Renovating a 1930s terraced property in the UK has been a big gamble. One of a long list of jobs was to lay paving to the front of the property. Whilst preparing it became obvious that new pointing was required below the damp proof course.
Black ash mortar was used in the construction of 1930s properties in the UK. Crumbling black mortar can be seen in the mortar joints to the front of the property.
How can you tell if you have black ash mortar?
It’s black and softer than cement mortars. Modern bricks are likely fired at higher temperatures making them harder allowing harder mortars to be used in modern buildings. Whereas older properties benefit from less dense and more flexible mortars. If Black Ash mortar is disturbed by using an angle grinder or mechanical action like scraping and gets in the air is smells and tastes like ash.
Due to the colour and chemical composition black ash mortar is not compatible with cement based mortars. Cement mortars will not look very good and weather more quickly.
A throw back to the United Kingdom’s industrial past from Victorian times to the late 1920’s and early 1930’s black ask was taken from collieries and added to mortar. Utilising a by product of industry by using it in construction solved a problem and was cost efficient. A good solution to the problem of getting cheap building materials when the country didn’t have a supply chain of commercial cement available. Furnace ash was commonly used in black ash mortar and ash lime mortars.
Portland cement was being developed at the same time but wide spread commercial use had not been initiated, probably due to the high barriers to entry for manufacturers, and was only used in ad hoc projects. One of the first uses of Portland cement was Eddystone Lighthouse. John Smeaton, the engineer, experimented with traditional methods of creating mortar.
Without sending a sample of my house mortar for testing it’s difficult to know if it is a cement, sand, black ash mortar or a lime, sand, and black ash mortar mix. Based on reports of other 1930’s properties I suspect it’s a lime, black ash and sand mix.
Black ash mortars have attracted some criticism because the ash used often produces weak sulfuric acid. Speeding up the corrosion of ferrous metals often used as wall ties for cavity walls.
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