Going to try some green sand: Free ingredients score!.

So I asked my friend Peter if I could use his sand mixer to crush some bentonite with it's steel barrel roller. Pete went up on the mezzanine floor and had a rummage around and came up with some 5 gallon drums of powdered foundry bentonite and sea coal that dated from 1983 or so. I came home with a 5 gallon drum of each to try and mix some green sand .

 

That supply looks like it should last you awhile! I guess I haven't been paying attention-what molding method have you been using?

Pete

 

That should be enough bentonite for up to 400Kgs green sand at 5% concentration. Up until now I've been using sodium silicate and phenolic urethane no bake resin...I think the green sand should be good for one off castings but will need some skill to learn to use it properly. I'll use it tomorrow in a steel bucket to hopefully to cast some round bars with leftover iron as I try to debug the furnace without the swirl disc on the plinth, it's been six months since last time which had too much swirl with the new air nozzle + disc. Leaving the disc out will lower the crucible two inches and let me run at full throttle without the slag volcano effect flying into the air all around .

 

So the first 10Kg batch of green sand was made according to the Navy foundry manual and rammed up in a stainless bucket around a 4" pipe. I plan to use this instead of an ingot mould for leftover iron, last time I made some dodgy sand-clay mix and had a lot wash out into the iron which is expected when you pour iron onto the sand bottom. This time I have a steel disc at the bottom to hopefully minimise the sand wash when pouring. On a foundry related note, Peter had a couple of steel barrels full of shell cores used for casting bronze tube for an old Royal Australian Navy contract in 1983: the green sand mould is rammed up as a vertical tube and the shell core is filled with steel blasting shot to weight it and prevent it from floating with a metal piece to keep it centered on top. When the bronze was poured, the steel shot would cool it faster and the shell core would collapse and prevent hot tears in the casting. He said the top few inches of the tube would be porous but a few inches needed to be in the lathe chuck jaws anyway and would be melted for the next casting. The tube was machined into a variety of plumbing fittings. The shell cores were shipped wrapped in 1983 vintage newspaper and filled with red cedar wood shaving, some of the newspaper car ads included.

 

I use play sand (silica sand) with 5% bentonite and some water. That works excellently for copper, brass and cast iron.
Bentonite can even be obtained by crushing cat litter (I did not try).

 

I imagine that the bentonite you were lucky to score is unlabeled and you have no idea whether it is sodium bentonite or calcium bentonite. And it probably does not matter a great deal as just having any bentonite is a plus and will work as a green sand binder.

I am lucky enough to have both and can tell you they are somewhat different in their bonding traits. Various authoritative sources recommend using some of each for an ideal green sand mix. As a long-term goal it might be worth trying to identify (not sure of an easy way, though there may be a simple method, without some sort of chemical analysis) what you have and to get some of the type you don’t have so you can use some of each. I found that for trickier patterns when I added in some western bentonite to my basic southern (calcium) bentonite mix, things got better.

At first I thought calcium bentonite alone was just fine. Then patterns became more demanding and the improvement due to added sodium bentonite became evident.

A link to basic information about the two:
http://smelko.com/?page_id=325

Denis

 

I’ve been mulling over how to distinguish sodium vs calcium bentonite. It occurs to me that flame testing might be the answer. I’ll try firing up an O/A torch and burning some of each. In theory the calcium should flame red and the sodium yellow.

Denis

 

IIRC, Jeff (aka Tohbo Mott) has some of both as well as mixed sand from Smelko, maybe he'd be willing put your theory to the test.

BTW, in my area at least, sodium bentonite is far easier to find than calcium bentonite.

 

Interesting, I never knew there were different cations available. Are kitty litter made from a specific type or is it any'ol clay?

Denis: It's worth a shot, but I suspect the black-body radiation will drown out any difference. You could try to make a water extract, then dip a wire in it and hold it in a propane flame. Another possible method would be to convert it to sodium bentonite (Wikipedia suggests it can be done by adding 5-10% of a sodium salt to wet bentonite).

 

I suppose it could vary from brand to brand but it's usually sodium bentonite.

I can remember doing exactly that in my grade 12 chemistry class, except that it was a natural gas bunsen burner flame.

 

I’ll have tried flame testing by this evening. I’ll use table salt as a reference color As it provides a good source for sodium. For flame I think I’ll try using natural gas stove burner as it is so generally available. A stainless wire (bike spoke or tig wire) will make a good sample holder. I’ll go to the foundry later today to get samples of the sodium and calcium bentonite.

Denis

 

If the flame tests prove inconclusive, maybe try a water absorption test, sodium bentonite will absorb more than twice as much water compared to calcium bentonite. I'm not sure but a well conceived and carefully executed test might even be able to determine the relative percentages of mixed bentonites.

 

Rocco has a point, what's important isn't the cation but the properties they produce. So the simplest might be to test for those directly. But a flame test is so simple it makes sense to try it first.

 

I forgot to add that it's supposed to be Western Bentonite from a foundry supply company, it came from the USA. It's a pale yellow colour if that's any help in identifying it. I can run some basic chemistry tests on the stuff if the borax bead test of flame test helps. Thirteen years ago I would have made a fused ceramic puck of the stuff and ran it through an X-ray fluroscope and X-ray diffractometer at my old job testing silica sand samples before the processed product got shipped overseas to be made into glass.

 

Western bentonite is sodium bentonite.

 

BTW, here's something you might like to read.

 

No joy on flame testing. I tried a natural gas stove burner, a MAPP gas torch (which is not really MAPP gas anymore), and an O/A torch. Though there was a slight difference in color, it was not definitive. There is probably someone here who has some chemistry background and can explain why flame testing fails. I suspect it relates to the fact that the an ionic portion of bentonite is large and complex compared to testimony NaCl, vs CaCl, for instance. Oh well, dang it.

With respect to color there is a slight difference, but again it is subtle and might even depend on how finely milled the bentonite is.

Denis

 

I was afraid of that. Well, what's next? Precipitation reactions? Calcium carbonate and sulfate are insoluble in water, same with calcium soaps.

 

That was an interesting read, thanks for posting it Rocco.

Well the borax bead test shows a white bead with the two samples of bentonite I have, no reaction at all.