Limestone Walls, and the Problems They Cause

Limestone is a popular choice for external walls in and around Perth. I’m not sure how the costs compare with other materials, but it’s a poor choice, for several reasons.

Initially it looks terrific, but before long, it starts to degrade. This begins with a blackening on the upper surfaces – the ones most exposed to the sun – and before long green plant growth becomes apparent.

Attempts to remove it generally fail, and if you try to use very high pressure water to remove it you start blasting holes in the limestone, as it is very brittle.

So what is happening?

The most important chemical process for life on the planet is photosynthesis.Carbon dioxide is converted to biomass (plant life) according to this simplified equation:

CO2 + H2O = C.H2O + O2

C.H2O is the basic formula of a carbohydrate, which wood is made out of.

So the bacteria that catalyze this process use sunlight to convert CO2 to plant matter and oxygen.

Unfortunately, limestone walls are also a carbon source, as they contain carbonate. So what happens is that the same bacteria that catalyze photosynthesis (cyanobacteria) do the same to your limestone wall. The carbonate in the calcium carbonate from which the wall is composed is literally converted to plants. It begins with a blackening of the surface, followed by the development of green plant material.

Contrary to popular belief this is not mould – it is plant life. And we know this because it occurs not where there is the least sunlight, but the most – on the top of walls.

And there is nothing you can do about it. It is possible to buy sealers that are supposed to stop this happening, but the problem is that because limestone is so porous it’s very hard to get it to seal completely.

The only real solution is to not use limestone in the first place. Unless you like the overgrown look that quickly develops, as some people do.

If you like the pristine, clean look, the best option by far is to use synthetic limestone – it’s made out of limestone ground in with cement. It looks great and will never degrade





































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































Breaking Bad #2: the science

How accurate was science in breaking bad?
In general it was pretty good. It is clear that the producers of the show have put some effort into ensuring scientific credibility, and appear to have consulted the scientific community.

But having said that, however, there were a few problems.

  1. Walter White is a physical chemist. We know that because his expertise is X-Ray Crystallography. The manufacture of Crystal Meth is very much the realm of a synthetic organic chemist. It is therefore rather unlikely that a physical chemist would be able to, off the top of his head, manufacture crystal meth to >99% purity on his first attempt. Most synthetic organic chemists wouldn’t even be able to do it without a substantial amount of research, trial and error
  2. In one episode Walter White makes a battery out of some acid, coins and bolts that he has lying around, and then uses it to start the RV. Even if he somehow managed to get the right voltage, there is no way that such a small “battery” would generate the several hundred amps required to start a car.
  3. On several occasions Walter pinches Hydrofluoric Acid from the school store to dissolve bodies.
    As the strongest acid in existence, and the most dangerous (S7 Poison, DG Class 8 (6.1), PG 1) there is no way in hell that a school would use such a dangerous chemical, and certainly not sitting loose on its shelves. Most industries that use this stuff (I used to work in a place where we used it to make a brick cleaner) store in a dedicated cage that is kept under lock and key, with warning labels plastered across it.

Shannon Lush gets it wrong #8

Amongst the many errors in the advice that Shannon Lush gave in her advice on 6PR was her advice not to use Oven Cleaners to clean ovens, but to use a mixture of bicarb soda/vinegar to clean ovens.

In other words what we are expected to believe is that the chemists who have designed products specifically to clean ovens don’t know what they are doing and that she, with no scientific qualifications whatsoever, knows better, and of course the very opposite is the case.
Oven cleaners work by a chemical process called saponification. That is, they literally convert the oil and grease in your oven to a soap.

Here is the reaction

This is why it all wipes up with a sponge so easily afterwards – you’re literally wiping up soap.

Bicarb will not do this as it isn’t alkaline enough, and especially after it has been neutralised by adding vinegar to it.