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Casting in tufa


#1

Hi all, I have been following with interest the discussion of
the Delft Clay Mold casting method. I, too, am looking for a
simple casting technique that will let me make more than one of
something [ruling out cuttlebone…whew says my loved one, who
hates the smell of toasted squid innards!]. Anyway, when I was in
New Mexico for the SNAG meeting in 1997, I saw some stunning
Native American pieces that were cast in tufa, a kind of
compressed volcanic ash. The surface texture of these pieces was
beautiful,finer than a sand-casting, brighter than a bead-blasted
surface: just the kind of thing I want to use under transparent
enamels. So I ordered a nice 8 lb. chunk of tufa from Indian
Jewelers Supply. Now I have this interesting stuff, and am
wondering if anybody else out there has used this material. Any
pointers, observations, frustrations, and so on would be greatly
appreciated!

thanks in advance,

Anne Hollerbach
@alhollerbach


#2
  1. Cut the material into squares or rectangles. Use a band saw
    if you have it, otherwise use a hack saw. You need matching pairs
    for each casting. Rub the faces together before carving, and true
    up all edges.

  2. Start carving your design on one face, using wax tools,
    X-acto knives, toothpicks, etc. Your design will have to be
    fairly thick with no undercuts. Cut air vents from the carving to
    the outside edge of the block, with the lines leading upwards.
    Carve one half of your sprue hole on the carved side. Place the
    matching blocks together and mark the sprue hole on the other
    side. Carve the sprue hole. The width of the sprue leading into
    your casting will be determined by how large the casting will be.

  3. Once your carving is complete, soot up both sides of the tufa
    with lampblack from a candle or your acetylene torch. This helps
    it to release your finished casting, and you will be able to use
    your mold several more times. Wire the mold shut by binding
    around the perimeter of the paired blocks. Set the blocks onto a
    fireproof surface. Heat the blocks by aiming your torch into the
    sprue. Switch to melting your metal and make your pour. Pour
    quickly so it doesn’t cool in the sprue. Let the blocks cool
    until you can touch them with your hands. Snip off the binding
    wire and unseat your molding. Check the molding for incomplete
    fills. If you have incomplete fills from a the mold not being
    carved deeply enough, you can recarve it.

  4. Once you have experience with carving the molds and have an
    idea of how large the sprue needs to be for various carvings, you
    can stabilize the molds before using them. To do this, you apply
    waterglass solution thinned out with water before you do the
    sooting operation. You let it dry, then soot up your mold and
    cast. This process makes the tufa really hard, and you won’t be
    able to do any additional carving, but it will last through
    dozens of pours without deterioration. Save one of your first
    castings in case the design is one you really like. It can be
    used to replicate in a rubber mold for shooting waxes.

Hope this helps, and if I’m not clear on something, contact me through
regualar email at @kpalchk


#3

Hi ; - Two references which deal with the problem of "
sandcasting in Tufa " are as follows . Oppi Untrachts superb
book - Metal Techniques for Craftsmen - ISBN - 0-385-03027-4 .
check pages 334 - 337 for a good traditional ( Native American )
illustration . And for those of us who can not import Tufa then
there is another reference in Tim McCreights less exhaustive and
less exhausting book " Practical Casting - a Studio Reference "

  • ISBN - 0-9615984-5-X . Pages 67 - 76 cover both how to cast
    using the manmade Tufa and directions as how to make up and
    handle it . There is another really good book by Tim McCreight -
    " The Complete Metalsmith " - ISBN - 0-87192-240-1 . Value
    wise I must recommend BOTH books by Tim McCreight . They would be
    worth it at twice the price . And for my final and absolutely
    last recommendation I would suggest that You contact Mr. Charles
    Lewton-Brain at Brain Press ( brainnet@cadvision.com )
    for these two books as well as others which he stocks and or
    publishes . They do good bidness ( I am a Texan ) and they are
    fun . - Most highly recommended . Please let the rest of know
    how you are doing . and p.s. make sure that you work the
    investment and powdered pumice mix outside or in a really well
    ventilated area so as NOT TO BREATH THE DUST . Respectfully Yours
    ; Robert Original Message From: Anne Larsen Hollerbach
    alhollerbach@norfolk-county.com Date: Tuesday, February 23,
    1999 10:09 PM Subject: [Orchid] casting in tufa

#4

Hi Anne:

Tufa is my favorite casting method. Using it is very simple, as
long as you follow some simlpe rules.

  1. Prior to carving your tufa, its a good idea to cure it first.
    I start out by putting the stones in the oven at 200 deg for an
    hour or so then up to 400 for two or three hours. This dries out
    the tufa and also makes it easier to work. (the chemical
    reactions are something I never bothered to learn, but trust me,
    cure it first) Allow it to cool slooooowly.

  2. Be sure to rub both surfaces that are to meet perfectly flat,
    (on concrete, a belt sander belt, whatever) then rub the two
    faces together til they form a perfect fit. Examine the surface
    for inclusions and plan your design around them if possible,
    they’re a real PITA to carve.

  3. When you lay out your design, start it far enough down the
    stone so that you have enough room for an adequate sprue. Don’t
    forget to match the sprue groove on the face stone. (Very
    important!)

  4. After your design is carved, (no undercuts if you want to use
    it again) cut vent channels about 1-1/2 mm wide starting at the
    very bottom of the design and curving up to the top. Make these
    vent channels all allong the sides of your design at about every
    1-1/2" 'til you get to about 1" from the top. Make sure they all
    curve upward, you don’t want your molten metal drooling out all
    over everywhere.

  5. I use my prestolyle torch for the next stage. Holding the
    finished carving in such a way as to avoid burning your hand,
    completely coat both mating surfaces with soot. To do this I
    "choke" my torch by wrapping my hand around the air vent where
    the tip screws into the handle. By cutting off the air supply to
    the acetelyne, and holding the stone above the torch, great gooey
    gobs of soot are deposited onto the face of the tufa. Lay it on
    thick. This treatment pre-heats the stones, acts as a releasing
    agent and as an anti-oxydent for the metal. ( Plan on lots of
    black greasy snow to fall in the room where you do this)

  6. Wire the two sooty surfaces together.

  7. Melt your metal (I use 60% scrap 30% clean and 10% fine
    silver) in a ceramic crucible using an adequate amt. of boric
    acid and pour at the slippery-mercury-but -not-spinning stage.

  8. Once the sprue stops glowing, cut the wires, open the mold,
    and place the casting in the pickle. Soak for a few minutes &
    Viola! trim off the sprue and vents and you’ve got as pretty a
    casting as you could ask for!

Oh by the way, don’t plan on using the same mold for much more
than 5 castings, the tufa just won’t stand up to the repeated
thermal shock, and starts breaking down.

I’ve demonstrated this casting method to my sons’ 3rd & 4th
grade classes and the kids were absolutely mesmerized.

Hank Paynter
Brook Hollow Studio