Lime plastering

Lime plaster is a traditional type of mortar used to cover the surface of internal walls and ceilings before painting. Thought to have been first used around 7000 BC, lime has long been the traditional choice for plastering buildings, characterised by its natural and rough appearance. As far back as 7200 BC, statues sculpted in lime plaster were found buried in a pit at the archaeological site of Ain Ghazal in modern-day Jordan. 

Lime plaster, mortar and paints have been used for thousands of years, and ancient lime plasters have been found in the preserved ruins of Pompeii, Italy, whilst Gypsum was used in Ancient Egypt in Pyramids. Whilst both are naturally occurring materials with historical use – they have very different chemical compositions and properties, making each suitable to varied applications (see ‘other information’ below). 

The Middle Ages (5th – 15th centuries) saw a widespread and gradual decline in the quality of lime mortars. After a 400-year-old Romano–British civilization, during the following Germanic Anglo-Saxon culture, the use of Roman mortars were gradually abandoned and forgotten. The decline was due to the increased use of impure sands, poor mixing, low kiln temperatures (resulting in incomplete burning) and an absence of natural pozzolanic materials (e.g. volcanic sands). As a result, during the 9th, 10th and 11th centuries the art of burning lime was almost completely lost.

During medieval times the quality of lime plasters declined, followed by a revival during the Renaissance. The Renaissance (15th – 16th century) saw a revival of culture and a renewed interest in ancient architecture. During the second half of the 1700s there was a renewed interest in Roman mortars because after so many centuries most buildings built with Roman pozzolanic lime mortars were still remarkably well preserved throughout Europe, despite harsh climatic conditions.

Non-hydraulic lime is made from pure limestone and is available as lime putty. It offers brilliant flexibility and breathability. Hydraulic lime is produced from limestone with natural impurities containing some setting agents — it comes in powder form and begins to set once it comes into contact with water.

John Smeaton was the first in England to scientifically investigate why certain limes would harden underwater. Commissioned to replace an old lighthouse near Plymouth, in 1756 he started a series of experiments to find the right lime mortar that would withstand the battling of storms off the coast of Plymouth. After testing 300 different lime stones, including “impure” lime stones naturally containing  6% – 20% clay, he discovered that the best raw material for a lime plaster that can set underwater (water lime) is, in fact, impure limestone that contains clay – being the key ingredient that makes lime resistant to water. The construction of the lighthouse was completed in 1759. 

Smeaton’s natural hydraulic lime (NHL) mixture was so successful that it became standard specification for government contracts for almost a century, eventually being replaced by Portland cement in 1867.

The next milestone in the development of hydraulic materials was the discovery of natural cement by James Parker in 1796. His invention was important because it showed that by firing an impure, high-clay-content limestone at relatively low temperatures (800 – 1,100°C) resulted in a burnt lime stone with hydraulic properties without the need for slaking. Once ground and mixed with water, the new cement mix hardened within 10-20 minutes, both above or underwater.

Parker’s new product was brought to market in 1798. In its promotional pamphlet, calling it Roman cement (presumably for its hydraulic properties, hinting that it could replace the pozzolans of its time). When Parker’s patent expired in 1810, his manufacturing process spread throughout continental Europe, with Roman cements becoming the leading cement during the first half of the 19th century.

Portland cements:

In 1824, Joseph Aspdin, a bricklayer or builder from Leeds, patented the material “Portland Cement”, named after its similarity to Portland stone, a limestone quarried in Dorset (Isle of Portland) with a reputation for quality and durability. Aspdin’s early Portland cement was very different from today’s Portland cement, essentially being an artificial hydraulic lime created from limestone used for repairing the roads and clay.

Lime plastering has seen a resurgence in popularity as more people look to restore old buildings, with its breathable and flexible properties making it ideal for such buildings being more prone to mould and decay. Easy to source and prepare and lasting thousands of years, lime plaster is therefore highly-suited to heritage buildings. 

• Plastering tools include: trowel, hawk, float, bucket, water, lime plaster mix, goggles, dust mask and gloves. 
• The basic ratio for making lime plaster is one part lime to two and a half parts sand, with the amount of water added depending on the desired consistency. 1) Start with one part of coarse sand. 2) Mix in one part of lime. 3) Then add another one and a half parts of sand. 4) Mix these materials dry for five minutes. 5) Slowly add in clean water whilst continuing to mix, and stop mixing when the desired consistency is reached
• Lime can be used by a mason to bed stones or modelled by a sculptor once the necessary aggregates have been added. In plasters, aggregates such as sand are added, typically in the proportions of around three-to-one for all but the finishing coat, principally to reduce shrinkage. A modeller using lime plaster, or ‘stucco’ as it is often known, has time to change his mind some time after he has used it, for lime plaster will set over a five to ten day period. During this period it must be protected from drying out too quickly or it will crack. Once set, stucco will last for centuries.

Allied crafts include pargetting, stucco and scagliola. 

• Higher labour costs – the process of applying the lime plaster takes longer and is more involved, resulting in higher fees. 
• Time concerns – lime plaster can take a long time to set, and the hardening process can take months. Hence, this slow and unpredictable set time can make it difficult to plan and schedule construction projects, making quicker setting options more viable. 
• Cost – lime plaster can be more expensive than other types of plaster, and tend to be more costly to purchase than cement
• Given the popularity of other modern plasters, It can be more difficult to find plasterers skilled in applying lime plaster

Building Limes Forum

Artisan Plastercraft

The Lime Plastering Company

A list of corporate members can be found on the building limes forum: https://www.buildinglimesforum.org.uk/ 

Gypsum plasters have predominantly been used in period properties for decorative plasterwork (such as cornice and ceiling roses), due to setting much faster than lime. Flat plasterwork (such as on walls) has usually been carried out in lime plaster, being a more breathable material than gypsum, therefore allowing solid walled properties (most buildings built in the UK pre-1919) to naturally ingress and egress moisture and helping to maintain a healthy internal environment and prevent damp issues. 

See also Pargeting, Stucco & Scagliola

1. https://www.buildingconservation.com/articles/gypsum/gypsum_plaster.htm
2. https://abdrylining.com/lime-plaster-guide/ 
3. https://www.rosieandthefarm.co.uk/blog/lime-plastering-part-1
4. https://unitylime.co.uk/about-us-blog-what-is-lime-plaster/
5. https://www.buildingconservation.com/articles/limeplast/limeplast.htm https://oldhousestore.co.uk/why-use-lime-mortar-in-old-buildings/#:~:text=Lime%20plastering%20is%20breathable%2C%20which,such%20as%20paint%20or%20wallpaper.
6. https://www.coreconservation.co.uk/why-roman-lime-plasters/