The A to Z of Lime: A Glossary of Lime Terminology

Whether you’re a seasoned professional in historic preservation or a novice enthusiast eager to delve into traditional building practices, this glossary of terms can help you navigate the labyrinth of lime-related terminology.

From deciphering terms like “hydraulic lime” and “limewash” to unravelling the nuances between “plaster” and “render,” we aim to demystify the language surrounding lime-based materials and methodologies with clarity and precision.

Whether you’re seeking to understand the characteristics of different types of lime binders or the intricacies of applying lime-based mortars and renders, this glossary equips you with a solid starting point to navigate the realm of building with lime.

Keen to learn more? Join us for our next free online webinar on Lime in Building, where you can learn more about the fundamentals of lime, including its historical context, the importance of breathability for solid wall masonry and how lime is used today.

A – F

Air lime

Air limes are a category of limes produced with pure limestones without clay and are often referred to by another term, non-hydraulic limes. As the name suggests, these limes do not have a hydraulic component. Instead, they stiffen and develop strength through carbonation by absorbing carbon dioxide in the atmosphere. We stock two air limes, quicklime, Lhoist CL90 and our mature lime putty.

Binder

The glue that holds the mix together, can be cement, lime putty, NHL, etc.

Breathable

A non-standardized term meaning the moisture management characteristics of a material, typically a mix of both capillary water absorption and vapor permeability.

Buttery/creamy

Very smooth texture when a tool is passed through the mortar, often air entrained but can be from a high free lime content with a finer sand.

Cavity wall construction

Cavity construction was utilised in coastal regions during the late Victorian period to try to stop water penetration; however, it became more popular in the 1920s. Two thinner skins of walls are held together with wall ties, often with a layer of insulation on the inside of the cavity up against the inner skin. In the 1970s, the energy crisis made in-cavity wall insulation commonplace. It’s not uncommon to find issues with moisture penetration-related damage in buildings with cavity-fill insulation, as this, in effect, turns a cavity structure into a solid wall structure.

Coarse stuff

Refers to coarse materials used for rendering, which are ideal for basecoats and scratch coats as the coarser aggregate provides additional workability, allowing for thicker layers to be applied and easier levelling.

Curing

Is the process where carbonation takes place and the material develops strength. It is an essential part of the process, and while lime mortars can cure for years, this term is usually referred to in the early stages after application when curing conditions are critical for strength development. For more on curing, see our helpful guide to Curing Lime Renders & Mortars.

Devil float

A standard wooden or plastic float, which features the addition of nails or screws that project outwards by a few millimetres so that the surface of the material can be roughened or scratched to provide a key for the next coat.

Dolomitic limes

A lime binder composed primarly of calcium hydroxide and magnesium hydroxide, and is free of additives or pozzolans. Due to limited mineral resources, it’s typically unavailable in the UK market. It is known for its higher water retention than calcium hydroxide and slower carbonation compared to high calcium lime. Researchers from Tongji University found that dolomitic lime binders were used to construct areas of the Great Wall of China.

Fat (on a surface)

The lime on the surface that the tool will bring up after passing over a wet material is termed fat or sometimes called laitance.

Fat/Fatty (on application)

A high free lime content, often sticky to tools and the wall, typically used at a stiffer consistency than less fatty mixes to prevent it getting too sticky.

Fine stuff

Refers to fine top coat plasters. The finer material gives you a smoother finish.

Float coat

This coat is the final coat of render, which is refined with a standard plastering tool known as a float. The float, which can be wooden or plastic, helps to ensure an even, smooth and satisfying finish.

G – K

Green

A relatively fresh or still damp mortar, often initially set but still weak.

Hanging wet

Stiffening slowly, often due to over-wetting the background, impervious masonry, or damp conditions.

Harling

A Scottish term for a thrown-on render with a highly textured finish.

Hydraulic lime

A lime that forms a chemical set on water addition, giving the mortar or render a workable time span, generally stronger than non-hydraulic limes, and available only as a dry powder.

Ironing in

A method to close the face of a cement mortar; not a good finish for lime.

Key

The creation of a rough textured surface, typically created by minor superficial scratches into a render, which provided a superior surface texture for the next coat of render to bond to.

