Lime, cement and plaster

preview of YouTube video

Professor Lewis explains  various interior and exterior concrete finishes.

Video Transcript

[Professor Miles Lewis]

I'm now talking about limes, cement and plasters. When Europeans settled in Australia lime was burnt locally, largely from shells from Aboriginal middens, the Aboriginals had left from their feasts whole piles of shells around the coast and they were used for lime. Later on limestone was more commonly used and in some parts of Northern Australia coral was used, they all have the same process involved of using calcium carbonate (CaCO3) to get that lime.

ABP-HBM-1.108 Handmade clinker brick 1

So in some of the early examples, we find this. This is mortar from another brick from Tasmania and the little bits of shell actually in the mortar and that show that this was burnt from shell lime in those early days. Now, later on, some natural cements were found which are rather stronger than lime, and have some capacity to resist water, say rain and so on, and then much later on, Portland Cement, which is the modern cement, was both imported from Britain and made in Australia and that sets underwater and has a very strong hydraulic capacities, can be used for harbour works and so on. But at first you're creating a building in a material which would dissolve in the rain so you tend to have not much ornament on the face of the building and a good coat of paint to protect it.

This is an early example. It's a very primitive building because you can see these are the remains of timber saplings. So it's timbers saplings on the outside of the building running horizontally and then this on the face, a smooth facing which looks quite sophisticated when it's finished because it's been made  totally smooth and ruled in lines to look like the joints in masonry, and that's the way lime is normally used. It was to imitate a masonry building and sometimes it had a coloured wash, sort of brown or creamy coloured wash to enhance that effect. So we find on this primitive building that being done but also on other buildings like this example here which is from a building in New South Wales. Again they were sapling and mud buildings but rather more sophisticated than the one I just showed you. And then again also, this which has been on the same effect, a ruled line, the lines can be ruled either with just a groove or with a crown, in them or both, but this one has been on brick as we can see the shape of the brick. So the same system is being used for all sorts of different wall finishes to give it the appearance of stone.

ABP-HBM-2.104_a Concrete Wall material 2

Now here's an example of a primitive form of concrete, quite strong. It's from a butter factory in Gippsland and you can see in here all the local pebbles, some river worn pebbles, local materials. Smooth pebbles are not desirable for concrete, it's better to have sharp broken stone than these smooth ones, but this is nevertheless pretty strong. In this case, though unusual detail here on this side, what appears to be some sort of burnt cork material, and this is because it was a butter factory and this was the insulation in the walls of the factory.

Now when you come to true concrete, it's a material of course very strong,

ABP-HBM-2.116 Concrete core sample 3

like this is a core sample which will be taken by an engineer and it cuts right through all the aggregate, the stones inside it, which are as I said sharp stones in this case. Usually, when we're investigating buildings for historical purposes, we're not so concerned to get a sample like this, which can be a standard size that can be tested for strength but to get the details of these stones, and the contents of the material. So we find rather different components. This here for example is a cement render, the face of the building has this pink paint on it, behind it is this little layer of cement and possibly plaster, and it contains within it hairs, commonly on the face of a building in fact on internal plastering as well, you would add either animal hair like horsehair and so on, or occasionally some other material like straw into the plaster to reinforce it.

Many buildings were made of solid concrete so in the 19th century when you speak about a concrete building you're not assuming reinforced concrete as you would today. It can be just solid lime concrete. But here's a variation, this material here contains coke breeze so it's concrete but instead of having stone like basalt chips in it, this is coke breeze which is a byproduct of gas making, so from the 1850s onwards there are these lightweight coke elements were available for almost nothing, and they made a very good insulating lightweight concrete. Not only was it used for structural walls but also for bricks.

Here's an example, this came from a building which had concrete bricks and then on the face of it rendered again to look like a stone finish, but in this case, with reinforcement to hold the rendering on tightly, of wire mesh. This coke breeze material was also used for reinforced concrete, but not for very long reason is that coke breeze: the product of gas making, contains a lot of sulphur waste in it, and the sulphur tends to corrode the reinforcement, so it's not good for reinforced concrete but quite good for mass ordinary concrete.

