Tuesday, 9 September 2008

Disaster!

About 15 years ago I made this buckle. It's cast silver, and represents the skull of a Babyrusa, which is a wild, Ethiopian pig. I had it on a black leather belt. It's not shiny because...well, read on.

Not long after we arrived in Winchester the other week, the bar attaching it to the belt came adrift at one end. This was clearly slightly inadequate soldering on my part, but should have been simple to fix. All I had to do was resolder it, pickle it and polish it up, before reattaching it to the belt.

The key words "pickle it" are where I went wrong. When you solder silver, you have to use flux to make the solder flow properly, and this results in blobs of a glassy slag stuck to the work when you finish. To remove these, you drop the piece into 10% sulphuric acid, which dissolves the slag. It does attack the silver, but much more slowly than it eats away the slag.

For decades, I've used pickle at room temperature, but recently acquired an old filter coffee maker to use instead. After cutting the top off, I now have a perfect pickle container which keeps the acid at about 85°C.

It's common knowledge that when you warm things up, chemical reactions go faster, which was why I started using the coffee maker. If you can get away with a few minutes, why wait half an hour? After soldering the bar, I dropped it into the hot pickle, and at around that moment, Per arrived.

Per was Jenny's first PhD student, many years ago, and is now Professor of Organismal Biology at the University of Uppsala in Sweden. He was visiting Jenny for a few days to work on a project they're collaborating on.

Some 3 hours later, I remembered my buckle in the pickle. As you can see, it is ruined, and not hard to work out why. For each 10°C rise in temperature, everyday chemical reactions go twice as fast. I'd normally leave a piece of silverwork in the pickle for 20 minutes or half an hour, and expect it to come out nice and clean.

At 85°C I calculate the reaction would go 20 times as fast, so a minute or 2 would have been plenty. I gave it 3 hours.

As you can see (especially if you click the image to see it big), the acid has eaten right in places. Severe worm damage! The bar I carefully soldered back on is so thin and flimsy, it's completely useless. The whole buckle is scrap. What a twerp!

Fortunately, I still have the original silicone rubber mould I used. I can make a new wax, and should be able to cast a replacement when we go for our silversmithing weekend in October. Hope so. I was always rather pleased with that buckle. One difference, which might be interesting, is that the place we go now only does sand casting, so the result will be substantially different.

The way I'm used to casting silver is the lost wax process. I usually start with plasticene on a sheet of glass to make the original. Then pour silicone rubber over that to make a mould. Get rid of the plasticene and pour molten wax in to the rubber mould to get a positive. Stick the wax in a steel cylinder and pour investment (a bit like plaster of paris, but heat-resistant) around that. Melt out the wax in an oven (hence "lost wax"), then pour molten silver into the space previously occupied by the wax. Break away the investment and there is your raw silver casting, waiting to be finished. Although there are many stages, they're all pretty simple, and you can get brilliant detail in your casting.

Although the sand used for sand casting is pretty fine-grained, it's still much coarser than the investment, so inevitably the end result will be rather different. Might still be perfectly fine. We shall see.

Oh yes, must not forget. There will be more silence. Sorry. All will become clear in due course.

6 comments:

Can Bass 1 said...

My dear sir! You seem to be going to an inordinate amount of trouble for a mere belt buckle. Try braces.

Lindy said...

First off I thought you found or sculpted a really cool dionasaur skull.

Second - my GAWD ARE YOU A TALENTED FREAK OF NATURE.

Third - Dude, I'm sorry.

Fourth - Hey it gave you blog fodder ! :P

(d-hor)

headless chicken said...

Oh dear, this all sounds very complicated and clever to me.....fascinating though!

Rob Clack said...

It may be a mere belt buckle to you, young man, but it's the product of my own fair hands! Braces do have a certain appeal, but only if they have flags printed on them, I think.

Thank you, Lindy, and you're quite right about the blog fodder!

Hello HC, haven't heard a peep from you for months. I notice your profile shows you no longer have a blog, which is a shame. I used to enjoy reading what you wrote.

A Mother's Place is in the Wrong said...

Hey Rob, just catching up at last, and see I have missed you again. Hope you are off somewhere having fun. That belt buckle is scarily good - look forward to seeing its reincarnation. M xx

Rob Clack said...

Thanks Margot. I've been struggling to get a new posting up, but blogger is having difficulty uploading a video clip and I can't seem to work out what to do. I'm determined to get it done this evening!