A Plinth that will Always/Never Stick to the Crucible

Discussion in 'Furnaces and their construction' started by Melterskelter, Jul 23, 2020.

  1. Background:
    One recurring problem in foundry work is having a crucible stick to a plinth. This can usually be avoided or at least reduced by placing a couple layers of cardboard between the crucible and the top of the plinth prior to firing the furnace. But still, sticking can and does occur especially if the plinth is not heavy in design. Recently, having made a lightweight Blu-Ram plinth, I have experienced consistent sticking of my crucible to my plinth. The underlying reason for the sticking is melting of the crucible glaze with consequent flow of the glaze down onto the plinth producing a gummy accumulation of glaze between the plinth and crucible. If the plinth is heavy, simply pulling up on the crucible breaks the bond and the crucible pulls cleanly. If the crucible is light, it produces a small downward force insufficient to break the gummy bond and rises with the crucible. Unfortunately, after a short time it lets go and falls back into the furnace. I've had this happen 5 times now since making a new plinth. Amazingly, and to the credit of the Blu-Ram, neither the plinth nor the furnace floor (also thin Blu-Ram), have shattered. I wonder if the white-hot Blu-Ram may be a somewhat soft and therefore tolerant of mechanical shock.

    Still wanting to be able to use a lightweight plinth and and realizing that sticking is to some degree inevitable, I am going to try a two-part plinth design. In some ways it is like an inverted cup and saucer. My intent is to allow the saucer to adhere to the plinth, hopefully, permanently. And the design of the saucer prevents glaze working its way between it and the cup portion of the plinth. I'll still put cardboard between the saucer and the cup just as additional insurance. But if all goes well glaze will accumulate on the saucer and firmly adhere to the crucible, but, because of an overhang, any additional drips of glaze should not find their way between the saucer and cup.

    I made the plinth by carefully pounding a mass of Blu-Ram into a seamless mass and shaping it into a solid cup-like shape about 6" tall and 4.5" diameter at the top. Then I used a 1.25" tube to core out the plinth so that I now have a thick-walled cone-shaped tube of BR as the plinth base. I also formed the saucer about an inch thick with a 5" diameter shallow round recess to nest over the top of the plinth. The saucer is also cored-out but to a smaller diameter than the plinth. I cored them both figuring that a ring shape has a better chance at surviving the expansion and contraction associated with furnace service.

    Both items will be fired in my kiln and placed into service this weekend.

    Here is the old plinth with nubs to allow heat circulation between it and the crucible. This plinth is hollowed out so that it has about a 1" wall. You can see the melted glaze, dark in color, which partially covers the plinth top. The new plinth is assembled next to it and has a melamine disc in the recess (white). The coring tube and a couple cores are also seen.
    Below the saucer is off the cup portion and flipped showing the recess as the melamine disc is to the upper right.
    Same as above but the melamine disc is in the recess.


    I am pretty stoked about the design idea. Time will tell if it performs well.

    Last edited: Jul 23, 2020
    Tobho Mott likes this.
  2. Zapins

    Zapins Silver

    You know I've never had this issue before. Silicon carbide crucible on ceracast 3000f.

    What materials are you using?
    Jason likes this.
  3. Super Salamander on a light Blu-Ram plinth. Never had a problem with heavy plinths that were slag-glued to the furnace floor. New furnace, clean floor, light plinth.

  4. My last iron melt had the disc and plinth stick to the bottom of the crucible, all that molten iron slag ran down the side of the crucible and stuck things together, it's not a pleasant happening but I was able to get it off with a tap and very little crucible damage, though a thin layer the size of a postage stamp broke off.
  5. I had sort of forgotten about your swirl disc. It is easy enough to drop some slag into the furnace when skimming. In fact, it is almost unavoidable. Sounds like this time a pretty good bit got away from you.

    Does the disc not usually want to stick to the bottom of the crucible just from melted glaze? Did you make the disc with the intention of sticking it to the plinth base?

  6. I'm using a clay graphite crucible so it seems to have less glaze than a silicon carbide and has never stuck to the plinth or the disc until the slag got under it. I'll try a piece of 1/8" fibro cement under the crucible from here on out, the modern asbestos free stuff.

    The disc was always a loose experimental piece to test the effect of forced swirl The next run will be without the swirl disc as the new nozzle seems to have enough swirl and this should give more height (4") above the crucible to stop the slag from getting blown around excessively.
  7. So, you can actually see slag being picked up out of the crucible by the vortex of swirling flame? That must be one heck of a swirl.

    How will the fibro cement disc help you? What special qualities does it have?

    I am hoping glaze and crud will more or less drip off the saucer edge and not work its way under the sauce. I did raise form the edge with a bit of a drip lip (sag) and not just a purely flat return mach as one might do to manage rain on house trim.

    Drip edge.JPG

  8. Al2O3

    Al2O3 Administrator Staff Member

    Denis, I can see how the overhang of the saucer may help prevent it from sticking to the plinth, but isn't it more likely that the light saucer will just adhere to the crucible? You might still be better off that way.

  9. From the first post :
    “But if all goes well glaze will accumulate on the saucer and firmly adhere to the crucible, but, because of an overhang, any additional drips of glaze should not find their way between the saucer and cup”

    Yes, I really hope the saucer will stick to the crucible. I actually made the crucible side of the saucer slightly concave to retain the glaze drips.

