When we think of a physical building, we tend to think of bricks, mortar, timber and stone as the materials that make it up. Perhaps we’ll also spare some thought for slate and glass. Paint, however, rarely gets a look in.

Although a mere surface coating, paint is vitally important in many buildings for preserving timber components such as sash windows and barge-boards, and metalwork. All these components are constantly exposed to the elements and they need a surface to protect them.

What is paint made of? Historically, paints that were applied to timber and metal on buildings were bound in natural oils. There are a few different oils that can be used, all of them derived from seeds or nuts (not crude oil!). Linseed oil—that is, the oil made by pressing flax seeds—was the one used for architectural painting. Compared to modern synthetic paints, linseed oil dries very slowly. Linseed oil wasn’t just used in architecture, but it was also used by the great painters of history from Albrecht Dürer to Zurbarán.

Linseed oil is only one component of a paint. The oil—the binder—is very frequently confused with the pigment, the dry powder that gives paint colour, body, and physical structure. These are two different things. When windows and barge boards needed to be painted, they would typically be painted white. Today, we use a pigment called ‘titanium white’, an oxide of titanium, which is safe and unreactive with linseed oil. However, titanium whites were only discovered in the 20^th century.

Before titanium white, the most reliable white pigment that could be used as an oil paint was ‘lead white’. Chemically, this is basic carbonate of lead. The resulting white paint was a very fast drying paint, because the lead compound reacts with the oil. It also formed a very stiff, rigid film that was hard wearing, rust and pest resistant. As a result, the lead white paint is an excellent protective layer for important wood and metal structures which are part of our architectural heritage. Some sash windows have survived since the 1690s, thanks to the use (and re-use) of traditional paints.

Unfortunately, as is well known, lead is a poisonous heavy metal. Consequently, the most important white paint used across the globe from antiquity to around the second half of the 20^th century was poisonous too. Dried lead-based paint films in good condition that are undisturbed are not harmful—they don’t passively ‘emit’ lead into the air (otherwise, you would have to visit the National Gallery in PPE). Hence one could live in a home painted with lead throughout and not be harmed. This is why civilisation got away with using the paint in general.

Industrial workers using the paints, however, began to see chronic negative effects from repeated exposure to large amounts of the material in the form of fresh paint and dust. This is why the first regulation on lead white paint came from the International Labour Organisation, which predates the modern United Nations.  Eventually, lead white paint was banned throughout the European Economic Community (later, the European Union), because of its toxicity.

In the United Kingdom, lead white paint has been prohibited to the general user. One important exception to this is for architectural and artistic conservation. In architecture, one has to get a permit (called a ‘declaration’) by enquiring to Historic England, Cadw, or Historic Environment Scotland. In Northern Ireland, this is dealt with by the Department for Communities. Declarations are only considered for the highest status buildings: Scheduled Ancient Monuments, Grade I and Grade II* listed buildings (excepting NI).

Ultimately, it is for contractors, building owners and occupiers and heritage bodies to decide whether using traditional lead-based paint today is a good idea in each case. The superior weathering properties of lead white paint need to be balanced against its health hazards and the fact that failed paint films will eventually need to be removed and renewed. Historic building owners should be aware of the likely presence of lead in painted surfaces.