air holes / blow holes in gold and silver

[philip]

Hi all
I am getting a lot of air holes / blow holes pits whatever you want to call them in both silver and gold when I melt it down to make wire or sheet. I use a clean crucible for each metal I use borax to prevent oxidation and melt using a maxi gas and oxygen mix. I also have mapp gas but i find its just not hot enough. I have started playing around with the delft clay and mould and have made some great items only to have them spoiled by the air holes which can not be polished out. Anyone any ideas how I can get better results?
Thanks philip

[josef1]

You cam get porosity (air bubble type effect) If you overheat the metal ,continue heating after it melts watch and wait till it starts spinning then pour.

[philip]

If I have over heated the metal can the damage be undone by re melting at a lower heat. Also I melted an old 9ct ring with a little extra scrap and it has turned a ver pink almost red colour that I cant shift no matter how many times I melt it or how much borax I use. Is if possible to burn gold?
Philip

[art925]

It could be that your gas mix is too oxygen rich. Reduce the oxygen from a fierce hiss to a fizz.

[art925]

Philip, I have found these articles as a start to your research ... http://www.professionaljeweler.com/a.../0699fys1.html http://www.ganoksin.com/borisat/nenam/craftim.htm

[edward soye]

For the red you mix a fresh batch of sparex and then with no borax on the piece warm the ring with the torch and let it go brown and then quench the ring in the fresh pickle solution, alternatively you can mix a batch of sparex and hydrogen peroxide 50/50 mix this can help remove the copper from the surface.

If you are not getting a great cast for your ingots you may have things too cold or too hot you need to judge and when the metal is freely moving in the crucible like Mercury then it is ready to pour if it spits or smokes a lot it is too hot. The ingot mold needs to be warm I put mine in a 350f oven for a while and I apply a bit of soot from the gas torch or candle flame to the mold, I was taught to use oil and they both work but I find the soot a bit better.
For the pits and holes don't be discouraged they happen to everyone. If you can recast and achieve a cast with less or no pits it is best but part of goldsmithing is to deal with these little #%##%%
I use a tool made by Vigor it is called a porosity killer it is a cushion shaped steel plate on a mandrel, you can make it but I wouldn't mine is ten years old and cost 19.00cdn
I use it to compress the surface and then once no holes are visible keep compressing in several directions, careful not to move too much metal and once it looks good give the piece a fine sand with 2000 grit emery and then a light polish and you are done. Some times when it is really bad I will compress and then aneal and then compress and then sand and polish. Not my favorite thing to do but when I was starting out and ask about porosity I got mislead to believe it was very rare b.s.


When I got to tour Cartier factory they said a polisher would apprentice for at least ten years before they were considered a master it makes you think!

[philip]

Thanks will have a look and letyou know how it goes

[Dennis]

Of course it is possible that the copper deposit is due to a fragment If iron in your pickle, or your melt.
See whether pieces of scrap silver turn pink too, in which case scrap your pickle. Dennis.