Hot Stuff

It has been a few weeks since my last entry with the Easter holidays, a trip to São Paulo, house building and most importantly last week end´s Ox Tail soup, with chocolate, cinnamon and star anise.... Delish!!!!!
The trip to São Paulo was twofold, to buy gemstones, some tools and consumables for jewelry making; as well as to visit friends and family. We also took the opportunity to show my work to a number of people and it was I am pleased to say, well received.
http://www.pinterest.com/essexvig/my-work/

I am almost at an end to the silver sheet that I had bought in London and Buenos Aires previously, so the time had arrived to start doing something seriously about producing my own material, as I have mentioned before you can´t buy sheet silver in Brasil.
Having researched what should be done I felt fairly confident that things should turn out OK. Looking at what I have ended up with after a few hours work I am quite happy with the results.
My first step was to season my melting dishes, one of the things I bought in São Paulo was a kilo of borax, which I noticed was from Argentina, which reminded me of a trip a few years ago to Gran Salinas an area of salt lakes near the border with Bolivia.

Despite it being winter the sun shines brightly and the local artisans who make and sell souvenirs carved from salt make sure they cover up well to shield themselves from the UV glare.

What has this to do with casting silver you ask, not a lot except we stopped at a local homestead for a meal and the owner worked previously in a mine locally that produced....Borax. He unfortunately had an accident in the mine and could no longer do the heavy work. 

It is a long way from anywhere, something to ponder when you use borax next.

I managed the seasoning of the melting dishes (I bought small medium and large as they were relatively cheap) without mishap except one dish had way too much borax but this was easily fixed by pouring off the excess. I allowed them to cool before use and the coating was a nice even clear glassy layer. 

So on to the casting, I found this video a few days ago and despite it being in French I picked up a few useful pointers. 

http://www.youtube.com/watch?=Bt6iyNsJdPA&list=PLO7Qs9JKDBsXEH7KlqoAnNM7TTmb6-xjL

The ingot mold was lightly oiled which stops the silver fusing onto it, then positioned at the back of the melting dish to preheat it. The only problems I had with the new ingot mold were with the clamp that fixes the two halves together, firstly it was painted, and secondly the knob on the clamping screw was plastic. So yes we had the sweet smell of burning paint and a plastic knob that squished when I went to release the clamp. No dramas however, the paint is almost all burnt off and I will drill a cross hole in the clamp screw for a tommy bar.

This is the model torch I have, it is a fairly large torch and runs off bottled gas, it proved more than sufficient to heat the silver in a reasonable time and provide enough heat to preheat the ingot mold which is quite large. 

This is the style clamp I have for manipulating the melting dish, it holds a range of sizes and clamps them very securely. 

Once the silver began to show signs of becoming hot, small amounts of borax were sprinkled over it. This does two things, firstly it acts as a flux and protects the silver from oxidising and secondly  it cleans up the metal, it turns a dark brown colour , this I understand to be the copper oxides in the silver.  

The first small batch was my bench scraps, this produced the smallest ingot of 13 grams, the problem here was lack of heat in the silver and not having the torch directed onto the silver whilst pouring. 

The second attempt, the mid sized ingot of 24 grams again did not fully fill the mold, I tried the larger mold as I thought the length/width ratio was too great in the first attempt. 

The largest ingot of 69 grams, my third and final attempt was reasonably successful, the main fault I can see is that I have been a little over generous with the borax and I have some inclusions from this. I don´t believe this will be a drama as they are on the outer edges, and a spell in the pickle pot should remove them.

I believe this cast was better for a number of reasons, however there are still signs that the silver was starting to chill looking at the edges of the ingot. 

I am fairly certain that for the first two casts the silver was not hot enough and not all the silver was completely molten, I thought of melting chocolate or ice cubes in a drink was a close analogy.  

So whilst there was molten silver there was also chunks that were not, and these would cause a blockage and prevent the silver from filling the mold. 

The melting point of silver is around 960 deg C. So if my silver was at this temperature the silver would be molten, but would have insufficient heat to lessen the chilling effect when the material enters the mold. So whilst the mold has been preheated it will still absorb heat from the silver resulting in it becoming solid or pasty. I have since read 1050 deg C is a good temperature to aim for, this ensures that the silver is all molten and you have sufficient heat in the molten silver for the chilling effect of the mold to be diminished. I don´t have any means of accurately judging the temperature of the melt but I used my stainless steel solder pick to check to see if there were any portions of the silver still in the solid state and once satisfied I held the heat on for a short period. 

For the first two pours I neglected to continue heating the silver whilst pouring, so I believe these factors contributed to the final successful ingot. 

The largest ingot measures 16 x 4 x 100 mm

My rolling mill does not have reduction and I am not to fussed about rolling everything down to thickness. I have seen on a few videos from Japan that after casting their ingot they forge the ingot to a size close finished size, then finish in the rolling mill. Whilst swinging a hammer is is not exactly my idea of fun, I think it is preferable to hours of rolling metal to size.