    I guess this approach is sort of like “if you can’t beat em, join em.”

  10. Al2O3

    Al2O3 Administrator Staff Member

    Missed that. Tougher to read'em all these days.

  11. Hi Denis, the fibro cement these days is reinforced with cellulose, in other words cardboard, so when it gets hot it should turn to a crumbly calcined cement powder. So hopefully a thin disc of the stuff under the crucible will aid separation. I have random bits of slag all over the furnace from earlier experiments but right now the 2" thick disc raising the height of the crucible to within 2" of the furnace lid, I get a slag tornado about 3" tall that sheds slag all over the underside of the lid and sprays bits into the exhaust stream at higher settings. Hopefully lowering the crucible will fix that now that I have the tuyere air nozzle fitted to induce swirl.
    Last edited: Jul 24, 2020
  12. Sounds like that should work well. I've not explored fibro cement. Sounds like something potentially useful in the foundry world.

    No worries, Kelly.

  13. Today I initiated the new plinth cup and saucer design. It worked as hoped. The disc (saucer) stuck to the crucible but released cleanly from the plinth (cup). Glaze did flow down onto the disc and dripped off the disc but did not work it’s way under the disc to foul the interface between the disc and plinth. That was just what was hoped for.

    No melted glaze on this side.
    D93FD7A3-D410-47D5-91DD-0399250F2CCE.jpeg F

    Plenty of glaze here that ran down onto the disc and caused it to adhere to the disc. This side is facing the tuyere, though the flame looks like it more or less misses the crucible and impacts the plinth. That might not quite reflect reality though.

    Mark's castings likes this.
  14. Bentation Funkiloglio

    Bentation Funkiloglio Silver Banner Member

    Try putting a dusting of bone ash on plinth before placing crucible on top. Cheap and easy. Works for me.
  15. Mark,
    I took your cue and got a piece of HardiePlank at the local lumber yard. Being cheap, I asked and, yes, they had a few good-sized broken pieces that I got for free.

    I tried it this morning as I had a 50 pound iron melt to do. It seemed to work very well. One caveat though, after ten or 15 melts my plinth is now pretty firmly glued to the furnace base by 1/4" thick layer of mixed iron slag and crucible glaze. So, it would not come loose from the furnace floor easily. But still, the crucible seemed to come loose from the plinth easily. And there was no evident residue from the HardiePlank in the furnace.

    To use it I cut a roughly circular about 5" diameter coupon of the plank and set it on top of the plinth.

    The saucer of BluRam has resulted in very clean separation of crucible and plinth with the saucer adhering to the bottom of the crucible as intended and essentially no glaze getting onto the top of the plinth. But, the saucers were good for only two or 3 melts and then they would break down and crack so that they could then not be reused. It is not that hard to make a saucer and the cost is negligible but it does take ten or fifteen minutes to pound one out and then it has to be fired. If they lasted for 10 or more melts, I think I would feel they were a workable solution. But for now, I think I'll use the HardiePlank discs and see how that goes.

    BTW, the BluRam lid now has 30.5 hours on it with no evident deterioration. That is the best lid I've had as the castable, IFB/Satanite, and wool/Satanite ones all showed significant damage and required patching long before 30 hours.

    Last edited: Sep 9, 2020
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  16. Al2O3

    Al2O3 Administrator Staff Member

    What's HardiePlank?

  17. Wiki:
    Fiber cement siding ("fibre cement cladding" in the UK and "fibro" in Australia) is a building material used to cover the exterior of a building in both commercial and domestic applications. Fiber cement is a composite material made of cement reinforced with cellulose fibers. Originally, asbestos was used as the reinforcing material but, due to safety concerns, that was replaced by cellulose in the 1980s.[1] Fiber cement board may come pre-painted or pre-stained or can be done so after its installation.[2]

    Fiber cement siding has several benefits since it is resistant to termites, does not rot, is impact resistant, and has fireproof properties.[2][3]
    Another quote on popularity:
    The Survey of Construction shows fiber cement siding grew from 9% to 20% of the U.S. new single-family house market during 2005–2018. The fiber cement market is expected to grow at a CAGR of more than 5% between 2019–2024, but the threat of substitutes like vinyl and wood siding is expected to hinder growth.

  18. Jason

    Jason Gold

    Next time you're in the homelessDepot, walk through the lumber aisle, you'll see it. It's either grey or tan wood looking stuff, comes in 1x4, 1x12 and they sell it in sheets. It's the exterior sister to hardi-backer used in showers. It doesn't rot, great for exterior siding applications.
  19. Hi Denis, good to hear some empirical evidence of the Hardiplank/fibro use in a furnace, I'd only heard about it secondhand from my friend who'd witnessed it in use with small (for a foundry) oil fired furnaces. The older asbestos based material was prized by plumbers to silver solder copper pipe fittings as the thin sheet would protect the wall from the oxy acetylene flames. I've tried modern stuff and it works once before crumbling to powder which in this case is actually desirable. Fortunately most of the asbestos bearing fibro has a visible honeycomb like texture on one side to distinguish it from the harmless sheet.
  20. Here is a 3 min video of a pretty clean and efficient way to make fiber-cement (HardiePlank) discs:

    Mark's castings likes this.

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