crucibles

machine#construction: casting#furnaces:

Crucibles  

A Crucible is a pot that you melt metal in.

Crucible Usage  

Crucibles should never be stuffed with cold metal - especially aluminum - as there is a strong possibility that the metal will expand and crack the crucible. Never leave metal to solidify in the bottom of your crucible, for the same reason. Crucibles should be inspected for cracks prior to each use and cracked crucibles replaced with new. I've heard that a sound ceramic crucible will "ring" when struck with a soft hammer and that a bad one will "clunk". I prefer a visual inspection, but it is dealer's choice. Store crucibles in a dry place and pre-heat them and the metal to be melted as your furnace or foundry heats up. Use the proper tools to transport and pour the melt and of course, use of safety equipment is essential. As they say "safety is job one".

Crucible Anatomy  

Have a look at
http://budgetcastingsupply.com/Crucibles.html

bilge crucibles - bilge crucibles were designed in order to decrease the surface area of the metal in contact with air. This only occurs when the crucible is nearly full. It probably isn't what you want.

A-shaped crucibles have curved sides like bilge crucibles, which increases the strength of the crucible (think: roman arch) but the cavity is not undercut, making the crucible easier to manufacture.

Crucibles generally come in two flavors, metal and ceramic.

*Steel Crucibles  

Steel crucibles should only be used at aluminum-melting temperatures and lower. They should be made at least 1/8" thick to avoid breakage while full of molten metal. I use a 1/4" thick pipe with a 1/4" plate welded to the bottom. Steel crucibles should be coated on the inside (and outside, optionally) with refractory wash (kiln wash) to prevent iron contamination and flaking.

Melting of metals is a serious undertaking for the home hobby foundry worker, your personal safety & well being could be jeopardised if you choose to use poorly made, thin walled steel crucibles.

Not everyone involved in the metal crafts, i.e. foundry work, posses the necessary welding skills, which enable the fabrication of good quality reliable steel crucibles.
The novice hobby foundry worker quite often chooses steel crucibles out of financial necessity, or simply because they may have seen pictures of makeshift crucibles made by others.... steel crucibles can be quite OK if they are constructed and skilfully welded
together, a good-sized melting pot can be quickly made in the home workshop for next to nothing.

But there are shortcomings with steel crucibles.

The fierce heat & hostile environment of the gas-fired furnace promotes the destruction of a mild steel crucible, I know this from experience, having used a steel crucible during the start up phase of metal casting, the steel melting pot was certainly an attractive proposition financially and also due to the ease of which one can be quickly obtained. But, the shortcomings were quickly made apparent.

The biggest drawback with mild steel crucibles is that of corrosion,which happens naturally when steel is heated to a high heat and allowed to cool, flakes of rust or iron oxide will peel off the crucible
external & interior walls every time it is placed in the furnace for a melting session.

Rust and scale can be removed from a cold crucible if it is hit sharply with a hammer or piece of steel rod, the scale will fall away, but there is always a quantity left and it will most likely dislodge during the melting session, and fall into the metal.

At the end of the session it is put aside to cool & corrode. Any rust flakes that end up captured in the molten metal and subsequent castings will make machining operations almost impossible to carry out.

The standing time that the crucible is idle between melting sessions may be anywhere between one to four weeks duration, this standing period is the time when heavy corrosion takes it's toll on a steel melting pot.

Graphite/zircon coatings can be applied to the inside of the crucible to help reduce corrosion, but it will continue to be a problem for the simple fact that corrosion is a natural oxidation reaction of iron & steel when exposed to the atmosphere

It doesn't take very long before small holes begin to appear in the walls or the base of the crucible (if the base is of a thin section), this pin holing action is potentially dangerous to the user, a pot full of molten metal that suddenly start to leak while transferring from the furnace to the pouring shanks, could easily spill onto your legs & feet. This is not the kind of thing you want to have happen on a weekend, or at any time for that matter.

Personal safety does not have a price. there are no compromises. There fore any one contemplating getting involved with metal casting as a long-term hobby should be prepared to spend a reasonable amount of money to purchase a high quality name brand silicon carbide, or a clay graphite crucible.
A five to seven year service life from a high quality silicon carbide crucible can be achieved at the hobby foundry level, if it is properly cared for.

Crucible longevity will more than repay you with safe melting practices and an assurance that your foundry melting session wont be marred with accidents & interrupted due to faulty or improperly constructed equipment.

It is a difficult decision for the hobby foundry worker who is generally on a tight budget and is always anxious to get the new foundry up and running before all of the proper tools & equipment have been acquired.

My advice to any one keen to start hobby foundry work is not to be too impatient, take your time to build the best equipment, tools, etc that you can, and concentrate on getting everything set up, do some experimenting and practice before you choose to carry out a full melt session.*1

*Ceramic crucibles  

I dont have any real experience with these, someone who does, please comment. *2

Ceramic crucibles are more brittle than steel crucibles, and need tongs that fit them perfectly. If a pair of tongs is shaped wrong, it will cause a point of high stress where it contacts the crucible, increasing the risk of fracture. It is generally safer to buy tongs made for your (bought) crucible, rather than try to make your own. Never make tongs that lift a crucible by the rim, since this is the weakest part of the crucible.

Alumina - white, low thermal conductivity, high purity, usually used for precious metals

Silicon Carbide - black, high thermal conductivity, not used for iron

Clay/Graphite - dark grey. Clay/Graphite crucibles need to be "tempered" before use. This involves heating them very slowly to drive off moisture without cracking them.

Hi, I have made several crucibles of my own with great success. I used a mixture of 50% grog to 50% fireclay and shaped the mixture by hand into the desired shape. I make the walls of the crucible at least 1/2" thick for strength and have had no problems with cracking. It does take a bit longer to melt compared to a steel crucible. I make my own tongs and have no problems.
Slip casting is another method of making your own ceramic crucibles.