POLISHED
PLASTER
OXFORDSHIRE
TADELAKT
LONDON
BERKSHIRE
OXFORDSHIRE
Vapour Permeable - Lime allows moisture in the walls to evaporate out, preserving the fabric it covers by keeping it dry. None breathing skins of modern cement mortars with modern masonry paints on the outside & modern gypsum plasters with emulsions on the inside can trap moisture leading to damp problems, condensation, mould & decay. Lime maintains an open pore structure when cured unlike cement which cures to a more dense pore structure makimg the material impermeable.
Energy Efficient - Dry walls mean less heat is conducted through. The problem with hard dense materials (cement, modern gypsum & dense stone) is once they get up to a certain temperature they stay there for a long time. Lime allows for a more stable temperature exchange.
Air quality - Lime has hygroscopic properties which help to regulate internal humidy, by absorbing high concentrations of moisture in the air in to its cell spaces & releasing once the concentration drops. Meaning temperature fluctuations created by unstable humidity are maintained.
Antibacterial - Lime is alkaline. Mould cannot deveop on its surfaces. The flow of oxygen through its cell spaces helps to kill bacteria.
Flexural strength - What makes lime unique over modern cement & acrylic binders, is its flexural strength. All buildings move and all building materials expand & contract with temperature fluctuations with their moisture content. Lime comes in to its own in this respect being able to withstand stresses & movement without fracturing, as visible on so many cementicous coatings. However lime is not elastic, if there is significant movement, any material has its limitations. But lime will not fracture in the destructive manor brittle cement mortars do, often cracking open the masonry it covers. Lime if pushed to the extremes will microcrack & over time will self heal through a process known as autogenous healing. So no need for unsightly expansion joints.
FUNCTIONAL BENEFITS
Lime is a very durable & versatile material with a range of functional & decorative properties which if applied correctly will last a lifetime.
With the emphasis of modern materials on preventing movement & vapour permeating the building, Lime accepts that moisture and movement are inherant in ALL materials, allowing for these 2 eventualities. This results in healthy adjacent structural components (masonry, timbers), healthy interior air quality and an energy efficient building.
The survival of ancient buildings is a testament to the functions of lime mortar
DECORATIVE FUNCTIONS
For decorative purposes, diluted as a limewash or as a rich fine stucco, it has a remarkable depth of aspect where the calcite crystals formed in curing (carbonation) allow light to refract IN TO the plaster giving the depth of a natural stone or marble. Combined with natural earth and oxide pigments creates an integral depth of color not attainable with other materials. Worked in Fresco (in to wet lime plaster) or secco coatings, a wide range of effects & patinas are possible. Synthetic resin or acrylics (plastics), will only reflect light off the surface, giving no depth of aspect, which is what we perceive to be fake & synthetic.
COMPONENTS
The Aggregate - Component provides the structure, (like the skeleton of the body) and plays an important role in providing the overall strength. Most aggregates are inert minerals such as quartz & marble, free of salt silt & clay impurities to insure a good close consistent adhesion with the binder. The aggregate also effects the colour and aesthetics of the plaster for decorative purposes.
The Binder - Binds this structure together by filling the voids between the aggregate (like the skin or flesh) and plays an important role in how the plaster will perform. Lime plaster cures via a process called 'carbonation' back to limestone. What makes lime unique over modern cement & acrylic binders, is its ability to allow 'the skin' to breathe and to flex, preserving the fabric it covers by keeping it dry.
Fibre Additives - Used to increase tensile strength, increasing a plasters flexibilty under load. Traditionally horse or goat hair was used & still is today, with the modern synthetic alternative 'polypropene fibre' now used a lot also. Very important for lath work & run in situ work. Also hemp shiv is used for hemp lime plaster
TYPES OF LIME
High breathability - High flexural strength - Ideally suited for soft substrates subject to movement - The highest quality lime for decorative purposes (venetian stucco) - Made from limestone deposits containing <95% puritY
Lime plasters cure through a process called 'Carbonation' returning the plaster in a cycle back to limestone
SLAKED LIME PUTTY
The burnt limestone (CaCO3) is heated in a kiln to temperatures around 900 degrees celcius, driving off the carbon dioxide content (CO2), leaving calcium oxide (CaO), also known as quicklime. This is then added to an excess of water (H2O) giving off excessive heat in a volatile reaction converting the quicklime in to a putty - Calcium Hydroxide (CaOH2) in a process known as slaking. This is then left to break down in to smaller and smaller particles over a period of months and sometimes years for very high quality putty.
As the quality of a fine wine matures with age, the same is true for lime putty. The longer the lime is matured in the slaking containers (without exposure to the air), the more the plasticity and water retentivity increases, meaning less shrinkage on drying & thus a reduced risk of cracking & for decorative purposes a more workable stucco with better adhesion, more consistency, stability and strength. Very important when applying techniques such as stucco veneziano using a number of very thin lime rich coats.
NATURAL HYDRAULIC LIME (NHL)
Hydraulic limes - have an increased set time - high compressive strengths - high flexural strength - A less open pore structure than putty lime & thus less permeability. Used for a number of scenarios such as coating exposed areas resisting erosion, less permeable substrates, areas exposed to freeze thaw cycles & many many more
Made from limestone deposits containing silica & alumina (clay) impurities, the higher the impurity the more hydraulic the lime classification. The more Hydraulic, the faster the set, the higher the compressive strength but the less breathable.
Hydraulic limes are classed based on the percentage of reactive clay impurities present.
NHL 2 - (6 to 12%) - Feebley Hydraulic
NHL 3.5 - 12 to 18% - Moderately Hydraulic
NHL 5 - 18 to 25 % - Eminantly Hydraulic
Hydraulic limes set with the presence of water as well as carbonation. So there are 2 processes here.
The manafacturing process requires a slightly higher and more controlled heat in the kiln at around 1250 degrees celcius where the silica & alumina components combine with some of the lime forming calcium silicates (belite clinker) & calcium aluminates. These are the reactive components in hydraulic lime which react with water to form calcium silicate hydrates. Micro crystalline structures between the calcium and silica create thousands of crystalline structures, forming a more dense network of crystals, resulting in greater strength but a less open pore structure than putty limes.
NATURAL CEMENT ; ROMAN CEMENT
Not to be confused with conventional Ordinary Portland Cement (OPC), ‘Natural Cement’ is an eminently hydraulic lime that is baked from clayey marls that have a high alumina content in addition to active silicas. The Romans exploited hydraulic lime deposits and would sometimes combine them with pozzolans to increase the hydraulic setting action even further.
With properties of a rapid set, self binding and high resistance to salt attack, natural cements are used for exterior statuary, run mouldings and as an accelerator for natural hydraulic limes
THE LIME PLASTERER
Ranging from a craft discipline of plastering walls & architectural features, to an artisan discipline creating decorative finishes, Lime plastering requires an in depth understanding of the materials, how they behave, their preperation & presentation to create long lasting stunning results in the same respect a chef must have an in depth knowledge of their ingredients, their source, preperation and how to combine these to create flavour.
It is an art of knowing correct timings of application in relation to the porosity of the substrate to which it is applied in order to control the curing process of the lime plaster back to limestone.
Lime is a high alkaline potent material which can only truly be managed with the upmost quality by experienced applicators. In the wrong hands with plaster being starved of its water too fast or not loosing its water fast enough can both prevent the lime cycle from completing.
For the things we have to learn before we can do them,
we learn by doing them.
Aristotle