Calcium
Calcium metal melts at 842 °C and boils at 1494 °C calcium spontaneously reacts with water to create calcium hydroxide chemistry understanding
Calcium carbonate
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Calcium hydroxide
used in liming rosin which can be used in wood or metal finishes.
Calcium Oxide
calcium oxide process
Traditionally hydraulic lime was used, as opposed to Type-S Air Lime used in this video. You can use many kinds of lime from Hydraulic, NHL3 to lime putties made from quick-lime. Wall prep Over substrate and then all subsequent coats, your wall should be clean, free of dust and well hydrated. Test by applying small amount of mix to wall, you should have ample amount of time to work. If not be sure to hydrate wall well, it may take up to several hours. Or try reduce amount of suction by applying an acrylic concrete bonder with water will reduce suction from existing substrate. If possible, seal room to add more humidity, allowing for longer curing times. Step 1 – Base Coat (optional) This does not need to be lime specifically, it could be stucco or earthen basecoat. I would skip this if going over conventional cementboard, especially if you red-guard it. It can be applied over an earthen or masonry wall. It can take many forms, but the purpose of this step is to: - Level and shape; and - Give an extra layer of water protection to our walls, strawbales in our case. Once applied, it is typically floated with wood or polyfloats to shape and level. Basecoat Recipe 1 Part – Hydrated Lime (Calcium Hydroxide) – We used Type-S here 3 Part – Mortar Sand – the quantity will vary depending on your sand Water – as needed Optional ingredients: ¼ Part – Fiber – I used an Acrylic Fiber. There are many synthetic and natural fibers out there that can be replaced. I used this type as I like the size and way it mixes into the plaster. Part – Water retainers – Methocellulose is a common one to add to thickness to your lime mix which helps slow drying time and allows for more working time. Again, there are many thickeners over the years, such as nopal (prickly pear), seaweed, blood and on. I also use Hydroxyethyl cellulose. Pozzalon or Portland – this is to give the lime plaster a hydraulic set. Application - can vary depending on aggregate sizes and what’s needed for base wall to get desired shape. In this video, I used sand screened through ¼ inch screen so could not be applied any thinner than ¼ inch. Most important aspect of application is to float to shape and level, using a wood or poly float, as they are more rigid than a steel trowel. Can be left to cure completely or if you add Pozzolan/Portland mix you can apply the Tadelakt mix soon after it has firmed up. Step 2: Tadelakt Mix Generally applied in 1/16 - 1/8 inch layers (1.5mm-3mm). Can apply as many coats as you feel fit. I personally have never done more than 4 coats. I only do the first coat with fiber and do not use again for all subsequent coats. Ingredients 1 Part – Hydrated Lime 1 Part – Lime Sand in this case Both sifted through windowscreen Water as needed Optional ingredients: Pigment - often needs to be broken down by screening through mesh or dissolving in alcohol. Fiber – refer to above. Fibers help prevent cracking but be careful with them as they can show up in finish. Make sure to leave time between coats and applying ample amount of mix. Only use in your first coat. Water Retainer – Refer to above. Tadelakt mix is already fat because of the amount of lime to sand is very high, so you don’t need much water retainer. I like to use it to give extra water retention and a creamy, workable mix. Pozzolan – I like using Metakaolin or white Portland Application - usually apply between two to four coats as needed, applied very thinly with a steel trowel or float, and giving time to set-up in between coats, but not dry completely. This drying time helps the lime carbonate and lets it shrink between coats, helps guard against cracking. Floating between coats helps to align the aggregate and to level the surface. Floating is optional but just the way I like to do it. Soaping – Use a soap high in steric acid. I use Black Olive Oil Soap in this video as it is the traditional soap used in Morocco for Tadelakt. There are plenty of other soaps that can be used however that have more stearic acid. If not already in liquid form, than grate or chop to dissolve. The soap needs to diluted with water - 1 soap to 10-20 water (I prefer more water). DESCRIPTION CONTINUED (as YouTube didnt allow enough characters for Tadelakt):
Calcium Oxide
Black Soap protects Tadelakts when mixed with Carnauba Wax. Use for cleaning different surfaces (tiles for example). This natural plant product is made from olive oil and olive paste. Moroccan Black soap is a spectacular, multi use product, that is superb for your skin. It has a high vitamin E content that revitalizes skin, acting against dryness, dehydration and aging.
