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I have a 1'×3' slab of soapstone that I'm placing on top of my radiator, creating a shelf:

Slab of soapstone on top of radiator

The radiator bears most of the weight, but the stone isn't stable on top of the radiator (the individual pieces of the radiator are rounded at the top, so the stone can wobble up and down).

My plan to solve this is to use iron shelving brackets to keep the stone stable, but I'm not sure of the best way to attach the iron brackets to the stone. My dad suggested a silicon sealant. Is that a good choice for bonding the raw iron to the soapstone? I see on Loctite's website that they don't recommend it for iron, so I wasn't sure.

Edit: Based on some more investigation, some silicon sealants release acetic acid which can corrode metals, but there are also sealants that don't release it (which I think are called "neutral cure"). I'm still investigating how bad the corrosion is/if the neutral cure version is appropriate for stone.

Edit2: Potentially I should use epoxy for this?


If someone with enough rep to create tags thinks it's appropriate, this post should probably be tagged with "iron" and "soapstone".

  • However you go about attaching these things, maybe give some thought to thermal expansion.... that is, the soapstone, adhesive and iron will all expand and contract differently in the face of the extreme heat (relative to ambient) of the radiator. – bobfandango Apr 21 '16 at 18:26
  • Comment about the edit: sink bolts or screws are the right answer. But if you must use glue, pay attention to the thermal rating. Epoxy and urethane have lower temperature ratings than silicone. Some epoxies can be high temperature, but many are not. Regarding silicone, there are "non-corrosive" kinds, but otherwise, acetic acid will only be released for a little while, and should not cause any issues. Silicone is what gasket sealant (for engines) is made of. Think about this, when was the last time you saw silicone causing rust issues? And with respect to thermal expansion... again, silicone. – Ben Welborn Apr 22 '16 at 17:18
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Soapstone is easily worked. I'd drill holes (not all the way through) and tap them with an ordinary tap, (or even a modified screw - soapstone is really easily worked) and use machine screws or stove bolts to fasten the shelf to the brackets.

To make a crude tap from a screw, grind the threads off the end and grind slots. Look at a regular tap for guidance. or just buy a regular tap - they are not that expensive. If using screws near the length of the hole you'll need two taps, a regular tap (to get started) and a bottoming tap to cut the threads nearest the bottom of the blind hole.

a tap.

a bottoming tap

  • I agree, but wanted to add that a grinder and sink bolts would be a more conventional approach. +Ecnerwal for knowing that the soapstone should be modified. – Ben Welborn Apr 21 '16 at 22:20
  • My inclination would have been to drill and then epoxy in threaded inserts. Soapstone is basically talc, one of the softest rocks, and I would be a bit paranoid about anything which applied concentrated force. Unlike wood, soapstone has little or no elasticity; like wood, it can split if abused. – keshlam Apr 21 '16 at 22:54
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Use the silicone, contrary to some ideas expressed it does stick well to clean smooth surfaces. So well, in fact, that mechanical scraping is often the only way to remove it once cured (actually there are some chemicals, like dimethyl adipate, that soften cured silicone).

If the brackets are securely mounted to the wall, silicone will grip like hell and hold that slab of stone firmly in place. Don't worry about the acetic acid, it smells a bit but won't hurt the iron. The soapstone might get discolored a bit but the underside shouldn't be a concern.

  • You might be thinking about latex/silicone caulks ("ALEX plus", etc.) Pure silicone does not stick to almost anything except itself. It is actually valued as moldmaking material (I used it as such a few times) because you can pour some silicone on a complex object, wait a few hours and easily pull the object out after silicone hardens . – Eugene Smith Apr 22 '16 at 6:08
  • No, latex/silicone does not stay as flexible and could crumble/crack in this application. I am talking about RTV silicone caulk. Sticks like glue. – Jimmy Fix-it Apr 22 '16 at 6:29
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    As seen as the ONLY thing holding most fishtanks together ("frames" on most modern ones are purely decorative plastic - the work is all done by RTV silicone.) – Ecnerwal Apr 25 '16 at 2:06
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I don't have enough reputation here to insert a comment, but I just want to say that the radiator will not work properly then. With radiators the heat spread mainly using convective transfer: the heat go up and you are blocking it with the stone.

  • That's sort of a goal for me—I actually want to block heat from rising up because a television is above it. I know a lot of radiator covers block the top, and just have a front-facing grille to let heat out, so I don't think I'm completely preventing it from heating the apartment. Thanks for your input; I upvoted you in hopes you'll be able to comment soon – MaxGabriel Apr 17 '16 at 22:08
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I would use a Polyurethane Construction Adhesive to attach the stone to the brackets. I would also install the brackets so that there is a 1 inch space between the top of the radiator and the bottom of the stone shelf. This would help reduce the heat transfer to the stone, limiting any stress due to thermal expansion of the stone.

  • I can't find the SDS for that product (locktite changes the names to keep their SDSs unlocatable), but generally polyurethanes have a low temperature rating - around water-boiling temperature (silicone is usually double or triple the boiling point). – Ben Welborn Apr 21 '16 at 22:04
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Silicone is not a particularly good choice for your application, since, one, it's soft (when fully cured, has the consistency of rubber), two, it does not actually stick to surfaces. (And yes, it releases some acetic acid while curing, but in minor quantities, it's a one time thing, and a few drops of acetic acid aka vinegar won't eat a hole through your painted cast-iron radiator.) Your linked loctite silicone is meant for making waterproof seals in gaps (e.g. between sink and countertop), not for structural applications.

Even most glues won't work too well between polished soapstone and iron. You could try the polyurethane adhesive suggested by the other answer, possibly after sanding both surfaces. A more permanent / sturdy approach would be to drill holes in the bottom surface of the stone block, use epoxy to glue a nut into each hole, and then screw brackets to those nuts once epoxy sets.

  • He's attaching to an iron shelf... not directly to the radiator. – Ben Welborn Apr 21 '16 at 22:22
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One part room temperature vulcanizing silicone elastomers are commonly of the air moisture reacting, acetic releasing type. This will typically not pose a corrosive problem, unless the acetic vapors are trapped in an enclosure (eg. electrical box). For critical applications in which corroision may be an issue, the less common neutral curing silicone formulations are utilized. Re adhesion: Again, silicones typically fall into adherent and non-adherent types. Silane coupling agents or primers can either be directly incorporated into the base silicone, or can be applied seperately. As previously stated, large size glass fish tanks are typically sealed/bonded using a silicone adhesive/sealant formulation. This material should adhere well to both (clean) soapstone and I assume properly painted iron brackets. Shelf can be seperated from brackets using thin sharp knife. Make sure to mount TV securely if earthquakes are an issue, and leave approx. 1" gap above radiator to help air convect and reduce direct heat conduction.

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