Putty? Slumpy? Skim? Get your head around the world of lime terminology with this quick reference guide to some of the most common and technical terms used when working with lime-based products.
A-F
Air lime
A category of limes produced with pure limestones, absent of impurities such as clay. Typically called non-hydraulic limes, these limes do not have a hydraulic component. Instead, they rely on carbonation to develop strength and stiffen by absorbing (not adsorbing!) carbon dioxide in the atmosphere.
binder
A binder is a material used to hold a material together. Examples include cement, lime putty or natural hydraulic limes.
breathable
A sometimes controversial term, breathability is a non-standardised term that references a material’s moisture management characteristics, typically a mix of capillary water absorption and vapour permeability.
buttery or creamy
This term is typically utilised to describe a very smooth material in texture and consistency, generally achieved with air entrainment or a high free lime content paired with finer sand.
carbonation
The process by which lime reacts with carbon dioxide from the air to harden and gain strength.
cavity wall construction
Cavity construction was introduced in coastal areas during the late Victorian period to prevent water penetration and gained popularity in the 1920s. It utilised two thinner wall skins tied together, often with insulation in the cavity against the inner wall. In the 1970s, the energy crisis made cavity wall insulation standard. However, buildings with cavity-fill insulation can experience moisture damage, as the insulation within the cavity can frequently simulate a solid wall.
coarse stuff
A common term to refer to a coarse material used for rendering, often as a basecoat or scratchcoat, as the coarser aggregate improves workability, allowing for thicker layers and easier levelling.
curing
A common term to refer to the early stages after application of a lime mortar or render, when protection of the fresh work allows it to stiffen and start to develop strength.
devil float
A type of wooden or plastic float with nails or screws protruding a few millimetres from the surface to roughen or scratch a lime render, creating a textured key for the next coat, which is generally only used before thinner coats to help reduce ghosting.
dolomitic limes
A lime binder made mainly of calcium hydroxide and magnesium hydroxide, without additives or pozzolans. It retains more water than calcium hydroxide and carbonates more slowly than high-calcium lime.
Limited mineral resources make it scarce in the UK.
fat – on the surface
A common term used to describe the lime brought to the surface by a tool passing over wet material, also sometimes referred to as laitance.
fat or fatty – material
A common term used to describe the high free lime content of a mortar or render, which makes the materials more sticky than those with a lower free lime content. Explore our range of fatty lime putty mortars or renders.
fine stuff
A common term to refer to a fine material used for topcoat or finishing plasters, as the finer aggregate suits a smooth and satisfying finish.
float coat
A term used to refer to the final coat of the render, which is refined for an even, smooth and satisfying finish with either a wooden or plastic tool known as a float.
flush pointing
The practice of pressing the lime mortar even or level with the face of the masonry units for a uniform and seamless finish.
G-K
green
A term used to refer to a fresh or damp mortar, often with an initial set but still developing strength.
hanging wet
A term used to describe a material that is stiffening slowly, often due to an oversaturated background or impervious masonry.
harling
A Scottish term for an external thrown-on render with a highly textured finish.
hydrated lime
A type of lime that has undergone the slaking process, where quicklime is reacted with water to make calcium hydroxide, including lime putty, CL90s and NHLs. In common use, this phrase typically refers to a dry powder CL90 hydrated lime such as the Lhoist CL90, which can be confusing if you’re not familiar with the terminology used in construction.
hydraulic lime
A type of lime that forms a chemical set with the addition of water, giving the lime mortar or render a workable period. Generally, hydraulic lime is stronger than non-hydraulic lime and is available only as a dry powder. Popular brands include Saint-Astier, Otterbein and LHoist.
ironing in
A method or the process of closing the face of a cement mortar, considered inappropriate for lime-based mortars.
key
A term used to describe the creation of a rough textured surface within a layer of render, typically created with minor superficial scratches to provide an ideal texture for the next coat of render to bond to.
k-VALUE
The K-value, also known as lambda value (often shown using the λ symbol) , measures the thermal conductivity of a material. It relates solely to the material itself and offers valuable insight into how effectively the material reduces heat transfer, independent of thickness. When assessing a material’s k-value, look for the figure expressed in W/m·K. The lower the K-value the better the insulation performance. Lambda values are more commonly found on renders where there is no set thickness.
L-P
laid up
A common term to refer to the area which has been physically covered with material when rendering or plastering. For example, a plasterer might say they have “laid up half the wall so far” when plastering a wall.
lean lime
A common term used to describe a lime that has been made from lime which contained impurities, resulting in less than 70% free lime present after burning.
lime putty
A traditional non-hydraulic lime found within fatty lime putty mortars, renders and plasters. We manufacture our own range of lime putty mortars and lime putty renders and plasters using our own high calcium lime putty slaked in pits onsite.
limewash
A traditional coating made with non-hydraulic quicklime, limewash is a classic coating that protects walls from the elements while providing a nuanced texture and soft, natural finish.
mortar
This is a generic term to refer to a material which is a mix of a binder and sand, a catch-all term for “building mortar,” “rendering mortar,” or “plastering mortar.”
