Ive been using my electic melting furnance, capeable to fire 1 KG graphite crucible for quite some time now!
My questions is: How many castings can the crucibles withstand before they should be replaced?
I know there are many variables involved, however do you have a rule of thumb of how many castings it can take? 10 - 20, 20 - 30 ? Will obviously only take this as a guidance, not definite.
I cast between 980 - 1050c, not more than 200 Grams a times, so not much slag getting accumolated… melting in a standard Electric melting oven, where oxygen can interfere when cooling down
Anyone perhaps has a refference picture a crucible used to the max, and signs for when you would replace?
Beeen casting with this i would say arnd 10 -15 times, dident keep track this time. However havent dared to push it much further than this,
someone on Ebay has them for short money, they work fine. When they start getting porous and pitted I stop using them. I certainly don’t want a pile of metal in the bottom of the melter.
This is not expert opinion, but mine have held up in spite of looking very rough. I think if they are not holding a lot of weight and you put your metal in first and bring it to a melt that will work better than if you just dump a lot of metal into a heated one.
Having said that, the saying goes, “discretion is the better part of valor”, so if you are concerned, get a new one and reuse the older ones in some other way. Like saw in half length-wise, file a flat top on the curved side, use it as a charcoal block for soldering.
Graphite from crucibles can be used for soldering, but are better suited to furnace soldering because the graphite is a supurb conductor of heat, and if to try torch soldering, the graphite will draw the heat from the metal wherever it touches. This can be useful, such as helping keep smaller parts from melting if they contact the graphite, but more commonly it will make it harder to get everything evenly hit enough for solder to melt. Graphite behaves thermally more like a block of copper or silver than it does like a charcoal block, which is not a good heat conductor. Graphite scraps can be carved into useful small ingot molds though. One guy I know uses his small cnc mill to carve small rectangular open face ingot molds that come out with rounded corners, a raised rim, and raised lettering that reads gold on the poured block of metal. Customers with scrap gold can have their scrap, no matter what karat or odd mix, poured into thìs. Solder on a bail, and they get a slightly rustic pendant or charm for little work or time from their otherwise questionable scrap, and my friend can use clean new metal to make their new item if they are doing that.
I have soldered on the back of a rectangular graphite mold for pouring ingots a number of times because the back is so flat and graphite is reducing. I didn’t notice a problem with it.
I tried finding one online to post an image and came up empty so I’ll have to take good care of the one I have. My pouring skill using vertical steel ingot molds leaves much to be desired. The open rectangle cavity is a much easier target.
The thing you really need to be aware of is that the lip of the graphite crucible, when deteriorated, can slip through the pliers. I was doing very heavy melts of bronze and had this happen recently. Fortunately, due to a variety of issues, I was pouring very close to the floor when this happened (twice), and the bronze didn’t splash too badly and the angle was away from me so the only thing that suffered was my ceramic tile floor. I finally ended up using a pipe wrench to pick up and pour the crucible. I kept using it because the body wasn’t in too bad of a condition. I think that the deterioration was due to the high volume of bronze along with the high temperature and the length of time it took to properly melt the metal. Just something to think about.
I do bronzes and we had a crucible of 100+ lbs slip thru the carrier “ring” (new crucible that was a few 10’s of mm smaller than the old ones….)!!! We poured outside, on a slight slope, covered with sand so 100 lbs.+ flowed down the sloped area on the sand. Nobody was hurt, no close calls but ALWAYS, ALWAYS - - - when dealing with molten metals, whether some number of grams or many pounds, BE CAREFUL and try to think of anything that could/might happen and have a resource or plan to deal with it. You will NEVER think of them all but the better prepared the safer the pour.
