iron corrosion on top of the blued
layer? In any case, I think that because the blue layer on your
object thus almost certainly will be an oxidation layer, cleaning it
with chemicals could be dangerous, removing this thin blue layer.
Probably the  way to clean the corrosion layer on top (if that is
the case) is mechanical. The BTA treatment I do not think is
necessary, BTA is used on copper alloys, its usefulness being proven
for this metal. I do not believe that it is inhibiting iron
corrosion.

To finish this answer: you can always make some tests with new blued
steel. Try your cleaning methods on these test pieces first (you can
also corrode them first) before you start on the Persian helmet!

Joosje van Bennekom
metalconservator
Stedelijk Museum Amsterdam


                                  ***
                  Conservation DistList Instance 16:27
                Distributed: Thursday, October 10, 2002
                       Message Id: cdl-16-27-002
                                  ***