Blacking aluminium

It's probably too late for Rik, but I've just come across this useful page in Engineering Workshop Data by Arthur W. Judge (Caxton 1950). I'll add it in case it's of use to anybody :

Dissolving caustic soda in water is exothermic, so I'd start with cold, then warm it up if need be. Caution, hot caustic soda solution is nasty stuff, be very careful and use goggles and thick rubber gloves

A couple of comments on the latest from Georgineer and Duncan W:

1. There are likely to be two difficulties with the Anodising recepies - the difficulty, now, of poppping down to Boots for a few ounces of chromic Acid, and then, not knowing what the current density should be. It is easy to measure the current in Amps, but no clue is offered regarding the relevant area. Is (or was) current density measured in Amps per square inch, or per square foot, or some other unit? Sorry, I can't help, here.

2. And yes, a useful reminder that dissolving caustic soda (or potash) in water generates lots of heat, so do it slowly. It is interesting, though, that washing soda shows the opposite effect. Chuck some in your bath (Like we did in the old days) and the water gets colder. Most noticeable if you take a handfull of soda and dip it into the warm bath water. The things we used to do before the internet was invented!

Cheers, Tim

Tim, I remember bathtime when I was youngster back in the 50’s, my abiding memory is of having to sit on undissolved washing soda crystals when getting into the bath, funny what memories we have stored away in our brains. Dave W

I would have thought that washing soda, being quite strongly alkaline, would be tough on your skin. We used sodium sesquicarbonate as bath salts, which I presume is half way between sodium carbonate and bicarbonate (more properly known as sodium hydrogen carbonate). Probably still tough on the skin, but we were young and resilient then.

George