Vitreous (porcelain US) enamelling a hood

Discussion in 'Fettling Forum' started by presscall, Jul 8, 2020.

  1. presscall

    presscall United Kingdom Subscriber

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    Patching really, of a red Bialaddin hood.

    This specimen, with the characteristic loss of enamel where spot-welds in the hood fabrication conduct heat resulting in hot-spots and enamel micro-cracking and loss. I’ve used a Dremel sanding drum to get rid of loose rust in the patches.

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    I cut a sliver of steel to fill that void. No need to silbraze it in position, it will be covered and be held in place by the enamel.

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    I’ve never done any enamelling before and with no justification to buy a kiln (£££’s) got to thinking that my MAPP blowtorch melts silver solder at a higher melting point than enamel (powdered coloured glass primarily) so why not try that?

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    On reading up on enamelling techniques I was pleased to discover that ‘torch firing’ is an established method. A possible snag was that it’s used primarily in jewellery enamelling, small pieces and not hood-sized.

    I found that as well as the blowtorch I already owned a chemistry lab tripod, something I couple up with a vintage naptha-fuelled Barthel bunsen burner occasionally to brew coffee.

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    Other essential was eye protection, since I visualised enamel perhaps cracking and getting airborne (it didn’t, but as well to take precautions).

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    Enamel supplies in the UK took me to WG Ball and I shopped for ‘groundcoat’ used to provide a base coat on steel ...

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    ... and a ‘Rosso Red’ enamel. There are a number of shades of red and orange vitreous enamel powders available from that supplier so it was a bit of a guess. The website colour chart of the fused enamel is the guide, the colour of the enamel powder is much paler than the finished result.

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    I shopped too for a green enamel to patch yet another Bialaddin hood.

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    I mixed the groundcoat powder with water to make a creamy paste ...

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    ... and coated the patches to be enamelled. I then fired the groundcoat.

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    The enamelling set-up. Torch flame is pointed upwards, from below. Apparently (online enamelling tutorials say) if the torch flame is played directly on the enamel it oxidises and discolours the finished result.

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    No need to let the groundcoat cool completely and I applied the red enamel as a powder, using my little measuring spoon to deposit a layer on each patch. Another reason to heat the work from below, since the torch flame would blow away the enamel powder before it had had a chance to melt.

    Workpiece still hot, to lighten in colour on cooling.

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    Finished result. Far from perfect, but a reasonable first attempt. Certainly encouraging to patch the green hood next. ‘Rosso Red’ turned out to be a decent match.

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    The usual darkening of the enamel on lighting up the Bialaddin.

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    John
     
    Last edited: Jul 8, 2020
  2. Tony Press

    Tony Press Australia Subscriber

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    That’s quite a good test of the technique, John!

    Good luck with the green.

    Tony
     
  3. presscall

    presscall United Kingdom Subscriber

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    @Tony Press
    A promising first go Tony certainly. I didn’t say how much enamel powder is used - very little in fact - and it’s not pricy stuff. Per hood, pence. The MAPP gas expended doesn’t break the bank either as it’s quite a quick firing.

    @ColinG , you could easily get through more expense with your hood painting trials! My guess is that a gloss paint will burn off pretty quickly.
     
  4. JEFF JOHNSON

    JEFF JOHNSON United Kingdom Subscriber

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    Well fettled!:thumbup::thumbup:
     
  5. ROBBO55

    ROBBO55 Subscriber

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    @presscall
    Looks like a good result for a first time, John. :thumbup:
    Did the enamel flow when melted or just tend to liquify?
     
  6. Henry Plews

    Henry Plews Subscriber

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    An encouraging result.
     
  7. presscall

    presscall United Kingdom Subscriber

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    Creating enamel powder ‘heaps’ in the pits left by the lost enamel and the fact that the top of the hood was horizontal kept the enamel where I wanted it once liquefied. The groundcoat enamel appears to act like a flux in soldering/brazing in helping the liquefied enamel ‘wet’ the steel and bond with the surface.

    With hindsight, I applied too much enamel at one go and expect better results with several (2-3) thinner layers. The enamel powder can be mixed with water like the groundcoat is and applying milk-consistency coats with an artist’s brush is the way to go.

    Green hood next!

    The enamel colour range is impressive and includes chocolate brown (I’m thinking of a Tilley PL53 hood in need of patching), yellows (pork pie Tilley cap).

    Enamelling a detachable cap would be easier in terms of access and speed of getting the enamel up to melting point - with less metal to leach away the heat.

    John
     
  8. phaedrus42

    phaedrus42 Subscriber

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    A game-changing technique, John! Some years ago there was a very informative post on the Coleman Collectors forum on home re-enamelling using a pottery kiln. The essence was that you pre-heat the kiln to about 970C, spray or dip on the undercoat on the clean bare-metal vent, bake it for about 10 minutes until the coat flows, remove the vent and allow to cool at room temperature, and then repeat the process for the top coat. It would be most cost-effective to do a batch of vents for every firing of the kiln.
     
  9. presscall

    presscall United Kingdom Subscriber

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    @phaedrus42 Undoubtedly that’s the optimum method, mimicking the way it was done in original manufacture for overall enamelling of a bare metal hood.

    Still, ‘torch enamelling’ is evidently a legitimate technique and achieveable in the home workshop rather than one specifically equipped for enamelling.

    I’ve started on the green hood with a thin groundcoat I’m going to allow to dry naturally - I warmed a skim of much thicker paste with a low flame on the blowtorch.

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    You see the difference, most of the paste on the red hood caked and made the top coat of enamel lumpy. Obviously wrong technique - with hindsight!

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  10. presscall

    presscall United Kingdom Subscriber

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    Progress.

    ‘Groundcoat’ enamel drying (water mixed in the powder evaporating).

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    Groundcoat fired.

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    Green enamel powder layer.

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    Fired. I know, there’s no colour match, but this was to practice on and I can acquire a better match of green enamel to lay over it - they’re opaque enamels.

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    Another layer fired on top of that.

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    Better colour match on this Bialaddin hood, which was similarly afflicted as the other two.

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  11. phaedrus42

    phaedrus42 Subscriber

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    Certainly a worth-while technique for preventing further rust while preserving most of the original enamel!

    Further uses come to mind, such as customizing and personalising...
     
  12. David Shouksmith

    David Shouksmith United Kingdom Founder Member

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    :-k John, if you're heating the hood sufficiently to melt the new frit, is that why the rest of the hood top is starting to look uneven / rippled or whatever? - because the original enamel is starting to melt also...
     
  13. ROBBO55

    ROBBO55 Subscriber

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    @presscall
    Thanks John.
    Your technique and result has improved already. :thumbup:
     
  14. presscall

    presscall United Kingdom Subscriber

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    Of the three I’ve done, I laid on far too much of the red enamel on the first one and all in one go, covering the centre ‘disc’ of intact enamel. Second one didn’t overlay the pre-existing enamel and the enamel patches I applied (two thin applications) weren’t so lumpy as the first one. In the case of the third, darker green hood, the heat caused the centre ‘disc’ of pre-existing enamel to bubble. Indisciplined torch flame application and turned up too much I put that down to.

    I can re-work these samplers - the lighter green 310 hood needs a colour match and there’s always this basket case to have a go at ...

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  15. malcolm race

    malcolm race United Kingdom Subscriber

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    Great tutorial John, well done.
     
  16. Thomas

    Thomas Subscriber

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    Very interesting, your post has renewed my interest in building a mini kiln, I didn't know where to purchase the enamel until now; thanks for sharing
     
  17. MYN

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    That's a great tutorial. Only problem for me is that these porcelain enamel groundcoats, frits and related materials are not commonly found around in my area.
    I should have no real problems building a gas-fired kiln or furnace that could attain average temperatures in excess of 1000deg C. I've done that in the past for other purposes.
     
  18. James

    James Subscriber

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    I would have thought that when you heated the hood the existing enamel would just start to spall off the metal. Sometimes that happens when my lamps are running due to the heat from the burner. Maybe its because the heat is on the lower part of the hood rather than on the upper.
     
  19. phaedrus42

    phaedrus42 Subscriber

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    When bits of enamel shoot off as the hood gets hot that is often because of rust and/or damp on the steel under the enamel.
     
  20. presscall

    presscall United Kingdom Subscriber

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    @James
    I think the spalling you speak of occurs when the enamel’s already on the verge of ‘letting go’ due to a succession of heating and cooling cycles and base metal corrosion.

    Already sound enamel doesn’t ping off whether the blowtorch is applied above or below the surface, but heating from below is preferable to check on the melt of the enamel powder, avoid discolouration from the torch flame and prevent the blast of the flame from blowing away the enamel powder.

    What @phaedrus42 said - posted while I was writing my reply.
     
  21. MYN

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    I've never tried the vitreous enamel touch-ups before. I'd certainly like to try these someday.
    Has anyone here ever attempted heating a piece enamelled steel to bright red(sufficient to re-melt the enamel)? In general, I'd be curious to know whether that would cause any discoloration or degradation of the existing enamel upon cooling.
     
  22. presscall

    presscall United Kingdom Subscriber

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    Yes, during the groundcoat firing on one of the hoods.