This is a vital ingredient in the art of fine lime plaster finishes and particularly the traditional plaster originating form Morocco: Tadelakt. During polishing, the “fat” penetrates the plaster, creating a second “skin” that naturally fuses with the lime. The chemistry leads to a permanent waterproof, shiny surface.
Use Carnauba Wax and Black soap for the “Savonite” protective finish on Tadelakt. The instructions for making “Savonite” are:
4 C. water
1 heaping Tablespoon Carnauba Wax flakes
3 Tablespoons Black Soap
Mix the soap and water. Bring to a boil. Add Carnauba Wax flakes
SCRATCH COAT
The first coat is known as the 'scratch coat', because the surface is scratched with lines to give a key for the next coat. The mix used is usually one part lime putty to two and a half parts of coarse, sharp, well-graded sand. If the grading of the sand includes more or less of a particular grain size the amount of lime may need to be varied slightly. An experienced plasterer will be able to tell instinctively whether another half part of lime or sand needs to be added. Another way to tell is to take a sample of the dried sand and measure the volume of water required to fill all the voids between the grains; the amount used is equal to the amount of lime required.
Hair can be added to the mix to give it tensile strength. Although this isn't absolutely necessary when plastering onto stone or brick, its benefit in the long term is that if the building moves or any patches of plaster detach from the substrate, the hair will help bridge over any gaps. Old plaster can sound hollow in places when tapped, but is usually still sound if it contains hair. When plastering onto laths, the addition of hair becomes a necessity, because plaster does not adhere well to timber once it has dried; it relies on interlocking fingers formed as the wet plaster squeezes though the laths and slumps over, so the tensile strength imparted by the fibres is vital.
It is important, before applying new lime plaster, to ensure that it isn't going to be sucked dry by the background it is applied to, as this will cause it to shrink and potentially fail. The suction can be reduced and controlled by wetting the substrate before applying the plaster. If the wall is very dry and porous it may need to be sprayed with a hose pipe a couple of times on the day before, and then once again on the day of application, but if it is less porous and the environment is relatively humid, spraying with a hand-held spray on the day of application may suffice. There needs to be enough moisture in the wall for it still to be damp to the touch after an hour, but no longer glistening with droplets of water.
The scratch coat should be no thicker than 15mm (5/8 inch). Any deep recesses or holes should be 'dubbed out' beforehand, using a stiffer (drier) mix, and allowed to dry to avoid deep pockets in the scratch coat. If plastering onto laths it is important to apply the plaster diagonally to the line of the laths, joining up each time with the previous area laid, to achieve a consistent key between the laths.
While still wet, the surface should be scratched with a three pronged lath scratcher or a single pointed lath (which is slower but gives a better job). The scratching should be in straight lines, diagonally to the laths or the line of the wall, in both directions, to create a diamond or lattice pattern. The quality of the scratching affects the keying of the next coat, so it should be done carefully to achieve an even pattern, and, on laths, particular care should be taken not to cut through to the lathing.
The scratch coat should then be left to dry and shrink before attempting to apply the next coat. In most circumstances it will need two weeks to dry out, but can take up to four weeks in some cases. Shrinkage cracks are likely to appear as it dries, but this is not a problem. The important thing is to avoid it drying too rapidly, which can cause it to fail. Exposed areas of plaster (adjacent to open windows, for example) may need to be covered with hessian or polythene, and the use of dehumidifiers should be avoided.
Once the pad of a thumb can no longer indent the scratch coat it is ready to take the next coat. At this stage the surface should still be slightly damp to the touch and will just need brushing down to remove any loose grains and then lightly dampened with clean water, using a hand-held spray. If it has been left too long and has dried out completely, more water will be required.
