I have had the same problem with too much dirty borax clogging up the crucible… after a while even trying to melt it out with oxyacetylene still leaves a thick residue of sticky borax… I haven’t had luck cleaning the crucible out except by grinding away the excess… Does anyone have a suggestion?
I try to use only the minimum amount to keep the molten metal clean.
Pouring an ingot hasn’t been too much of a problem for me. I have an adjustable clamp together mold that will pour ingots from square to rectangular and of different widths, as the two parts slide together before getting clamped into place… the mold also has different size holes to pour rods to make wire.
because its a top pouring mold, the metal assumes a rectangular shape and is smooth on both sides and bottom… only the top of the pour is not flat…I have had some borax sticking to the ingots but not much at all… I’ve tried using a open pour mold but haven’t had much success as with my top pour mold. I do use oxyacetylene to superheat the metal prior to pouring and do heat the mold until it’s smoking hot… oil or Vaseline does well and carbonizes… the mold has to be smoking hot, hot enough that the smoke almost fumes out completely, but never dull red hot, I would say near 500 degrees F, like an oven cleaning setting… I’ve found that preheating the mold and superheating the metal gives a good pour… if the mold isn’t hot enough, the metal will not pour completely and fill in the mold… if the metal is not hot enough, it cools too quickly to pour well also.
My problem isn’t with borax sticking to the ingot but sticking to the melting dish or crucible.
Pure PMs like Gold and Silver do not need Borax at all as long as the melting dish has been glazed with Borax.
Borax will dissolve in boiling water and maybe a little dilute Sulfuric acid.
I don’t know what this will do to a melting dish though.
my melting dish is coated with a brown dirty borax flux that is very sticky and syrupy when heated to try to pour most of it out… when cooled down, it’s like a glassy ceramic glaze. I’ve soaked it in water overnight to try and get it to dissolve but that hasn’t cleaned it much at all… I think that the melting dish can withstand soaking in a bucket of water for a week…or longer as necessary to get the borax gunk to slowly dissolve… I don’t know if anyone has a good idea on how to get rid of it… please respond if you know a technique that works faster…
you will see a chart on the solubility of borax in water. What I get from the chart is that borax dissolves better in water over 140 degrees, and the hotter and the more water the better (within reason) as you get a saturated solution in smaller amounts of cooler water, meaning the borax would stop dissolving.
By putting it in a bucket and changing the water daily, the solution won’t become saturated. Something worth trying, rather than boiling it in a big kettle on the stove… acidifying the water with a little muriatic acid might work and should not harm a ceramic fused silica melting dish… boric acid is more soluble than borax so it’s definitely worth trying… I don’t think I can clean it any faster… but thank you very much.
What is used in refining is hot dilute Sulfuric acid and time.
Since I have not done it myself I have to rely on the description by others.
And hot in this case would probably mean 50-70 centigrade and 5-10% Sulfuric.
Time is also an factor, I really do not think this can be rushed much.
What is your intention here, just clean the dish or are there values trapped in the gunk?
If you have another melting dish you could use that in the mean time.
The important thing with melting dishes is that they need a proper glazing prior to the first use.
Which metals do you melt in it?
I have used it to melt sterling, fine silver and gold. I agree that hot water always works better than cold.
If there’s anything trapped in the gunk, it would be negligible… Since I already have poured ingots, I don’t need anymore right away… I have a lot of time to try and get it to dissolve. H2S04 is less corrosive than HCl… I could use NaHS04, pickling acid instead as I don’t want to have to buy either pure sulfuric acid or car battery acid…I’m in no rush to clean it, so just letting it sit is fine with me… I also understand and correct me if I’m wrong, that melting dishes as made fused silica, so that they should be resistant to prolonged immersion.
Is it common to use same dish for several metals?
That is a big no no in the refining community where one goes for high purity.
I would suspect it may cause alloy offsets risking brittleness and such.
This might be wrong though.
it could be that i used 20 mule team laundry borax to coat the dish… it’s not dehydrated borax and fluffs up before it melts down into a sticky paste…would soldering flux work better?..it’s much more expensive…
You need to heat it until it gets thin enough to flow.
Anhydrous borax is just ordinary borax that has been heated enough to drive off the crystal water.