Guide to Lime Terminology

Air limes - 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.

Binder - The glue that holds the mix together - lime putty, NHL,

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

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.

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

Dubbing out - Filling small hollows or isolated areas in a wall to create a level surface for rendering.

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) - 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 topcoat plasters. The finer material gives you a smoother finish.

Float 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.

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 to bond to.

Green - 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.

 

Natural 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.

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.”

Lime Putty - A wet form of non-hydraulic lime.

Limewash - A thin layer of lime used historically as 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."

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 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. 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 material before this time, it will likely stiffen, and you may lose workable consistency.

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 be required.

Setting up/firming up/pulling in - Mortar (or render) stiffening from substrate suction.

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