FLOATING COAT
The second coat is known as the 'floating' or 'straightening' coat, and is used to bring the surface to a level plane. The mix is usually slightly less rich than that of the base coat, typically one part of lime putty to three parts of coarse, sharp, well-graded sand, and normally without any hair. Again, it should not exceed 15mm (5/8 inch) in thickness. A level surface is achieved using long 'floating rules' or 'straight edges', passed over the wet surface to remove undulations.
In the best quality work, wooden blocks (known as dots) are temporarily applied and plumbed and levelled; lines of plaster (known as screeds) then join between the dots, and are levelled using a floating rule; finally the spaces between are filled using trowels and levelled with a floating rule with its ends bearing on the screeds. This method was used in finer quality Georgian and Victorian buildings.
Once it has begun to stiffen up, the floating coat needs to be consolidated by 'rubbing up' the surface using a wooden float to counteract shrinkage. This is likely to be required once or twice on the day of application, and may be necessary on the following day as well. The timing depends on the speed of drying. Sprinkling the surface with water, using a brush, assists the circular rubbing action if it has dried out too much.
Once the surface has been compacted, a 'devil float' (wooden float with nails or screws driven through the corners to project about 2mm) is rubbed over the surface to form a key for the finish coat. It should then be left for about a week or so before it is ready for the final coat.
FINISHING COAT
The final coat is known as the 'setting' or 'finishing' coat. It is usually thinner than the other two coats and uses a finer sharp sand. The mix can vary depending on the hardness and the type of finish required; the richest mix being three parts of lime to one of fine sharp sand, and the leanest mix being one part of lime to three parts of sand. More sand will give a harder finish and is more suitable for open textured float finishes; more lime will give a softer surface but allows it to be polished smoother. For standard work, a mix of one to one is suitable.
Setting coat finished with sponge floats and lined out to imitate ashlar Patch repair (top left) to fine 18th century plaster, gauged with gypsum to achieve a completely flush finish between old and new
The thickness of this coat can vary between 2mm (1/16 inch) and 5mm (3/16 inch). In order to achieve an even finish on the surface, it needs to dry out consistently, so is applied in two or three very thin layers. The additional time and labour required for this is well worthwhile because it controls the drying of the surface and allows the plasterer to achieve the finish required.
Before applying the setting coat the floating coat needs to be lightly dampened with water to control the suction. Each layer is skimmed on as thinly as possible, working in alternate directions each time, and is laid over the previous one as soon as it has had a chance to 'steady-up' (usually in about half an hour). When the work is firm enough it should then be scoured to compact and consolidate the surface.
A straight-grain float (that is to say, one with the grain running the length of the float) can be used for the scouring process, but if a very flat surface is required, a cross-grain float is better. The 'cross-grain' prevents the edges wearing down and ensures that any projections are shaved off as the float passes over the surface. Cross-grain floats are only used to rub or scour over a surface and any specification that refers to using them to apply lime plaster is clearly wrong.
Depending on what is required, the surface can then be worked over using either a trowel, to achieve a fine closed finish, or a combination of wooden and sponge floats, to create an open textured finish. Some water is likely to be required in this process, splashed on with a brush.
The rubbing and scouring process required to achieve a suitable finish can present a problem when patching up to old plaster. The first important point to note is that the exposed edge of the old plaster is likely to suck more moisture out of the new plaster than the wall itself, so more water will be required to control this suction at the edges. Once the area has been patched, particular care will be required when rubbing over the setting coat, to avoid forming an indent at the junction with the old plaster.
In some cases, where an absolutely blemish free surface is required, the setting coat mix can be gauged with Plaster of Paris (a form of gypsum) to minimise the need for scouring. However, the decision to add another material needs to be taken carefully, and the visual compatibility of the repair needs to be balanced with its technical compatibility. This brings us back to the need for traditionally skilled plasterers who understand the materials they are working with. Without these skills we are lost.