Cement Pitch Bowl

Repousse & Chasing | | |

There are various suggestions for DIY pitch bowls out there, bowling ball, skillet, etc. I am interested in making a cement bowl, has anyone done this? Tips? Suggestions?


Rich Waugh's picture

I haven't made a concrete

I haven't made a concrete pitch bowl, (I use cast iron ones), but I have made a number of similar objects from concrete. Based upon my experiences I'll make the following suggestions:

1. Don't use the ready-mixed stuff in the bags, such as Sakrete. The aggregate in that is too large for what you're doing, unless you're planning a pitch bowl the size of a bath tub. Instead, mix 1 part Portland Cement with 2 parts sharp sand and 3 parts pea-sized aggregate. Mix this together thoroughly in the dry state before adding any water.

2. Add to the above mix some shredded polyester fiber reinforcing. You can get this at most places that sell supplies for concrete and plastering work. It is made from recycled soda bottles and adds tremendous tensile strength to thin sections of concrete. Without it, concrete has little tensile strength, that is, resistance to breaking from stress.

3. Use just enough water to make the mix plastic, but not at all "loose." The more water you use, the longer it takes to set and the weaker it ultimately is. Additional working to get it shaped and compacted will be better than more water.

4. Your concrete bowl will need to set for 30 days to achieve maximum strength. After two weeks it will have about 75% of full strength, but don't rush it. When you use the pitch bowl, the forces pushing it out from the inside can be considerable and that is the direction it is weakest. Be patient.

5. After the bowl has set completely, you can sand and/or wax the outside to achieve a surface finish that will be comfortable to move in a support ring. I use old solid rubber tires from scrapped toy wagon wheels and such for support donuts.

I hope this is some help to you. Let us know how it comes out.

Rich


fireflyjewelrydesign's picture

What would you use to sand

I like the cast iron bowls as well and that is what I have used so far but I wanted to see if I could figure this out. What would you use to sand the bowl and finish it? I think I might have used too much water in my first attempt. I'll post some pics of the mix if I can figure out how to add that to my post...


visitor's picture

I have one that I made in a

I have one that I made in a workshop with Marcia Lewis and people used them that week. I think that it was a ready mix cement that we used. Latter today, I will hunt up her book and see if she has directions in it. It is not really a bowl. It is a cement hemisphere with the pitch piled on top. After it had set up for a while, we stuck holes in the top so that when it hardened, the pitch would have "finger" holes to grab. Garden tractor wheels make a base that allows the hemisphere to be rotated to different angles.

marilyn


Rich Waugh's picture

Marilyn,Yes, in a heavier

Marilyn,

Yes, in a heavier section such as the hemisphere you describe, you could use the concrete mix and get away with using it almost that same day. I would still suggest sifting out any aggregate over 1/2" mesh, just to get a smoother fill into whatever mold you're using. Lots and lots of vibrating or joggling too, to get a smooth surface to ride in the support ring.

I was describing a method to make what "firefly" asked about - a bowl, or hollow form with relatively thin cross section. That's a different kettle of fish, so to speak, from a solid form.

Rich


fireflyjewelrydesign's picture

pitch fingers

This is an interesting take on the idea. Sounds easier.


marilyn's picture

The cement was a smooth mix

The cement was a smooth mix so perhaps it wasn't what I had said. After it had set for a bit, we pushed pencils or fingers about an inch into the top. After it hardened, we removed it from the container and piled the pitch on top. That is why we made the depression. they keep the pitch on the hemisphere.

marilyn


fireflyjewelrydesign's picture

Take one!

So here is what I've tried so far...
I took a basketball and cut it in half and partially filled it with a mixture of portland cement and water. (Later, I found out this might not be the most sound way to approach this project, but I guess this is how one learns.) This made a big wet blob of mush that didn't look too hopeful. I tried to sculpt it to no avail and then I abandoned it for a few hours. Later when it had dried a bit I could scuplt it into a bowl shape with a relatively level ledge. It is now drying, but I don't know if this is the end of this experiment.
edit: I just checked on it and it has a crack in the rim! Guess I will have to try again...

I would love to hear what Marcia's book has to say on the subject. I'm really out of my realm of expertise here. ;)


Rich Waugh's picture

If you use straight Portland

If you use straight Portland cement without any sand, aggregate or fibers, it is guaranteed to crack every time. There is simply too much shrinkage upon setting. Cement does not dry, it sets hydraulically through a chemical reaction between the cement and the water. After setting, it still retains a significant amount of free water that then must dry out of it - this causes the shrinkage. ONce it is completely dry, it still has water in it, but it is chemically bound to the cement. Firing it above the calcining point will release that water and turn the cement back into a powder form, essentially.

As I mentioned in my first post, you want the 1-2-3 mix of Portland, sand and gravel to be stiff - like say, cookie dough or just a bit softer. Then you won't have the problems of manipulating into the mold and it won't crack up when setting/drying. The fibers make a huge difference on this as well.

The basketball is a pretty good idea for the mold, but it will be too floppy to let you really work the comcrete into shape, unless you back it up. Place the basketball half in a box or barrel dep enough to hold and big enough to allow a full inch or more all around it, then slowly pour dry sand around the thing. Once you get sand up to about the level of the rim, you can dampen the sand with a water mist from the hose to "set" it. Sort of like building a sand castle at the beach, if you've ever done that. Now you have a stable mold to work your concrete into.

Never try to speed up the curing of concrete! Doing so will result in cracking, spalling and general loss of strength. Concrete actually develops its highest strength when cured under water, so keeping it moist for the first three to seven days is recommended. This is particularly true in thin-section work such as your bowl project.


fireflyjewelrydesign's picture

Thanks!

This is great info, thank you!


Gene-ious's picture

cement

Hi, I use a lot of concrete with my steel work, It's a basic mix of 2 parts sand 1 part pea gravel and 1 part cement. When casting you need to have your mix fairly wet. Take a look at www.revelstonecreations.com.au


man of mettle's picture

mix your own

i use tupperware and a wood frame to keep the spaces. it worked for a lampshade i made during the summer. usually it is best to mix your own crete using sand, portland cement and perhaps fiber... and there are plenty of recipes on the web. sofa lamp 1 keith