Today I got two 200ml crucibles from Amazon. Not the best, but for copper temps these are OK. They last about 20 melts and recently one was cracked which resulted in fouling the furnace (and burner !) with copper. I repaired the furnace and made a new burner and tested it successfully. I think to let them last a bit longer, I dry out all moisture before the first melt, because crucibles can attract moisture when stored in warehouses before shipping. I put mine for a couple of hours in my woodstove and they got about 250 C.
I've bought and used the same type of crucible bought direct from the manufacturer. They don't use as much graphite in the clay mix which also has lumps of grog (crushed fired clay) in it so you get a network of fine cracks from the first firing as there's no vacuum impregnated glaze either. So in a nutshell they cut corners on the clay content, graphite content and don't even glaze the fired crucible. The lack of glaze severely affects durability as slag can attack it.
Not Glazed??? even Lmine.com budget crucibles are glazed, when they gave a case lot deal that is what i used to buy , about half the cost of a Salamander , but sized a little small , but the life expectancy really made them not a deal. V/r HT1
I have been taking my Amaz*n crucibles slowly up to a dark red or orange once empty before using them. They go from black to more of a silver color. I have yet to have a name brand crucible.
Indeed, it is the same crucible. Left a new one which I received this week and oven- (woodstove-) dried yesterday and right one used a few times in both gas and electric furnace heated to 1200 C for melting bronze. The same network of cracks on the outside appeared. The same appeared to the one which finally deceased last week fouling the furnace with copper and was used almost 20 times. I am not going to use these for cast iron, for that I'll use Salamander ones.
Ive used cheap crucible, I have stopped, because of the risk to my furnace , if an crucible completely failed , I doubt I could rescue the furnace, and the cost of the repairs or replacement has to go against the savings on Crucibles , not a good trade Metallab, i would not run a crucible with a random smattering of cracks like your pictured one , but Marks with the uniform spider web is OK as long as the wall thickness is beefy enough, but I much ather just us Super Salamanders and not have the concern V/r HT1
HT1, you are right. Today I melted cast iron in a brand new Salamander and heated it over 1400 C and the glazing remained intact. On the inside I saw that above the liquid level were a few bubbles. So for cast iron temps I'll only use premium crucibles.
I believe in being kind to my crucibles. I always stick a small half inch propane burner in the tuyere and let it heat up everything for a half hour before I load the crucible, stick in the oil burner, and pour in the heat. It may not be necessary, but it has served me well. After a few cracked Chinese crucibles, back when I didn't know better, I bought Starrbides. I pre-heat them and after ten years of numerous melts of Everdur they are perfectly sound. But then there is good fortune. I bought a #40 Chinese crucible from Budget Metal Casting back then that turned out to be a winner. Go figure. It's really only a #30, but it has held up well for years. Most Chinese stuff is junk, but then some of it is really good. Richard