non-hydraulic lime
A lime that gains strength and hardens through the process of absorption of atmospheric carbon dioxide through carbonation. Unlike a hydraulic lime, if left underwater, this lime would not set or harden. The two most commonly used non-hydraulic limes are lime putty and quicklime.
passes
A term used to describe the number of times the material is applied to the wall in a single 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 there has been two passes, separated by about an hour, this would remain one coat because each pass of the material will be pulling in and curing together as a single coat.
prick up
Another term used for a scratch coat, or first coat of render applied and often used in reference to application onto wooden lathing.
plaster
A building material used for the protective or decorative coating of walls and ceilings. In the Southwest, plastering typically refers to any fine finishing coat. In other regions, a plaster may refer more generally to a material used for internal work.
pointing
The process of applying a lime mortar into the joint between masonry units, such as bricks or stones. As the lime mortar is softer and more porous than the masonry materials, moisture will move through the joints instead of the stone or brick, supporting moisture management and thermal performance.
plastering and rendering
The process of applying a lime render or plaster onto a wall or ceiling. This terminology can differ based on your location or the person you’re speaking with. In the Southwest, “rendering” typically refers to any materials used for building or levelling, while “plastering” denotes the fine, smooth finishing coat. In contrast, in other regions, “rendering” may refer to external work and “plastering” to internal work.
pozzolans
A category of materials that can be added to lime-based products to change the characteristics of a mortar once hardened. These materials are typically based on reactive silica or alumina’s which combine with the free lime. Pozzolans can impact the workability and performance of a mix, which is why they are not always utilised or appropriate.
quench or quenching
A term used to describe when a dry lime requires time to absorb water and reach a fatty, workable consistency or state. The time required for this will vary by brand, but generally, most limes are quenched sufficiently within approximately 20 minutes.
quicklime
Chemical formula CaO. This type of lime is made from limestone, a rock rich in calcium carbonate. When slaked with water, Quicklime will produce a violent exothermic reaction, creating lime putty or hydrated lime.
R-Z
raking out
The process of removing an old or damaged mortar from the joints to repoint with a fresh material. Explore our Drymix, our range of building and pointing mortars.
render
A building material used for the protective or decorative coating of walls and ceilings. In the Southwest, “rendering” typically refers to any materials used for building or levelling. In other regions, a render may refer more generally to a material used for external work.
r-VALUE
The R-value measures a material’s thermal resistance, indicating how well it insulates by resisting heat flow. It’s determined by dividing the materials K-value by it’s total thickness in metres. When assessing a material’s R-value, look for the figure expressed in m²·K/W. A higher R-value signifies greater effectiveness at slowing heat transfer. R values are more commonly found on boards which have a set thickness in manufacturing.
scratch coat
A term used to refer to the first coat of render, that is then ‘scratched’ typically in a diamond shape to provide a textured key for the next coat. For particularly uneven walls, more than one scratch coat may utilised.
setting up / firming up / pulling in
A common term to refer to a mortar or rendering which is stiffening from substrate suction.
sheltercoat
A type of material, typically a blend of limewash and stone dust, designed as a sacrificial coating to protect the underlying masonry, such as stone.
slaking
The addition of water to quicklime to create hydrated limes, such as powdered hydrated lime, NHL or lime putty.
slumpy
A common term to refer to a mortar or render that hasn’t held its shape, sags, or runs after placement.
skim
A common term to refer to finishing plaster, typically gypsum plaster.
solid wall construction
A traditional method of construction, many solid walled buildings were built with natural stone, blocks or bricks where there is no air gap between the inside and outside of the structure. It’s also not uncommon to find early vernacular buildings built with cob or earth also referred to as solid construction. When considering the age of a building, it’s common to find that the thicker the wall, the older the building. However, this rule varies by region across the UK, with some of the thickest walls we’ve seen being nearly two metres thick in Cornwall.
substrate
A term used to refer to the background or underlying layer a material will or has been applied to. For example, a wood wool board would be a substrate.
timber frame construction
A method of construction that uses a structural framework of timber (wood). It’s popular for the speed at which buildings can be built and has been a topic of industry debate concerning its role in the UK’s goals to reach Net Zero.
U-VALUE
The U-value, also known as thermal transmittance, is the rate of transfer of heat through a structure. Expressed in W/m²K, the better-insulated a combined structure is, the lower the U-value will be. It’s worth noting that you won’t find a U-value listed on a singular product, as when calculated, it takes into account the thermal resistances of each material making up the total structure.
void filling grout
A liquid mortar poured, pumped or injected in solid masonry walls to fill voids and reinforce masonry walls. Explore our range of lime-based masonry grouts.
weathered or struck pointing
The practice of pressing the lime mortar into the joint to create a slightly recessed profile for a classic finish that adds depth and dimension. This type of finish is also more effective in combating water penetration by directing moisture away from the joints.
wind-driven rain
Wind-driven rain occurs when wind pushes falling rain diagonally, often in exposed or coastal areas, leading to water saturation of building materials, which is why it’s a factor in determining a lime mortar or render.
