There
are three important versions of lead corrosion: Red, Brown and White. In addition, there are other factors that can
weaken the lead came.
Red lead is a corrosion
product that appears as a bright red surface, is dangerous, and requires water,
air and often wood, to form. Sometimes water in the manufacturing process can
develop red lead. The chemical
composition of red lead (Lead (II, IV) or triplumbic tetroxide is Pb3O4
or 2(PbO.PbO2). It
is a bright red or orange crystalline or amorphous colour.
Red
lead is virtually insoluble in water or in ethanol. But, it is soluble in
hydrochloric acid as is present in the stomach.
When ingested, it is dissolved in the stomach’s gastric acid and
absorbed, leading to lead poisoning. It also dissolves in undiluted acetic
acid, as well as in a dilute mixture of nitric acid and hydrogen peroxide.
When
inhaled, lead (II,IV) oxide irritates the lungs. In the case of a high exposure,
the victim experiences a metallic taste, chest pain, and abdominal pain.
High
concentrations can be absorbed through skin as well, and it is important to
follow safety precautions when working with lead-based paint.
This
means that anyone dealing with read lead needs protection against skin contact,
and breathing protection. Methods need
to be implemented to ensure no dust is raised, or that the area is thoroughly
cleaned after dealing with red lead. Clothing should be discarded or washed
separately from all others.
White lead corrosion, Lead(II)
carbonate, is the chemical
compound PbCO3. It occurs naturally as the mineral cerussite. It is a curious compound, as it
is soluble in both acid and alkali. It
is possible, but rare, for excess whiting left on the lead to give rise to this
form of corrosion. Generally, it will be neutralised by the weather.
Brown lead corrosion appears as a brown to dull red colour.
Lead(IV) oxide, commonly called lead dioxide
or plumbic oxide or anhydrous plumbic acid …, is a
chemical compound with the formula PbO2. … It is of an intermediate
bond type, displaying both ionic (a lattice structure) and covalent (e.g. its low melting point and insolubility in water) properties.
It is an odourless dark-brown crystalline powder which is nearly insoluble in
water. …. Lead dioxide is a strong oxidizing agent which is used in the
manufacture of matches, pyrotechnics, dyes and other chemicals. It also has
several important applications [e.g.,] in the positive plates of lead acid batteries. Source:
wikipedia
Air,
water and salt are needed to form brown lead. This means coastal areas and
those with driving rain are prone to this kind of oxidisation. Lead dioxide also forms on pure lead, in dilute sulfuric acid. So, with the
acid rain that we are all subject to, it can form in almost
any situation, but will be more obvious on areas exposed to the prevailing
wind. The corrosion is soluble in strong acetic acid.
Tin corrosion also has a brown, almost copper appearance, very
similar to brown lead. The tin corrosion
will be confined to the solder joint and surrounding area rather than all along
the length of the came.
Corrosion resistant lead
The
ideal composition of lead to resist corrosion is 98.5% lead with up to 1% tin.
This, combined with fractions of a percent of antimony and traces of silver,
bismuth and copper provides a combination of metals and trace elements to resist
corrosion of the lead as well as stiffening it.
Conservators indicate that, for whatever reason, cast lead incorporating
trace elements is the most resistant to corrosion. This is evidenced by the longevity of
medieval lead cames.
Solder
composition
Conservators
also indicate that the higher the lead content of solder, and the better the
match it is to the lead came, especially the almost pure lead came, the more
resistant it is to lead came fracture at the margins of the solder joints.
Stretching the lead came, rather than simply straightening it, not
only weakens the lead, it leaves very small pits in the surface of the lead. These
small pits allow the elements of the environment to penetrate the lead’s
surface and act as sites for the beginning of corrosion.
Stretching
also causes stress points near the solder joint. The stretching creates stress along the
length of the lead. When the lead is
heated in the soldering process the molecules of lead sort themselves into a
stress-free arrangement. As heat does
not travel far or fast in lead, there is a stress point formed a short distance
from the soldered lead joint where the already stressed and the stress-free
lead meet.
Conclusion
Clearly
there are a range of factors that relate to the resilience of lead came. 98.5% lead with trace elements including tin
and antimony provides the greatest strength and resistance to corrosion. Stretching the came can lead to general
weakness and introduce pits into the surface forming sites for corrosion. Stretching
can also lead to stress points near the solder joints.
All
these indicate that resilient leaded glass windows can be produced by:
the use of lead came with 1.5% of trace elements,
the use of high lead content solders, and
the straightening (rather than stretching) of the came.
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