L – P

Laid up

Refers to the area you have physically covered with material when rendering or plastering. If you were plastering a wall and were halfway through, you might say, “I have laid up half of the wall so far.”

Lean lime

A lime that has been made from a limestone with impurities which means that there is less than 70% free lime present after burning. The result is a lime which is not very pleasant to work with, and is often considered the opposite to a ‘fatty’ lime.

Lime putty

A wet form of non-hydraulic lime.

Limewash

A thin layer of lime used historically as a modern paint to protect renders from weathering and as an aesthetic finish.

Mortar

A mix of a binder and sand, a catch-all term for “building mortar,” “rendering mortar,” or “plastering mortar.” View our range of lime mortars, renders and plasters.

Non-hydraulic lime

A lime that, if left underwater, will never set or harden, increasing in strength only by absorption of atmospheric carbon dioxide through carbonation.

Passes

Often confused with coats, a pass is distinctively different. A pass is the number of times the material is applied to the wall in one coat. For example, you could have one coat of plaster in two passes by first applying a pass of plaster, allowing it to pick up for 30-60 minutes and then applying a second pass over the top. While the material has had two passes, separated by about an hour, it still remains one coat because each pass of the material will be pulling in and curing together as a single coat.

Pick up

To dry and pull in moisture from the background, into the material.

Plaster

Often used interchangeably with render; in South West England, it’s often the decorative final layer of several coats on the wall.

Pointing

The application of mortar to a joint, the gap between two bricks, blocks, or stones, to replace weathered material or improve water-tightness of a structure.

Plastering and rendering

These terms can vary depending on the area of the country you are in or even who you are speaking to. In the Southwest, it’s common to refer to ‘rendering’ as any course material used for building or levelling and ‘plastering’ as the fine, smooth finishing coat only. In other areas, ‘rendering’ refers to external work, and plastering refers to internal work.

Pozzolans

A finely divided powder which can give a hydraulic-like set to a non-hydraulic lime, increasing strength.

Quench or quenching

Quenching is when a dry lime needs time to fully absorb water and to come to a workable fatty state. While this can vary by brand, generally, most limes are quenched enough to use in approximately 20 minutes. If you try to use a material before this time, it will likely stiffen and you may lose a workable consistency.

R – Z

Render

The layer of mortar applied to a wall to flatten it, helping to protect the wall itself and provide weather resistance.

Scratch coat

Usually the first coat of render, that you scratch afterwards to give it a key for the next coat. For particularly uneven walls, more than one scratch coat may required.

Setting up/firming up/pulling in

Refers to a mortar or render stiffening from substrate suction.

Sheltercoat

A blend of stone dust and limewash for better weather resistance, colour, and higher thicknesses of application, often used for preservation of specific stones or statues.

Slumpy

A mortar that doesn’t hold its shape well and sags after placement.

Skim

A slang term for plaster, often referring to gypsum plaster.

Solid wall construction

A historic method of building where there is no air gap between the inside and outside of the structure to isolate water from getting into the building. Generally, the thicker the wall, the older the building; however, different areas of the country can feature various types of solid wall construction. For example, many brick buildings in London often feature 225mm thick walls, whereas stone buildings in Cornwall are generally around as 600mm, although we have seen walls nearly 2m thick!

Substrate

The base from what you are working on; in plastering this refers to the wall or ceiling.

Timber frame construction

Built similarly to a standard cavity wall construction, but the inner skin is replaced with a timber frame. The inner skin of the building is wrapped with a breather membrane to prevent excessive moisture penetration.

Void filling grout

A liquid mortar poured or pumped into the center of a wall to fill voids.

Wind-driven rain index

Wind-driven rain is when a horizontal wind drives falling rain diagonally towards the ground. Wind-driven rain is common in exposed or coastal locations and can cause water saturation of the building fabric, which is why it’s a factor considered when choosing a lime mortar. The Centre for Environmental Data Analysis (CEDA) calculates the annual index of wind-driven rain, which sums up all wind-driven rain spells for a given wall orientation and time period using hourly weather and climate data to calculate the volume of rain blown from a given direction absence of any obstructions, with units of litres per square metre per year.

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