|Continuing the conversation on natural building materials, is an overview of clay plaster. Clay could be considered a vernacular New Zealand building material, given our land’s richness of the substance. It can sometimes be used as a structural building material and acts as a natural solution for cladding and rendering.|
Clay is the oldest form of plaster and has the lowest environmental impact, as it requires no processing. Clay’s simplicity goes unmatched by any other material for both its structural benefits and promotion for a healthy living environment .
Clay’s unique makeup assists in regulating both humidity and temperature by absorbing and releasing moisture. It makes rooms more tolerable in humid weather, and when the weather becomes drier it releases moisture into the room helping to improve air quality .
Clay surfaces can also help moderate temperature swings, allowing wall surfaces to remain cool in summer and warm in winter. The more clay used in a room, the greater the benefits .
Additionally, it is suitable for those prone to allergies, attenuates electromagnetic radiation, and neutralises odours .
Clay based plasters are made up of clay (the binder), mixed with various sands and crushed stones (the aggregate) which form the bulk of the mixture . Water is then added to create a workable material for plastering.
The type of finish depends on a variety of factors, such as the size of the aggregates, the thickness of the plaster, and the tools/ methods used to finish the plaster:
Hard Trowel: Using a trowel to polish the clay gives it a smoother feel.
Skip Trowel: By not covering all of the base coat and letting some of it show through, you are skipping as much of the top coat as you would like.
Sponged: When you use a sponge to compress your wall, it gives it a suede look and brings out the sparkle of the marble sand.
Burnished: By burnishing, or highly polishing with a trowel, you are giving the plaster a very smooth finish.
Clay plasters can be applied to most wall substrates, including modern materials such as plasterboard, OSB, plywood, and more. Various techniques are required to get the plaster to grip onto the substrate, but a more compatible substrate is preferable to get the best result 
This means porous, absorbent, and slightly rough surfaces such as earth, brick, wood fibre, hempcrete, straw, and bamboo matting. In a New Zealand context, wood fibre boards are suitable as an alternative to plasterboard .
Clay plaster is 100% natural, containing no VOC’s or cement; is a more environmentally friendly choice than traditional gypsum plaster; and is fully recyclable .
Other benefits include:
– naturally absorbs odours
– mould resistant
– is naturally anti-allergic
– can be used with a range of absorbent backing surfaces; brickwork, cement sheeting, gypsum plasterboard and plaster undercoat
– acoustic insulation properties
– natural fire resistant qualities
– a non-dusting finish
– long drying time means that it is workable for longer
References: Earth Studio. Natural Plaster  eBoss. Rockcote’s Earthen Natural Clay Plaster Coating  American Clay. Original Plasters
For more information about clay plaster products in New Zealand, see: