Found Dreaming

I have been having some ferocious gas attacks so I finally narrowed it down to either my plaster, or my sand. Turns out it was the sand. Beach sand. That's all I have available here, other than the crap the construction guys use to mix concrete which is full of dirt and God only knows what else, and pool filter sand of course, at a price Bill Gates couldn't afford. So I tried brick dust and plaster. Perfect!

I am thinking that it's the ground up sea shells in it that are the problem. Sea shells are about 95% CaCO3 The remaining 5% includes calcium phosphate and magnesium carbonate and soluble and insoluble proteins.

The question is: What happens when 2200 F bronze hits these chemicals? My guess—and it's not even an educated guess; I never went near a chemistry class after high school—is that carbon dioxide is released.

Too bad. It sure is beautiful sand.

Richard

I can't help you Rasper, the last few times I poured brass I got a reaction and that was with Plaster of Paris. I've been trying to take a sculpture class and at the end we would pour Everdur to cast whatever our wax piece ends up. It's always full before I can sign up.
My only thoughts would be, maybe there is organic material in the sand. Rinse it well and heat it as much as you can and rinse again. I imagine it is CO and CO2.

I'd say it's a combination of both your sand AND that plaster. I don't know what sea shells will react to in terms of temperature but plaster I do know something about. Jammer, your plaster has to be "set" then heated gradually to force out water content. I do this at 500 degrees for half a day, then every couple hours after tweak the temp up a couple hundred degrees more till the kiln reaches 1100 degrees. Then I hold it at that temp for another four hours. Then, I gradually turn down the throttle in increments of 100 degrees or so every hour till I'm back to 500 degrees then shut the throttle down alltogether and let 'er stand and gently cool inside the furnace. Meanwhile, I have preped and had ready a sleeve to put the mold into once I remove it (still hot) out of the kiln. Once in the sleeve I pour dry sand around the mold between it and the sleeve to catch any bleeder cracks that will form during plaster burnout. At this point I have my melt near ready to pour, as well. Ideally you pour melt into a plaster mold that is fresh out of the kiln but only if the plaster has been heated sufficiently for a length of time, an at a temperature, that will force out chemically bound moisture from the plaster molecules. If there is the slightest degree of chemically bound moisture still left in the plaster mold you will have severe gas bubbles as a result.

Rasper, check with other members here who "I think" has a Navy Foundry manual (mine's in storage at the moment). In that manual is an instructional tutorial on how to create a good foundry sand mix from beach sands and other raw materials found literally on most coastlines. What you put together obviously isn't working. See what that manual has to offer as a substitute to what you've been trying.

I ran a test using two identical investments burned out together in the kiln, the only difference being that one was composed of the sand/plaster based mix and the other of a brick dust/plaster mix. The brick dust mix poured perfectly; the sand mix was a mess as you can see.



Richard

glad you found the problem,

your work is awesome

Thank you.

Some simple research in Wikipedia reveals that calcium carbonate (sea shells), when heated to 1517 F., forms calcium oxide and releases carbon dioxide. It would do no good to fire the sand in a furnace because the reaction naturally reverses itself over time; the calcium oxide combines with carbon dioxide in the air. That means no more beach sand for me. Fortunately there are plenty of broken bricks around here. And I have a helper who can bust them up.

Richard

Looks like a pretty definitive result. What ratio of Brick Dust to Plaster of Paris are you using?

F.C. wrote: Rasper, check with other members here who "I think" has a Navy Foundry manual (mine's in storage at the moment). In that manual is an instructional tutorial on how to create a good foundry sand mix from beach sands and other raw materials found literally on most coastlines. What you put together obviously isn't working. See what that manual has to offer as a substitute to what you've been trying.

Its good to see that you have solved the problem Rasper I have copied a few pages from the Navy manual on sand but cannot upload them, the site won't let me attached them as a PDF, if someone has an answer or if they know how to attatch them I can email the PDF's to then so they can upload them to the library section

Mike

save em as a jpg. Harry needs to set up a files area for uploads and downloads.

I mixed 2 brick dust to 1 plaster.

R