    On cooling it looked the same as it did before firing.

    The enamel no doubt melts (when the metal is red hot it’s not possible to see if it has) but doesn’t ‘run’, pool, or spread into the areas lacking enamel.
     
  23. MYN

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    Thanks for that info @presscall . Just what I needed to know.
    I might be able to construct an LPG-fired kiln that'll heat the entire piece uniformly to enamelling temperatures. Besides, its easier for me to slowly and gradually ramp up and down the heat to minimize any thermal shocks. That way I might be able to heat it without the flames impinging on the part. I've made such furnace-like stuffs in the past not for enamelling but for calcining kaolinite to convert it to some forms of metakaolin. In another occasion, I've made some sodium chromate in a similar construction by fusing and oxidizing green chromic oxide with molten sodium carbonate and hydroxide in a clay crucible. The entire 10-inch crucible attained a vivid, yellow-white glow from visual inspection(almost as bright as a lantern's mantle at full throttle).
    A little off-topic the above :content:, but I'd guess I might attempt some vitreous enamelling stuffs at home if I could get my hands on the proper frits.
     
  24. presscall

    presscall United Kingdom Subscriber

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    @MYN That’s a perfect set-up you’re describing, a controllable heat-soak of the whole component. On reflection, zapping any part of the enamel, pre-existing or newly laid-on, so that it gets red-hot quickly can induce bubbling. Not pronounced, but I detected some. Controllable and avoidable even with a blowtorch, but more so with the kiln you’re proposing.

    Looking forward to your photos and observations when you get that done!
    I find myself looking around for projects now! Launched into the (potentially) ambitious stuff when I should have knocked out a keyring or two first. The ‘stove’ side of my hobby suggests a stove control key might get the treatment - I’ll get ribbed by the CCS attendees at our next camp-out for being too ‘flash’ probably, but what the heck.
     
  25. MYN

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    That's sure a useful tip. Controlling the rate of rise of temperatures might be easier said than done. Especially with my inherent impatience and doing it with just a simple naturally aspirated burner without any thermocouple feedbacks on the actual kiln temperature.
    This will take some practice and persistence.
     
  26. presscall

    presscall United Kingdom Subscriber

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    @MYN There’s another issue you’d have to factor in and that’s any turbulence of the gases in the kiln and how they could disrupt the surface of the enamel they impinge on. Think ‘effect of wind rippling surface of water’. Torch firing with the flame playing on the back of the metal avoids that.

    Enamelling kilns I’ve seen on the market are electric, so again no blast of flame hitting the enamel surface. A gas-fired kiln would need to be large enough to facilitate some sort of internal baffle arrangement or inner chamber physically (though not thermally!) isolated from the heat source to give a relatively turbulence-free space around the object to be enamelled.
     
  27. phaedrus42

    phaedrus42 Subscriber

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    Unlike in the case of pottery, for enamel work, the rate of rise and fall of the temperature of the kiln does not really matter. Modern pottery kilns use a programmable RAMPS temperature controller to set the temperature curve for the whole baking and cooling cycle. For enamel firing, the oven is pre-heated to slightly above the melting point of the enamel. The prepared cold work piece is put into the hot kiln for a few minutes until the glass powder has fused and is then removed and allowed to cool at ambient room temperature.
     
  28. MYN

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    Useful tip. I guess that's gonna require some extra thermal power from the burner(s). I could just house the piece in an earthenware pot or something. From my previous experience with calcining, I'm pretty sure the heat would soak in sufficiently to fuse whatever's suppose to fuse.
    Sounds simpler than pottery then. Glad to hear that. Although I'm quite familiar with automatic control stuffs(partly from my profession), I'd not be inclined to invest on a temperature controller or some high power-rating controlled thyristor drives for this purpose. That'd be over-ambitious in my opinion.:)
     
  29. phaedrus42

    phaedrus42 Subscriber

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    Yes, it is almost, but not quite, entirely unlike pottery ;) :lol:

    Digital temperature controllers and 40 or 60 amp solid state relays are cheap as chips these days.
    Just pick your thermocouple with care. Many don't survive 1000C.

    Ramp/Soak PID temperature controllers for pottery kilns are a bit more pricey but these days one can achieve that functionality cheaply with an Arduino or ESP32 microcontroller.

    Electric Kiln Controller - Arduino Project Hub

    KilnShark
     
    Last edited: Jul 12, 2020
  30. presscall

    presscall United Kingdom Subscriber

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    @phaedrus42 @MYN
    Love this discussion, thanks for your insights. My tripod and blowtorch seem quite spartan now ... :)
     

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