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!
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.
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.
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.
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
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
Tufa is my favorite casting method. Using it is very simple, as
long as you follow some simlpe rules.
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.
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.
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!)
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.
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)
Wire the two sooty surfaces together.
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.
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.
Thanks Rob for bringing this forward. I just ordered some tufa and plan on giving it a go. This thread is really informative! My only question for those in the know would be about adding a stone to a cuff. Would it be best to form a recess in the mold to set the stone or simply solder a bezel onto the cuff. Or will both work and it’s a matter of “the look” I’m going for.