Expert: Thomas - 4/27/2004

Question
Hi Thomas, I am a tool & die maker and have often thought about making a small mould and melting all my old gold rings and broken chains into a small gold bar using our electric heat-treat oven. Can this be done, if so what is the melting point and things to be awhere of. Thanks' Don  

Answer
Dear Don,

This should be an interesting project!  I am not familiar with the temperature ranges of the oven you will use.  All the melting we do is generally by torch or with a small electric melting pot.  Before going into temperatures for melting the gold scrap items, I will make suggestions about melting gold in general.

First of all, you must have a suitable container for the metal while in the furnace.  For most gold, a fired ceramic clay or fired silica crucible or melting dish will work just fine. A graphite crucible will work for yellow golds but not for white gold since the nickel in the alloy reacts with graphite and can contaminate the melt. Graphite crucibles look something like the carbon used for brushes in electric motors.

If you plan to melt the metal in the oven then pour it into a form for the bar, crucibles similar to those found at these web sites will work.  Search for crucibles, melting crucible, or casting supplies.  You might have some similar heat capable material on hand to use for the melt.  I will make a suggestion later on how to make a temporary crucible or bar mold.

Both of these companies should show crucibles.  (The links are not live and must be copied and pasted to the browser.)
Contenti Co.    http://www.contenti.com
Alpha Supply  www.alpha-supply.com

Ideally, the crucible should be lined with borax glass. This means taking boric acid/ borax powdered and heating the material in the crucible to a melt, allowing the boric acid to flow around and form a glass like coating on the crucible. This procedure protects the crucible and protects the metal from flake off of the crucible. For one melt, you can get by without this process.

MELTING PROCEDURE. The metal is placed in a suitable container (crucible), several pinches of boric acid are placed on the metal, and all is placed in the oven and heated to melt. The boric acid is the same as boric acid powder found at pharmacies.  The boric acid acts as a flux to prevent oxidation of the metal during the melting process.  This is necessary for a clean melt.  Ideally, an oven with little oxygen or even a cover gas will produce the cleanest melt.

Heat the metal to about 100-200 degrees f. above the melting temperature. Overheating will damage the melt by affecting the alloy content of the gold alloy.  About 100-200 f. is the recommended temp over the melting temperature to allow for a “flow” of the metal.  (Some alloys have a flow temp slightly above the melt temperature.)  If you plan to pour the metal into a mold, this heating over melt temp is essential and pouring must be done quickly before the metal begins to freeze.  Hot stuff! Be careful!

When melted, the metal will tend to go into a half ball shape or at least have a rounded top surface. The strong surface tension causes this.  If you have a melting container of a bar shape, the metal will likely fill the recess but will still need some rolling, milling or hammering to reduce the cooled metal to a flat bar shape.

HOMEMADE CRUCIBLE/ POURING MOLD.  The kind of plaster used by jewelers and industry for “investment casting” may be used for a temporary crucible or to form a bar shaped recess for pouring the molten metal into.  The material is mixed with water to the consistency of a quite thick milkshake, poured into a tray for a form and allowed to cure. After about 45 minutes you can carve a shape into the plaster, either as a crucible or for a mold for the molten metal.  Let dry about an hour then place in an oven and heat to remove totally all moisture. Raise the heat slowly to prevent cracking and hold above 300f. or so for an hour to drive out all moisture. This should be done soon before using so moisture from the air will not resoak the plaster.

A melting crucible made of this investment plaster will do in a pinch but must have a backing of some sort for strength. A metal tray capable of taking the melting temps will work.  Sprinkle some boric acid in the homemade crucible before using.  Overheating this material above 1400 f. will start deterioration of the plaster and can contaminate the metal. So, the use of such a material for a crucible is limited. For use as a mold, the plaster is ideal.

Commercial Ingot(bar)Molds.  The commercially made open and two piece molds for pouring metal are made of soft steel.  As a die maker, you could fashion one likely better than the commercial models.  To help insure a good pour, the mold may be “sooted” with something like the soot from an acetylene torch. The mold does best when heated to about 400f – 500f to allow a better flow of the molten metal.

MELTING TEMPERATURES OF COMMON GOLD ALLOYS.
The melting temperature depends on the alloy. Gold is mixed with silver and copper plus trace amounts of other metals to form jewelry alloys. White gold often has nickel or palladium and the melt temp is higher than for most yellow golds.  The various alloy recipes cause some variation in melting temperatures. For this reason, I suggest you shoot for a melting temp of the highest melting gold alloy in you lot of gold items.  Here are general melting temperatures:
10k yellow   1617f-1650f (881-891 C)
14k yellow   1524f-1606f (829-874 C)
18k yellow   1679f-1765f (915-963 C)

White golds generally melt at higher temps.
10k white  about 1822f (994 C)
14k white  about 1737f (947 C)
14k white with palladium(not common) 1968f (1076 C)
18k white about  1704f (929 C)
18k white with palladium(not common) 2006f (1097 C)

Don, I hope this information helps! If you need something more specific, please use the follow-up to get back or ask again for a clean page.  Best wishes in your project.

God Bless and Peace to All.          Thomas.               4/28/2004 6:37 PM