Monday, 9 October 2017

Lead Corrosion

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.

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.