Repairing an enamel on a cast iron sink - can I do this with heat?
July 8, 2018 7:37 PM   Subscribe

I have an old cast iron sink with a few chips. What is the plausibility of using an enamel powder normally used in jewelry or ceramics and a gas torch to fill these chips?

I have done a bunch of reading on repairing these things and the three accepted methods, in order of preference and inverse order of cost are to use a two part epoxy resin, to have the thing sandblasted and apply a two part epoxy resin to the entire thing or to have it sandblasted and properly re-enamelled by someone able to do that. I don't much like the first option as it is reputed to not last long and the second two are more money than I want to throw at this, particularly given I could just ignore the chips.

So I got thinking that the vitreous enamel on the sink is quite similar to the enamels used in pottery and jewelry, and sometimes they get applied with a gas torch. I have a workshop, I have a gas torch (and can get hold of a road-works sized one or oxy without trouble) and I have a good amount of manual skill. Can I fill the chips with an enamel powder or liquid glaze of a vaguely appropriate colour, heat up the area and get the general vicinity viscous enough for the old and new enamel to flow and adhere? The chips are in a location that means that gravity can be used to assist things rather than dragging liquids somewhere I don't want them.

Searching suggests that if anyone has tried this they haven't been game to talk about the experience. Does anyone with knowledge in any of these areas have advice they can share? I'm most concerned about oxidation or burning of the enamels from a direct rather than general heat, lack of adhesion and any other problems people can think of. The sink is not installed and can be in whatever orientation necessary, I can access the underside and pre-heat the cast iron to avoid massive thermal gradients.
posted by deadwax to Home & Garden (3 answers total) 1 user marked this as a favorite
massive thermal gradients

are going to be your problem, I would think, given any kind of spot-heating process; given the rather extreme temperatures it takes to fuse vitreous enamel, I would expect to see horrible cracking and crazing in the non-heated sections, and you might even crack the cast iron itself.

If it's only a few chips, you might get away with spot fills using a polymer powder coating rather than going all the way to vitreous enamel.
posted by flabdablet at 7:35 AM on July 9, 2018 [1 favorite]

Adhesion and temperature control are going to doom this effort. I have used a variety of commercially available jewelry frit and in general once cooled enamel is hard to work with, even with the same frit and hardware. I have never 'repaired' and have always had to replace any damaged enamel. Note: enamel was not something I specifically specialized in - but in my travels its exceptional for anyone to bother with a 'repair' it's generally been replace. Reheating enamel can change its color, and it might not bond with a new frit of different chemical composition. I have ruined other folks prized personal jewelry, and kept one piece on my bench as a reminder.

But the big issue is temperature. Proper enameling requires good temperature control over long periods of time, the differential contraction on large glass/ceramic will cause the hot area to just break/shear/peel away from the cooler area outside of the cone of heat a torch makes. Outside of beadwork most glass work and all enamel is kiln fired. And it will be impossible to get the underlay metal hot enough to get adhesion - that will act like a giant heat sink. Worst of all even if you got everything hot enough - improper cooling will make the enamel much more fragile and prone to shattering. Even for small bits of enamelwork cooling can take a very long time with temperature controlled in deliberate steps down.

I have used the epoxy for repairs - get nice 3m 600 grit and 1200 grit sand paper, follow the directions and give yourself more time to let it 'set' before touching it.
posted by zenon at 7:55 AM on July 9, 2018 [2 favorites]

the differential contraction on large glass/ceramic will cause the hot area to just break/shear/peel away from the cooler area outside of the cone of heat a torch makes.

To understand why this is so, look at the coefficients of thermal expansion for various kinds of glass vs cast iron.

You'll notice that cast iron has a higher coefficient than any kind of glass; so as an enamel-coated cast iron piece cools, the cast iron base metal will want to shrink faster than the coating, and that shrinkage of the base metal will keep the coating in compression. Once the piece has cooled to the point where the coating enamel's viscosity becomes high enough to resist further flow, the coating will stay in compression, and this is part of what gives an enamel surface its strength.

It also explains the way the surface fails when it does fail. Bang an enamel surface hard enough and a chip will fly off, leaving a little cone-shaped depression behind. In effect, the compressed surface around the chip has squeezed the chip out like a cherry pip between your fingers. Typically, there will be no cracking extending beyond the chip zone, because all that enamel remains in compression.

But because the surface did compress and flow plastically for part of the cooling process, it won't end up with enough internal compression to deal with expansion on reheating. Once you get an enamel-coated surface anywhere near the temperature at which the enamel softens enough to flow again, the underlying iron will already have been trying to expand faster than the overlying glass for some time; this puts the enamel surface into tension, and it will crack and flake and pull itself off the iron as a result.

Ask anybody who has accidentally left an expensive Le Creuset pot on an active hotplate with nothing inside it. The results are not pretty.
posted by flabdablet at 12:25 AM on July 10, 2018 [2 favorites]

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