More slag and cast iron

[kelvin2164]

I've been doing some more cast iron castings. Skim the slag off the top, ok, but after pouring, there is a huge amount of slag stuck to the bottom of the crucible.
A few questions. Is this actually slag? Can I add something like borax to act as a flux. I thought all the slag would float to the top? Why is it at the bottom. And how do I get it out without breaking the pot (see pic).
I believe slag has a large silica content. I've been putting the cut off feeders and sprues back in the pot, after cleaning. Could this be a problem, getting a bit too much sand in the mix.

 

[10K Pete]

Are you putting any limestone in? That's the stuff the mills use for flux...
If you aren't cleaning out, but putting the junk back in each time then it's no surprise you have the problem. Foundries use a calculated mix of scrap and new minerals when melting CI. And it's not all CI as they also use steel scrap and mineral additives. Check out a few of the foundry supply outfits and see what they're selling for ingredients.

Pete

 

[abby]

You are presumably melting in a crucible furnace not in a cupola ? there are crucible additives available which will increase the fluidity and machinability of your castings , these are often available in small quantities from ebay sellers .
It is hard to tell from the photo but I suspect that what you are seeing is "skull".
Even commercial foundries get crucible skull which is a layer of cast iron that has solidified around the inside walls of the crucible.
You would find it almost impossible to melt steel in a crucible without induction or arc , the temperatures involved (1600°C) are really not for the home foundry , steel can be melted in a cupola furnace to produce cast iron and as pete says limestone is used as flux.

 

[kelvin2164]

Thanks guys, I'm getting there. I'm like a kid full of questions.
If this is skull. Why has it solidified when it should have the same melting point as the rest of the cast iron. My last pour must have been a bit too cold as the iron didnt flow properly and left unfilled voids. Would this have contributed as I've now done a few cast iron pours, but non so bad as this one. I know you shouldn't stir the pot, but would poking a steel rod down the side of the crucible, while still in the furnace, be a good idea to get this stuff to float to the top.

 

[10K Pete]

Re-melting of metals involves a lot more than just adding heat. When scrap is melted come of the constituents can come out of the mix and not want to go back.
The use of other materials can act as fluxes and/or catalysts to cause some of that to re-melt properly. But some things just won't go and form 'slag' which is an all inclusive term for the left overs. Including impurities and junk.

About half of junk won't float and will go to the bottom.

Again, I suggest you contact one of the good, established, foundry supply outfits, that cater to iron casters, and tell them what's happening and have them make recommendations of materials for additives.

Pete