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What is causing my castings to be porous?

Posted: Thu Mar 08, 2018 4:36 pm
by jscarmozza
What could be causing my brass castings to be porous? I cast a pump body for a test pump last fall, when it was finished I was able to push water through the casting walls. In the same lot of castings I made some simple brass blocks for use where needed. I milled two of them to square them up and immediately saw that they were loaded with imperfections. Any idea what I could be doing wrong? Some of the flaws are tight and almost look like cracks, some are voids, both occur in the same casting and all have the same surface appearance and texture as the outside of the casting. John

Re: What is causing my castings to be porous?

Posted: Fri Mar 09, 2018 2:19 am
by Harold_V
Could be caused by many things. Starting with oily material when melting is one of them.

This link may shed some light on the subject. ... stings.pdf


Re: What is causing my castings to be porous?

Posted: Fri Mar 09, 2018 4:16 pm
by jscarmozza
Thanks Harold, a lot of information to digest.

Re: What is causing my castings to be porous?

Posted: Sat Apr 07, 2018 3:34 pm
by Pipescs
There is also the chance that you need to de-gas you metal prior to pouring

Re: What is causing my castings to be porous?

Posted: Sun Apr 08, 2018 8:22 am
by jscarmozza
Charlie, about 20 years ago the local highschool ended shop classes and I bought some of their machine tools, including their foundry equipment. Along with the furnace, tongs, ladle and crucibles came large plastic jars filled with powder, one white, one pink, and a number of foil wrapped cakes, all of it unlabeled. I'm assuming the powder is aluminum and brass flux and the cakes are for de-gassing, but I'm not sure what the stuff is so I never used it. With all that being said, I think I'm going to buy some new flux and de-gas chemicals and start using using them; I have the bad habit of using dirty scrap as a metal source. My last brass pour was yellow brass sourced from old brass water pipes, complete with painted exteriors and silted de-zincified interiors. In my own defense, I always pig my aluminum scrap into clean ingots, I don't pig my brass for fear of losing zinc content, hence there's a lot of crap in my brass melts. Thanks for the tip, John

Re: What is causing my castings to be porous?

Posted: Sun Apr 08, 2018 6:10 pm
by steamin10
While I have not cast anything in a while, the problem of porosity is as old as time. Todays home caster can get tight castings rather easily if some very basic rules are followed. When you violate the rules, your product will show it. Aluminum follows a similar path as brass and bronzes, but copper based alloys are a bear to keep clear of gas simply because of the nature of copper.

To degas aluminum, start by not putting gas in. I know this sounds silly, but turnings , pop cans, thin sheet, painted items are all a no-no as this all promotes dissolved gas in the melt. It will not hold it and will bubble out, as it changes from liquid to solid state. Steel bolts and rivets will actually dissolve in aluminum, and as the iron content goes up, fluidity and strength go down, until you wind up with pasty gobs that dont flow at all. The best path here is to melt fast with high heat and a reducing to Neutral flame, and pig your recovered material.. This forces the gas out of the metal and gives you a more precise pig to match your project weight with. I use several brick shaped channel iron pigs about 8 inches long to pour my excess into and store. They stay clean enough and store well in this form. All my heats are cleaned and degassed using pool chlorinating tablets crushed and swished into the melt just before skim and pour. ( a rusty pig mold will blow aluminum on the ceiling, ask me how I know. Lightly oil your mold before you store it, clean it if need be.)

For Brass and bronze, especially turnings, burn it with a torch. If the sample gives you a bright blue-green flare, then it has zinc in it and is fairly common brass. Color alone will not tell you, Good bronze will turn red and puddle without much flair. It will be tin based and may contain lead, but that is the way it is made. There are many types of copper based bronzes, so it is a good idea to keep scraps labeled so you dont start out in the woods. Of course there is the purchase of bar metals, but what fun is that.

Brass will burn its zinc off when heated and support this weird looking flame at the top of the metal in the furnace. To stop or slow it down crush some beer bottles and place a 1/2 cup of this on the melt to form an air barrier. It will stop burning and you will not be contaminated by the smoke which is poisonous and can give you metal poisoning if too much is inhaled. Called caster augh, or foundrymans delight, fluelike symptoms will plague you for a couple of days if you are unwary. While working the melt the glass can be hooked off in a thick glob when its time to pour and stir, and is not a large concern. Brass uses a commercial degassing agent swished in before skim and pour.

With any pouring of metals remember to melt fast, pour fast and full without mixing a lot of air into the stream and keep you risers high and full to get pressure on your parts. A lot can be said about sand porosity and being able to vent air from the cavities being filled with metal, all will have an effect on the finished product. Sand that is too damp is instant death to tightness of brass castings as the moisture will make steam and combine with the zinc while you are pouring.

The art of casting has been done for several millenia, you would think the home guy could do it in a snap. In point of fact, it is an old skill that is easily lost, like black smithing.

Re: What is causing my castings to be porous?

Posted: Mon Apr 09, 2018 6:29 am
by jscarmozza
Hi Dave, good to hear from you.
Yeah, 'burning it off' isn't going to cut it, I'm going to have to pay attention to what I melt and how I melt it.
On a related matter, my surfaces were getting progressively rougher with each pour, I knew what I had to do but was too lazy to fine screen all of my molding I added a little more water to the sand to make it pack better. It didn't, and as you said may be one of the porosity causes. I finally bit the bullet and fine screened all of my sand, there was some coarse sand grains and a lot of clay balls that wouldn't break down in mulling. The result is fine uniform sand that that packs with low moisture content. John

Re: What is causing my castings to be porous?

Posted: Sat Apr 21, 2018 7:26 am
by BClemens
Try some Petrobond sand mix. Works fine for bronze. You can see a fingerprint in your casting! No moisture to steam....