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:
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.
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.
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.
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.
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.
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.
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-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.
For more information on the different uses of lime and its applications, get in touch with one of our trusted team of experts.