Originally I intended to wainscot and possibly fully or partially line the walls but after removing some of the render around a bulging crack high in the wall and after experiencing how hard it is to drill holes in the bluestone (basalt) for the new fireplace surround I decided I'd commit to strip it all off and re-render.
I will be replacing the existing rough render and it's coating of non-breathing acryllic (a.k.a. plastic or latex) paint with traditional lime render to a smooth level finish and a lime wash. This will restore the two important abilities of the walls to:
regulate the humidity in the room by allowing the walls to absorb/release moisture from/to the air, and
allow the render and mortar to self-repair fine cracks that naturally occur with expansion and contraction.
Of course, there are other reasons to re-render in traditional lime.
the light that reflects off calcium carbonate is different. Softer but brighter. See box ->
the satisfaction from living with something closer to the original.
The beauty of lime
Limewash is uniquely beautiful, incredibly bright and gives off such a depth of colour that it appears to shine. It is impressively vibrant at night and can lighten up the darkest of interior rooms or heavily-shaded courtyards. On the Atlantic coast, light houses have been limewashed white in living memory, as have day marks for shipping. Whether used on stone and earth houses, cathedrals and dry stone walls, or palaces and field barns, limewash has stood the test of time and has protected and decorated the structures of the world.
So what gives limewash these wonderful qualities? The beautiful luminosity of a limewashed surface is due primarily to the reflection and refraction of light. The intense brightness of non-pigmented limewash is largely due to the reflection of light back into the viewer’s eyes. A more subtle, but startling quality, is the refraction or bending of light through crystals of calcite (the carbonate mineral that forms through the process of carbonation). Light refracted through calcite splits into two rays, one fast, one slow, and the visual effect is a doubling or twinning of the light emitted by the crystal.
The intensity of the light itself is not changed, but when compounded by millions of microscopically-small calcite crystals increasing over time as the surface continues to carbonate, the effect is a surface that appears bright and vibrant with subtle internal texture and variance. This mottled effect provides depth and interest to the viewer
34. The crack that started it all.
35. Three-quarters of the first wall stripped after approximately 12 hours of effort. The old render comes off easily when chipped at using a crowbar.
A horizontal 400 mm wide band of cement render applied at the level of the wood blocks was harder to remove but fortunately could be pried off in small chunks after a little loosening without any damage to the stonework. Similarly around the doorway, which is a later addition, and will be harder to remove
36. A close-up of wood blocks embedded along the wall at chair-rail height. According to a former student pupils faced this wall in class, so perhaps these are to support blackboards attached to the wall. I don't know yet if they go all round the room, in which case they may be to support wainscoting that has been removed in the past
37. Wall completely stripped of old render. Hard cement render around the door will be removed when the doorway is rebuilt. This is a stitched panoramic, things aren't as wonky as they appear here
38. Fireplace with render stripped from wall. Continuing to remove render around the room and still considering whether to flat render walls or leave some stone exposed.
39. Clues to previous colour schemes. The yellow plastic paint at bottom is the main reason for the complete strip and re-render. This patch of wall was around and under a timber moulding installed just before the yellow colour was applied. Under it is the off-white colour, which may be an old lead paint, it has that linseed oil look about it. Below that is the teal colour. I found other areas of this that were much thicker and much more the colour of copper oxide and penetrated right through the plaster so I'm imagining it may be a lime wash
I decided to buy matured lime putty rather than make my own. The saving in time to make and store and mature it is worth the additional cost and I get a known quality product.
40. Just an idea but increasingly unlikely
Any stone left exposed will need some kind of sealer applied to the lime pointing to minimise dust. Traditionalists suggest "lime water" as the recommended stabiliser. This is a saturated solution of calcium hydroxide in water, about 0.2% or 2g per litre. It is usually made by adding some lime putty to water, mixing really well and then leaving the excess to settle before decanting the clear liquid that is the lime water.
I guess it works the same way lime scale deposits work and anyone who has cleaned a shower stall knows how hard it is to remove lime deposits.
I started out assuming and not properly understanding what "suction" referred to in plastering but fortunately I haven't started rendering yet. It is the action of the substrate in drawing water out of the render and high suction causes the render to dry too quickly making it brittle etc. It is not what makes the render stick to the substrate. High suction requires treatment to reduce it, usually by spraying with water enough to dampen but not leave the wall dripping wet.
The sequence on the lounge room walls will be:
a haired scratch coat
a coarse stuff coat
normal finish coats
It's very difficult and/or expensive to get hair in Australia so I'm going to use a synthetic fibre instead. It's a polypropylene fibre used in cement work including stucco and it has some advantages, including not needing to be slowly teased into the mix.
I have bought a couple of 15kg bags of fly ash to use as the pozzolan if I need it.
I've trialed some mixes of render to get a feel for the preparation, application and the effect of the dampening treatment through the initial curing stage of the products in different situations. Also looking at the colour change of the product as it cures.
3:1 course sharp sand and undrained lime putty. This is a well graded clean sharp sand pale yellow in appearance. I'll measure the actual void ratio for this material and adjust, but as it was it would work as a scratch or course coat.
3:1 river sand and drained lime putty. Predominant grain sizes vary from 1mm up to 5mm, with a few pieces over that size, medium sharp to smooth grains. Yielded a very grainy mix and not very sticky on the trowel. As with the sand, I'll measure the actual void ratio of this material. I think this mix would be ok for filling between stones and dubbing out.
To measure the volume ratio of sand to space between the grains, or void volume, I used the saturation method. I put 150 ml of bulk material into a small bottle and then slowly poured water into it until the water started to saturate the top surface of the material. For this measurement a tall narrow vessel is more accurate than a wide flat vessel. The volume of water used is the volume of the voids. For both materials this was within one or two ml of 50 ml, so the 3:1 basic ratio is confirmed for both types of sand.
After that I tried a 2:1:1 mix of river sand, sharp sand and lime putty. As expected this mix is a little smoother than the straight river sand mix. I used this to do a minor repair to a corner of the rendered plinth around the outside of the bluestone walls where some render had broken off, so I will see how it looks after a few days.
I don't need to colour-match any of the interior product but will for the exterior re-pointing where it's clear that ground charcoal and ash has been used to blend with the bluestone. This accentuates the straight lines of the pure white lime ribboning that was applied originally.
Results after a couple of years - yes it's been that long
The sharp-sand-lime mix has worked very well, it is hard and takes some wear while the river sand mix does not take wear as well. The corner repair did not work. It came away so I need to work on getting the bond right.