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Thread: "Korina" finish

  1. #1
    Luthier&Pickup maker ret. Soundfarmer Pete's Avatar
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    Default "Korina" finish

    Maybe a long shot but I`m building a couple of electrics inspired by the `58 "Korina" V and Explorer.
    Limba (Terminalia Superba) isn`t easily available here so I`ve used Idigbo (Terminalia Ivorensis) which is a close relative with similar properties.

    Just wondering if anyone knows whether the original guitars were given a color coat or whether that lovely orangey yellow is a result of wood/cellulose ageing gracefully........I don`t like those orange tinted maple necks (or fake sun tans come to that

  2. #2

    Default Re: "Korina" finish

    I don't know about the old Gibson finish schedules.
    But, if you do some experiments with potassium dichromate (Van Dyke crystals) or potassium hydroxide (potash), it should accelerate the natural darkening of the wood. Ammonia is also traditional for darkening some woods.. works great on eucalyptus. Sometimes you get crazy things happening, like African mahogany turning bright red when wiped with potash. But in theory, if you get the color where you want it, the darkened wood should stay pretty much the same color over time, which isn't necessarily true with trans-tint or similar dyes.

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  4. #3
    Registered User sunburst's Avatar
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    Default Re: "Korina" finish

    Marty, why would (extremely toxic, rather unpredictable) potassium dichromate be more color fast than (less toxic, more predictable) transtint?
    (BTW ammonia reacts with tannins to darken wood, so tannin content is needed for that process.)

    Sorry, no knowledge of the Gibson "Korina" finish.

  5. #4

    Default Re: "Korina" finish

    I agree, I don't like working with potash or other serious stuff if it can be avoided.
    My thinking was that if you want wood to look like it's 50 years old, if you can accomplish a similar change in the wood, then it should stay that way, right?

    I just got some potash last week and was going to play with it carefully in very weak solutions. I mentioned potassium dichromate because it's been used by some well-known mandolin makers- probably because it was mentioned in the original Siminoff book (IIRC).
    Last edited by Marty Jacobson; Apr-20-2013 at 6:30pm.

  6. #5
    Café habitué Paul Hostetter's Avatar
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    Default Re: "Korina" finish

    The color is mostly a natural yellowing of wood and lacquer. I remember Gibson's korina instruments when they were new and they were disturbingly white.





    There are two good articles (which, alas, don't quite agree!) here and here.
    .
    ph

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    Paul Hostetter, luthier
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  7. #6
    Luthier&Pickup maker ret. Soundfarmer Pete's Avatar
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    Default Re: "Korina" finish

    Thanks very much Paul.....fifty years down the line, my babies should look rather nice!

  8. #7

    Default Re: "Korina" finish

    I don't know what pigments, dyes, or stains are used in Transtints, but metal oxide and salt pigments have been recognized for their permanence for many centuries. If I could go back to where my childhood happened in the 40s and find one of those old John Deere's, I'd have no doubt that the Cr2O3 green paint would be the same color today. Or I might find an Allis-Chalmers with that chromium pigment red, also still the same. And the chrome yellow that makes the highway lines last until they're worn off -- that's another example. Of course treating wood with dichromate is a little more complex than just adding a pigment. The darkening seems almost certainly oxidation by the chromate, but in the earth's outdoor or dry environment, there's not much that happens to reverse oxidation. No danger of anything un-rusting without some severe treatment.

  9. #8
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    Default Re: "Korina" finish

    Potassium dichromate is not just "extremely toxic"; it is among the most potent carcinogens known to man (and woman). Right up there with things like benzo-a-pyrene. Dichromate salts ((Cr2O7)2-, orange), and their very close cousins, chromate salts ((CrO4)2-, yellow) are "Chromium(VI)", aka "Chromium six", compounds. They are strong oxidizing agents. That means that they do their coloring by taking electrons away from other things. In the process, they are "reduced", meaning that they gain those electrons themselves to become something other than Cr(VI). The most common products of the process are chromium(III) compounds, most often chromium(III) oxide, Cr2O3, which is a deep green color. Use a little too concentrated dichromate solution to color your instrument wood, and you will notice a definite greenish tint in the result, evidence of the presence of the Cr2O3. So, not only do you risk carcinognesis, but you also get a green tint in your colored wood. Is it worth it? I don't think so. Look up the MSDS sheets for Cr(VI) compounds. Do you have a glove box to work with the stuff?

    "TransTint" dyes, also repackaged by Stew-Mac under another name, are metal complexes in a glycol ether solvent, or at least some of the colors are. They are not particularly safe either, but to my present knowledge, at least they are not Cr(VI) compounds. You will note some warnings on the label. Good idea to use disposable gloves with those things as well.

    http://www.Cohenmando.com

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  11. #9
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    Default Re: "Korina" finish

    Luthier Michael Stevens solved this issue on my electric mandolins by using real Korina (Limba) on the back sides and neck. I love this wood for both the look and the tone.

    Click image for larger version. 

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    You can see a few recent guitars he's done in Korina here.
    Paul Glasse
    Driftwood, Texas
    http://paulglasse.com

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  13. #10
    Resonate globally Pete Jenner's Avatar
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    Default Re: "Korina" finish

    Strueth Dave it seems you need gloves just to look up the MSDS!!! [reaches for sunglasses]
    The more I learn, the less I know.

    Peter Jenner
    Blackheathen

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