The ceramic shell
technique" by
Jill Varani
Making the
First Wax
Step One: Sculpting the
Original
Step
one in the sculpting process is to create your original sculpture. This
can be done using any material but traditionally artists use either clay
or wax. Both materials have their advantages and drawbacks.
·
Clay is a good medium because it is soft and
easy to mold. It is great for working smaller pieces, which is what we
are focusing on here. However, there are two major disadvantages: it
needs to be kept moist or it will dry out, and it requires an armature
to hold its weight.
·
Plasticene is a more expensive kind of clay
that won’t dry out, but it also cannot be fired.
·
Wax is also easy to use, and for smaller
pieces it doesn’t require an armature because cold wax is strong enough
to hold itself up. It is not as soft as clay and therefore more
difficult to model. It can be warmed under a lamp until it is soft
enough to mold in your hands, and cooled down in order to smooth the
piece and add the finer details. Another
advantage of sculpting the original in wax is that you don’t
necessarily need to make a mould.
For
information on armatures, see:
·
From Clay to Bronze by Tuck Langland
·
http://tx.essortment.com/armaturewire_rkxg.htm
·
You can also sometimes purchase pre-made
armatures at Sculpture supply stores. Barnes in Australia, (http://www.barnes.com.au)
supplies both pre-made armatures and armature wire. Be aware that the
armature wire sold in sculpture supply stores is just a soft aluminum
wire that can usually be found at metal merchants for less than a
quarter of the price.
Step
Two: Making a Mould of Your Sculpture
The mould is the hollow form
into which you will pour molten wax, that will then harden in the shape
of your original sculpture. This is made in two parts or layers. The
first layer is a silicone-rubber skin that catches the details of your
sculpture. The second layer is a plaster backing that supports the
silicone.
The
silicone component of the mould can be made as a single skin that covers
the entire piece, or it can be made in parts. A single skin will need to
be carefully cut to free the sculpture; therefore, a complicated piece
with many angles, undercuts, and holes would be better off made as a
piece mould.
Before Beginning the Single Skin Mould:
·
You need to set aside a block of time. The
silicone must be done all in one go, layer by layer. If you wait too
long between layers, it may set and the layers won’t bind together. It
takes between three to seven layers of silicone, with at least a half
hour wait between each layer. You can extend the half hour safely for up
to six hours, but be careful that you don’t wait too long, or you will
have to start over.
·
You need a clean space to work. The mould will
need time to set overnight, so make sure it is somewhere out of the way.
It will be a messy job, so lay down newspaper or plastic.
·
Think about where you want the opening of the
mould to go: the opening of the mould goes where you don’t paint
silicone or build plaster. If you secure the sculpture to a base and
paint silicone around the entire thing, then the opening will be at the
bottom of the sculpture, where it is secured to the base. This could be
a problem for a piece with a smaller base than peak, for instance, a
standing human figure. It would be wise then to make a sprue on the
thickest part of your sculpture, and secure the other end of the sprue
to the base (for definitions/how to, see the section on “Sprueing your
wax”).
·
The sculpture needs to be secured firmly to a
base. For small pieces, the best way to do this is to take a flat metal
tray and pour a thin layer of molten wax into the tray so that the
bottom is covered. Once the wax has cooled very slightly—after only a
minute or so—set your sculpture onto the middle of the tray, so that it
sinks into the warm wax. Let the wax cool down the rest of the way, and
test to make sure the sculpture does not move.
·
It is handy to put the sculpture on a
turntable in order to view it from all sides so as not to miss any spots
when coating it in rubber.
·
If the sculpture is made of anything besides
wax, it should be coated with a release agent to free the mould from the
sculpture.
·
You will need the silicone rubber and its
component parts: silicone rubber (RTV), the catalyst, plus the
thickener, which is useful for the final layers. Barnes (http://www.barnesonline.com.au)
in Australia, and TopMark (http://www.topmark.co.nz)
in New Zealand supply various silicones and accessories. “Trade 26
Silicone RTV” and the associated catalyst and thickener are a good
choice. Be aware that there are less expensive alternatives to silicone
for flexible moulds, including latex and some polyurethane rubbers.