Now, in reinforced concrete of course you have good quality concrete and you have rods of steel to take the stress of the appropriate points, and this is a very small piece of that,

ABP-HBM-1.306 Jenning's brick 4

you can see where the steel has come out and this is important because this came from a very important building, the Dennys Lascelles Austin Wool Storage in Geelong long now demolished, a really major piece of engineering work in world terms, but its problem was the reinforcement didn't have enough cover, you have to have enough thickness of concrete over the reinforcement to stop water penetrating and the reinforcement rusting, and also, in this case, the quality of the concrete was questionable, and there were sea breezes coming in, so salt came in. So you can see here the reinforcement is not bonded properly with the concrete. Ideally, you get a firm fix between the concrete and the iron.

In the 20th century, systems were devised for building in concrete, not necessarily reinforced at all, but building in units of concrete and this is one of the most famous of them: this piece here is a Knitlock patented by the architect Walter Burley Griffin the American architect, who came to Australia, patented in 1917, and what you're seeing here is two units, which slot together.

ABP-HBM-2.105 Knitlock concrete 5

The ribs make them join together so they're both cast with a perfectly good flat face. Then you put them together to create a wall.

Knitlock System 6

Now these are called the tessaral units and in the wall you have the whole wall built in that quite thin dimension of tessaral units, but at intervals, vertical ribs called vertebral units, vertebral like a spine, which gives the strength to the wall and that's the basis of Griffin's patent.

On the face of the wall, in the 20th century you might in the arts and craft tradition try and make it a cottagy by having a finish like this, this is rough cast, so it has perfectly ordinary cement render first and on the face of it, this finish which has small stone that has been thrown against the wall to give a sort of rustic effect. Now from there, we should turn to the internal plastering finishes, commonly inside plaster would be plastered over laths not layers but laths, these are ideally split timber. Split timber is better than sawn timber, because it has a better bond to the plaster, but sawn timber was sometimes used. So these are placed over the whole framework in the wall, with small gaps between a plaster is plastered over and it forces its way into the grooves between and therefore holds onto the to the wall, and that's the standard way of plastering right into the 20th century.

However, there were various substitutes and this is one, called expanded metal.

ABP-HBM-2.101 Expanded metal lathing 7

Expanded metal achieves the same effect, the plaster can be pushed through it and therefore will bond. Now it's called expanded because it's made from a single sheet of metal which has a number of little slots punched in it, and then pulled out sideways to create a diamond pattern which makes this a very useful mesh, the same material came to be used even for some forms of reinforced concrete, although originally it was devised for plastering.

Now, the finished plaster may be common gypsum plaster but at some points with this elaborate decorative work or where there's some corners that might be damaged by passing traffic, Keene's cement is used. Now Keene's cement, although it's called cement, is a form of hard plaster and another one Martin's or Parian cement are used for the same purposes. Skirtings for example, which you may think are going to be of timber often proves to be of this hard plaster or cement.

ABP-HBM-5.301 Skirting 8

This is an example of skirting. It's not Keene cement it's an ordinary sort of concrete cement but you can see how the wall has a line of plaster which is the surface of the wall proper and then the cement skirting has been put on the face and run in plaster to give a finish which when you look at it you might assume is timber but of course, it's not. And then the same way for mouldings like this you may need to again use Keene's cement to get greater strength. These are quite difficult to make and I can't describe it in detail here, they have undercuts which means you tend to make them in a silica gel mould, which can then be peeled off to give that a complicated shape.

Image & 3D model references:

1:  Small handmade clinker brick of red/orange clay, with a roughly rectangular handmade frog. Mortar - attached to flat face - contains pieces of shell and charcoal.

2: Concrete wall material from the Butter Factory in Cowarr, Gippsland. River pebbles visible. Burnt cork visible on one face which was used as insulation.

3: Cylindrical core sample of a reinforced concrete wall with slender reinforcement rod passing through the centre

6:'Knitlock' system: D L Johnson, The Architecture of Walter Burley Griffin (Adelaide 1977), p 59 [edited].

7: Concrete reinforced with expanded metal

8: Section of skirting; exposed face with several layers of paint, the top layer is green.