Making a Castable Refractory mold
Sunday, November 18th
I'm taking a colored clay class online with Curtis Benzle and part of the process involves making
a mold out of a material new to me - castable refractory. It is a type of cement usually used for
outside oven construction due to its heat tolerance.
This is my first attempt and I wanted to share my process. I hope for success but happy to share
mistakes as well.
I purchased my castable refractory on Amazon. Manufacturer Rutland. Since this is a new process
I liked that I could buy a small amount. In this case 12.5 lbs. I noticed after I received it that it said
2200 degrees on the label. Since I will ultimately be firing in the form to Cone 6, I hope it works. Fingers crossed. I understand that you can purchase from other sources that fire to 2400 degrees. I will know that the next time but continued with what I had.
I built two forms out of red earthenware. One is small - around 4" diameter and 1/25" deep small bowl. The other is a larger oval bowl - around 10" long x 6" wide x 5.5" tall.
I placed them on a sheet of plastic and built thick walls (approx. 1.5" thick and approx 1" taller than forms) around to fence the material.
I sprayed the forms and walls with vegetable spray based on several online sources. It puddled so I did sponge it a bit in recessed areas (photos 1 and 2).
I put on a mask and took the CR cement and water outside. I added the cement mixture to a larger bucket, then added the quart of water as instructed on the label. I mixed this together using my hand covered with a rubber glove. That allowed me to feel where dry pockets of the cement were at the bottom and made mixing easy.
I then brought the cement inside and poured gently around my forms. The small one went smoothly but when I put the remaining mixture into the larger form, the foot seemed to be barely covered (photo 3). I had used the entire 12.5 lbs so I decided to add additional clay around the top in order to push the cement towards the center to build up more coverage around the foot (photo 4). I fear this might not work out well and may result in a funky, sharp edge castable refractory form. I'll know more tomorrow.
1) Small bowl with fence, sprayed with vegetable spray
2) Oval bowl with fence, sprayed with vegetable spray
3) Oval bowl with CR material, showing minimal foot coverage.
4) Oval bowl with CR material after adding inserts around top to try to make form casting work.
Here are my final two forms with castable refractory setting up. I will check consistency and dryness in the morning. I am covering with a light dry cleaning bag overnight. I remember reading that slow drying is best.
This brand seems to be setting up more quickly than I expected. It isn't dry but after only about 2.5 hours, it is slightly stiff.
Monday, November 19th morning
I checked them first thing and they seem hard. I can tell there is still a lot of moisture in the cement but it feels stiff enough to proceed.
I removed the outer ring of the small bowl easily. I think the vegetable spray really helped here (photo 5).
5) Small bowl fence removed.
6) Oval bowl with fence removed.
7) Oval bowl messy top removal.
8) Oval bowl top edge with CR material.
The outer ring/fence removed easily from the large form as well (photo 6). As suspected, the last minute addition of clay strips to the top of the large oval form is a mess. I removed the largest pieces but I am hoping if the clay dries a bit more, it might shrink a little and make that clean up easier, fingers crossed (photo 7).
And finally this morning I flipped both forms hoping that allowing air to get inside might facilitate removal of the forms. I noticed some overflow of the castable refractory on the top edge of the large form. This was easy to removed with gently scraping with a putty knife (photo 8).
So I'm now going to leave them alone. I will of course check them through the day. I'm hoping that the clay forms will pull away from the CR form assuming the clay will shrink as normal and the CR will not. They are both still pretty wet. It's a rainy day here in Cincinnati so I'm happy drying will be slow.
I checked on them after about 5 hours and the small bowl had definitely pulled away from cement. When I turned it over it fell right out. So this one is done and ready to dry (photo 9).
Since the CR cement was hard, but still wet I decided to do my best to clean up the form. I used a chisel and a sanding block to remove as much of the clay and rough edges as possible. I think it ended up working pretty well afterall. The shape is less heavy and should dry faster and more evenly with the "stairstep" shape (photos 10 - 12). It took a lot of work to clean it up so I wouldn't recommend doing it this way, but I'm happy to have two forms to move forward with. Waiting for oval form to dry and pull away to be removed.
9) Small bowl released easily.
10) Chiseled sharp edges.
11) Sanded with sanding block
I had for kiln wash.
12) Final with form still inside.
Tuesday, November 20 morning
The oval form fell out with minimal cleanup to do. Now to wait for drying and first firing.
I hope to use this time to make the color clays and get them ready.
While cleaning my large form further I actually broke thru a thin area on one the sides. I'm looking into seeing if there is any hope for repair but I suspect I'll just have to throw it away and start over. Oh well. As usual with these types of situations, I did learn a lot
13) Small hole
13b) Attempt to patch hole in side of first oval
form - fingers crossed. Curtis did comment that the form was still usable - just cover the hole with a small piece of thin cardstock before placing the porcelain slab.
Tuesday, December 4
I was able to get back to this process today. I cast two new forms and attempted to patch the hole (photo 13b) in the form I made a couple of weeks ago. I am using a new castable refractory that is labeled as able to to to 2400 degrees - Meecos Red Devil 1611. This was purchased on Amazon as well.
Not great success. I don't know if I didn't add enough water or if this is a rougher CR mixture but the outcomes are very pitted and rough (photos 14 - 16). I"ve emailed Curtis to see if he knows of any type of patch material I can use to smooth the surface and any other advice he has to offer. I will post those comments once I get feedback.
14) Exterior of new casting
15) Interior of new casting
16) Close up of broken, rough edges