Glossary of Lime terms

Glossary of Lime terms

When it comes to different forms of lime, its qualities and uses, there is often a lot of technical terminology. This can make the terms difficult to understand – particularly as some of them actually refer to the same thing. For example, harling and roughcast are often confused, but the two terms actually carry the same meaning.

Here is a comprehensive glossary of terms relating to all things lime to help you:

Hotmix lime mortar
Roman cement
Butter coat
Roughcast/wet dash
Coarse stuff  
Knocking back up
Stipple coat
Compressive strength
Lime putty
Thin coat renders
Dry dash
Monocouche renders
Tyrolean finish
Fine stuff
Natural Hydraulic Lime
Vapour resistivity

Vapour permeability



Breathability is the colloquial term for  how a wall dries out, through vapour permeability and/or capillary wicking.

Butter coat refers to the fresh render coat into which dry aggregate (or dry dash) is thrown.

Coarse stuff is a combination of lime putty and coarse sand in a typical ratio of 1:3. It can be used for backing coats in plastering.

Compo refers to a mixture of sand with cement or lime.

Compressive strength is the resistance of building materials, including lime products, to compressive loads. It is often used by engineers in structural calculations. It is also used as a proxy to indicate other performance characteristics of the material, like breathability, which may be suitable for concrete but won’t work for lime-based materials.

Dry dash is a popular method of rendering in the 20th century. Clean, dry pebbles are “dashed” or thrown into a soft butter coat before it sets – not to be confused with roughcast which is a traditional method, where the pebbles are in the mix and fully coated by the lime or cement.

Fine stuff is sieved lime putty, mixed with fine and aggregated to produce a finishing plaster. The mix ratio is often 1:1.

GGBS (Ground-granulated blast-furnace slag) is a by-product of steel and iron production. The GGBS, once ground to a white powder, is  a type of cement and used to be called Slag Cement by the Victorians. It is often blended with Portland cement to produce highly durable concretes with good chemical resistance. It can also be added to lime to give extra strength at the expense of breathability. Additionally, GGBS contains a number of trace heavy metals which can produce interesting, rainbow-like effects in lime mortars under the right conditions.

Green is the firm, fresh state of a mortar, plaster or render. It typically refers to the state a few hours after application.

Harling is a mixture of lime or cement with aggregates, including coarser particles. The grading of harling varies dependent on the mixture. The mixture is then cast or thrown onto buildings as a wetter mix. The thrown action gives improved adhesion and leaves a rough textured appearance which provides a large surface area, allowing vapour exchange and enhanced weather protection. The term originates from the Middle English term ‘harlen’, meaning ‘to drag’ and is commonly used among Scottish plasters today.

Hotmix lime mortar is a method of mixing quicklime and sand together. It is not a type of lime, but rather a method of mixing lime mortar. It was often used historically to produce crude lime mortar quickly, easily and as required. Quicklime is mixed with sand and water and it is normally used whilst hot (hence the name). It is one of several methods of making mortar using lime.

Hydrate refers to a controlled amount of water being added to quicklime to form a powder rather than a putty. It results in lime which is stable and safer to handle than quicklime. This can be done to hydraulic or non-hydraulic limes.  

Hydraulic any lime or cement which will harden under water, rather than by reacting with air.

Knocking back up is the re-mixing of the mixture once it has started to stiffen.

Knocking up is the mixing of the lime and sand mixture until it becomes workable.

Lime putty is made by adding an excess of water to quicklime. Lime with hydraulic character will set underwater within a matter of days, making it impractical for making putty. In comparison, non-hydraulic limes will remain plastic if stored underwater and will improve with age. The word ‘putty’ comes from the French term ‘potée’, meaning ‘polishing powder’.

Monocouche render, as the French meaning of the name implies, is a one coat render. It only requires one visit to the wall, and is achieved in two wet passes, often using a spray machine. This can achieve good compaction with no slumping.

Natural Hydraulic Lime is lime which sets underwater, because of naturally occurring impurities in the bedrock like clay or silica, rather than because pozzolans have been added.

Pargetting is decorative rendering and plasterwork, often in the form of raised relief patterns embellished with free-hand sculpturing or moulding. The term is particularly associated with the English counties Suffolk and Essex.

Pozzolans are siliceous or aluminous materials, for example burnt clay or volcanic ash, that react with calcium hydroxide (lime) to produce a hydraulic set. They can naturally occur or be manmade.

Quicklime is the result of burning limestone in a kiln at temperatures around 800 or 900C traditionally, though modern kilns may run in excess of 1200C. Pure limestones produce nearly pure Calcium Oxide (CaO), while limestones which formed with impurities produce quicklimes with complex chemistry, resulting in quicklime which will be hydrated to form Natural Hydraulic Lime.

Roman cement was first discovered by Rev. James Parker in 1796. Parker found that burning specific marlstone formed a product that was highly hydraulic, in comparison to the limes and lime/pozzolan mixtures available at that time. Although it is much harder than traditional lime, it is much softer and more breathable than modern Portland cement.

Roughcast is a rough finish coat “cast” or thrown against the wall, common in Wales and West England. Very similar to Harling.

Splatterdash is a rich mixture of lime (or cement) and coarse sand, made to a wet, slurry-like consistency and thrown or sprayed onto a wall as a preparatory coat.

Stipple coat is similar to a splatterdash but is trowelled into place and then pricked up using a brush, dabbed onto a wet surface to provide a heavy texture. The name originates from ‘stippelen’ in Dutch, meaning ‘to prick or to speckle’.

Thin coat renders are not traditional renders. They are, in essence, a gritty paint, supplied either dry or wet, with a high content of a single size grit. This often means the product is extremely thick and heavy, meaning it cannot be applied by brush and therefore must be applied by hawk and trowel.

Tyrolean finish is a splattered texture finish which is hand-applied using a tyrolean machine which flicks the wet mixture onto a wall. The term originates from the traditional alpine, but modern Tyrolean render is made with white cement and is exceptionally hard.  

Vapour-resistivity is the inverse of vapour-permeability. It refers to a material’s resistance to allowing vapour to pass through.

Vapour-permeability is the rate and ability of water as a vapour (i.e. a gas), to pass through a material. All materials, including cement mortars, can be measured for their rate of vapour-permeability. Some materials are more effective at allowing vapour to pass through them, such as lime mortars and woodfibre insulation.

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