well, they were just the raw plaster, there wasnt any sort of support structure on the outside, the pieces were set into a tarpaper mold and poured separately on 3 occasions, then the plaster investments were left to dry for 2 weeks then melted out to reclaim the wax at 400 degrees, left to cool, then burned out at 1000 degrees overnight and whenever the kiln hit 1000 it simply shut off and let the investments sit until cool, around 250 degrees and immediately poured. there might have been too much water in all of them because even the investment that survived was very brittle. thing is, i was having problems getting the investment material to stay liquid long enough to pour, it seemed the plaster was setting up very fast for some reason.

i was also wondering, would it be possible to poor aluminum into an investment that was only plaster? or would the investment just fail?

The last time that I used plaster of paris to make a mold was for casting a deflogulated clay body. I got two things out of the experience. One was a word that I haven't used for thirty years, until now. The other was that plaster has a short working time, that working time can can be varied by the temp of the water. Plaster has other problems; long cure time, when it does finally cure it has almost no strength, not to mention that after all of that it shrinks and then cracks. I'm not sure how it stacks up for thermal shock, or breathability. Can it be used as an investment? Sure! Your whole investment process and burnout are going to make allowances for all of those variables. It's probably best done in ren faire garb with lots of mead ale or stout. They are not optional. I draw the line before casting in a kilt. I've made investment, it can be done. Silica gypson talc feldspar and maybe dolmanite could be part of the mix. None of them breath very well. Todays investments are well worth the money, are greatly consistent, are produced to solve or eliminate many casting problems. They are not cheap. shipping is the major cost factor. It's nice to remove variables from the casting process. and the good stuff is only a phone call away.
enjoy Brad

I'll second what Brad said about using real investment rather than homemade stuff. There is simply no substitute for the real thing when you're talking about the temperatures necessary for clean burnouts. In college, we used the homemade stuff for sculptures, but for my jewelry work where it mattered, I only used the best that Kerr had to offer.

That said, in making investment from plaster and sand, it really, really matters what kind of sand you use. It must be sharp sand, and washed, not the playpen sand or beach sand. It also matters how fresh your Plaster of Paris is. Old stuff will fail in different ways, all of them bad. If it's been sitting around for more than a few months, get new.

The cracking issue indicates a watery mix, alright. To extend your working time with the mix, use VERY cold water, and have the plaster itself cold before you start, if you can. Cold weather helps, too.

One thought - did you mix all the dry ingredients first, then add water? Or did you add the dry mix to the water? As I recall, when we did it, everything was mixed dry in a cement mixer, then added to a pre-measured amount of water. Additional water added during mixing as needed, but sparingly. For big castings, the wax was slurry coated first, and allowed to set hard before additional investment was built up around it. This minimizes thermal cracking, I think. It also allows re-inforcing of the investment with SS wire or screen as you go, if that seems advisable. I can't remember the exact success/failure ratio for the process, but I do recall it was considerably better than 1:3, more like 90-95% success.

Why were you using 400° for the wax recovery temp? Doesn't the wax melt at a lower temp? If you have any margin of error on your temp readings, you could inadvertently calcine the plaster before you intend to that way. I don't know what type wax you were using, so this may be not germane. Just a thought.

as I work as an artmaster(for a big old firm) all I can say is that any good artcaster make their own and the only way is to try alot. burning slow, binding well, you shuld be able to blow throu the investment with mouth...

Well, it got me.
I put three models in a flask, added investment. I use Kerr.
Let cure for two hrs, then did the burn out.
It was my typical blow out!
Pile of superhot investment on my vaccum table.
Arrgh.
I have been guessing on the water mix. Kinda like thick cream.
Now I have to get more models, and do it again.
Arrgh.
Should have measured better.

A flask big enough for three models should "cure" for a lot longer than two hours - more like overnight. Then the burnout has to be slow enough to get rid of the water, both "free" water and chemical water, without generating steam. A flask that is 3" diameter by 4" high would take me at least five hours to get up to casting temp (1100F).

Rich

I went back, and felt the pieces.
They did feel damp. I brought the temp up to 1350* for 2hrs, but it was not long enough.
I have an overnight burn going last night/today. It spent all night at 1100*, and it will spend 2hrs at 1350*, then two hours at 800*. The castings have little detail, so I run the mold cooler than fine detailed models.
We shall see.
I was amazed at the dampness of the investment after many hours at that temp.
I believe, I had too thin a mixture. Too much water to start with.
With the larger flasks, I will let it sit over night.

Yeah, I went back later and felt the pieces that had slumped.
They had a dampness to them!, and were quite crumbly.
I guess too much water, too short a time to set up, and even though I was at 1350 for several hours, it was not enougth with that much water.
Life is learning!
Got another flask in the oven. It went over night at 1100*, now the jump up to 1350 for an heat soak/decarbonize, then to 800* for the cast.

In the beginning was the Floyd, and the Floyd was with him.

You asked this same question on another blog post and I gave a more thorough answer there, but I'll recap it here. I don't see yo taking your time to get the oven up to temperature. You have to take the flask up to a bit over 250F over a period of a couple hours and let it sit there to drive off the free water. Then you take it up to 600F and soak it for an hour per inch of radial thickness. This removes the chemical water, calcining the plaster. If you fail to do these steps you're going to cause the investment to disassociate and fall apart later. After you have gotten to this point you can take it up to 1100 or so to burn out all the wax and organic residues. It takes an hour per inch of radius at this temp once again. It is now ready to cast.

I don't take my investment above 1200 if I can help it, and I don't bother droppi9ng the temp before I cast. The cooling that happens while I'm getting things set up to cast is minimal, too. As I noted before elsewhere, I've done hundreds of these and never had one blow out.

I'm not saying my way is the only right way, by any means. I'm only saying that it has worked for me for years.

Rich

Rich,
Thank you for your complete responses.
After last nights disaster, I did another casting, with the three remaining models.
I took your advice, I went with the radius/inch, and after letting it air cure for two hours, I heat soaked it over night. From that point, it was a small matter to bump the heat to 1350*, and after an hour, drop it to casting temp. I do not like "shoking" the mold/flask, so I dropped the temp in stages. After running to the shop, doing an errand, the Kiln was at the ready point. I have an electric furnace, which I really like. Throw the switch, set the temp, and go have a cup of coffee. I like the carbon crucibles. Helps pull the O2 from the metal during the melt.
For those who are wondering about the Kerr Electromelt, GREAT!. I got mine for next to nothing from a jeweler going out of business. He went through crucibles like no tomorrow, I noticed the lid was ajar. I fixed the hinge, and it is great. Long crucible life. I keep a crucible for gold, and a crucible for silver. I keep them apart.
At any rate, with the flask on the vaccum, I poured the metal. A little flux on the button, and into the quench water.
I like to burn out over night as it burns out the carbon from dewaxing. I have found, no carbon, easy to polish. Carbon in the cavities, and polishing can be a mess. I use stainless shot for polishing.
You about dialed in the issue. Excess water. My mix was like heavy cream, not milk shake. This last one was like milk shake.
Perfectomondo!
I appreciate the info. Small things helped.
Mostly, I gotta stop with the hurry stuff. I always do them twice when I am rushing, at 6$ a model, it adds up.
Thanks for the help.
I appreciate it.

In the beginning was the Floyd, and the Floyd was with him.