Posted by bruce paul fink on January 12, 1999 at 12:22:15:
Bernard Porter wrote:
I THINK I am getting an oxidized surface on my castings =
the surface of the aluminum is a dirty grey, (pitted and scaly - requiring removal of at least 1/16 inch) I have been using regular hardware-store plaster of paris for my cement in my investment (50/50 with flint). I'm studying your previous mails and will experiment at length in getting rid of possible hydrogen in my melts and getting the mould DRY DRY. If you'll stand still for it, I'll be back after I've experimented further.
The Plaster Of Paris part is okay just probably much more expensive. I use the cheapest #1 moulding plaster currently @ $16 per 100 lb. It's only a binder anyway and is basically the same thing.
Did you originally mean 'aluminum' and 'bronze'? I read your post as 'aluminum bronze'. There is a great difference as aluminum bronze is a type of brass or bronze alloy and has very different characteristics from aluminum OR bronze. If you meant aluminum (as it appears to be above) there is a possible other solution to your pitting.
To the best of my understandings..(I will try to use general and condensed terms to illustrate) here is my thinker resolution. This doesn't come from an intelligent source but from many years observation and having conquered the problem so it's now pure pleasure pouring aluminum. Kind of like a mint julep on a breezy summer day vs. a bowl of Texas Chili while sitting in a sauna.
(they usually don't use meat don't you know).
The investment mold has to be dry... but that may be misleading.
As the plaster is mixed it only requires a very small amount of H2O to make it chemically set but takes a lot more to mechanically make it fluid. So when you burn out a mold you are doing several things.
1... Getting out any foreign wax or plastic or whatever you have in it that is the positive of the piece to be cast and
2... Removing the mechanical water (sitting in a dry room or the sun would eventually do that and
3... removing the chemical water. This is the part that allowed it to become hard and be the binder for the investment. I take molds up to 900 F for that and hold them there for from 4 to 7 days and nights depending on the mold sizes, the media to be removed be it wax, plastic or wood, etc. and the kiln sized load. The heat source can be low but must be continuous.
(I use an oil burner running with .75 gallon per hour for a normal 6 day run and that does all the molds packed in a kiln 5 ft. high x 4 ft diameter. This kiln can be larger when needed but that's an average.)
I always put the molds in wet...
as wet as possible since water is a better heat conductor to a mold than an air dry mold is and the wax will warm and be squishy-movable vs. tree root forcible in the rock like mold. (Cuts down on any flashing. See also my other posts on styrofoam spruing and good open drains and foamed wax cores if solid.)
If there is ANY chemical water left or if there is any wax residue left in the plaster there is likely to be pitting, boiling, gas released from the mold and even a violent BURP or explosion like a geyser that will blow the metal right back out the top at worst or just bubble away at less worse.
Both are real bad.
Both make for casts less desirable (being real kind here... personally I call them poorly shaped remelt ingots).
In regards to the aluminum..... it melts very different than other metals and I will attempt to relate, bear with me.
If we had a chart with time on one axis and heat BTU's being
absorbed on the other axis, the line across would be near straight. A natural melt chart like most metals have consists of a fairly constant rise in temperature and BTU absorption from a cold ingot to a softening and then melting ingot so you then give it a little more time to over heat before pouring as the heat loss / viscosity will drop down consistently also.
A MELT CHART FOR ALUMINUM ALLOYS LOOKS ENTIRELY DIFFERENT
(if you use scrape it's most likely an alloy,pure aluminum is as soft as lead and bends and sags so much it is seldom found as scrap except in maybe soft pie pans of the disposable kind, etc.)
BTU HEAT absorption and TIME are again consistent but as the aluminum soaks up the heat it seems to stay more nearly hard and then get like a softer putty. Time passes, BTU's still are absorbed but the metal remains putty like for a longer time. Minutes pass but all of a sudden checking shows it's liquid. TIME FOR CAUTION Do NOT overheat now as the melting curve has just taken a squared jump up the scale of viscosity. It's time to POUR.
And as the metal sits in the mold it again cools for a longer time than seems normal before it suddenly starts to get putty like and then harden. It cools to a harder stage again in a reversed and squared chart line. Had you given it another 10 to 15 minutes of soaking up the heat it would have only further extended the period of time it would sit in the mold as a liquid.
SO here's the rub.
If you heat aluminum like bronze... once it is liquid DON'T give it another 5 or 10 or 15 minutes to soak up the BTU's as it not only does NOT want it or need it but will set in the mold TOO long before solidifying. This gives it too much time to do the things metals do when liquid and are able to molecularly mess around, separate into individual weight layers of metals, open up to later variations of uncontrolled crystal forming and in some cases seek other combinational alloy groupings... and if there is any H2O or wax residue even way far into the investment body it can do funny things to the surface. Create soaking gases and surface wise make one feel real grumpy later.
At least that's what my mother told me.
bruce paul fink Sculptor / Designer
Woodstock, CT USA