·
You will need a scale for measuring and
mixing, and disposable plastic cups and a plastic spoon or knife for
mixing.
·
You will also need latex gloves and several
disposable brushes.
The best book on Bronze casting. A must to every
sculptor
USA
UK
Making the Skin Mould:
·
Begin by mixing the silicone and catalyst
according to the product instructions; do not mix in the thickener yet.
If you are working with a small sculpture (Approximately 20x10x10 cm or
less), then only mix 50g of silicone. For sculptures that are a bit
bigger, mix 100g, and beyond that, use your best judgment, keeping in
mind that you want a layer of silicone no thicker than 1 cm. Also, if
you are making a mould for a very large piece, you could try mixing 100g
at a time and painting it on in sections.
·
Paint a very thin layer of the silicone
mixture onto the sculpture, beginning from the top. The silicone will
slide to the bottom until it has cured. It may be necessary to keep
moving silicone back to the top to keep an even coating over the entire
surface. If you see bubbles forming, you can help free the air by
prodding the bubbles with a needle or with your brush.
·
Wait approximately 30 minutes until the first
layer begins to set. Test the layer before painting the next: if the
silicone comes away from the piece when touched, it needs more time. If
the silicone gently smoothes back into place, then move onto the next
layer.
·
For the second layer, repeat the procedure for
the first layer, keeping a thin even coating over the entire piece.
·
For the third layer, mix twice as much
silicone with a proportionate amount of catalyst, and mix in the
recommended amount of thickener.
·
Paint on an even coating of silicone. The
third and following layers are meant to thicken the silicone rubber mold.
The thicker the mold, the stronger and also less flexible it will be.
For a small piece, you want 1 to 2 cm thickness. For a larger piece, you
will want a thicker mold. The main thing is to reduce or preferably
eliminate all undercuts on the mold so that you need less plaster parts
in the next step.
·
Continue this process until you have reached
the desired thickness of silicone rubber.
·
Let the silicone cure completely. This amount
of time will vary depending on the type of silicone rubber RTV you are
using, so refer to the product instructions.
Making the Plaster Support
·
Start by cleaning the space around the mold,
and removing any unwanted silicone bits from around the mould. Cut
around the base of the mould to remove the excess silicone rubber.
·
For the plaster mould, you will need water,
plaster, a bowl or bucket and a plastic spoon for mixing the plaster.
You’ll need enough clay to cover half the silicone covered sculpture, a
rolling pin or bottle for rolling the clay, and a knife for cutting the
clay. You will also need a small brush, some petroleum jelly, a marker,
and a sponge.
·
Begin by deciding how you will build your
plaster parts so that they can be easily taken apart. Notice any
undercuts or places where the plaster will get stuck trying to come off
the mould. If it helps, mark the line where the plaster will separate.
·
Roll out the clay and cut it into strips no
more than 3 cm wide.
·
Use the clay strips to build a wall along the
plaster parting line. Use the excess clay to lump behind the wall,
leaving one plaster section free of clay. The wall should be firm so
that it won’t move when the plaster piece is built against it.
·
Use the back of a pen or marker to press
“keys” into the clay wall. The keys are small indentations that will
help the plaster pieces refit together easily once the mould comes
apart.
·
Mix the plaster according to the package
instructions. Mix enough for at least 3 cm thickness of plaster where
you are working on the mould.
·
When the plaster is still liquid, fill the key
holes and any areas that might prove difficult to fill later on. Add
more plaster as it begins to set.
·
Build an even section of plaster. Take special
care to make the edges of the section thick enough. Before it hardens,
make sure the top of the clay wall is visible. Run a knife along the
parting line.
·
Add knobs of plaster to the section: these
will help hold the rubber bands in place when putting the mould back
together.
·
The final step is to smooth out the section of
plaster. The easiest way to do this is take a damp sponge to smooth out
the plaster.
·
Let it set completely.
·
If you have more than two sections of plaster
to build, then continue this process of building a clay wall along the
parting line. Remove the clay wall from the plaster section previously
built, and use the petroleum jelly to act as a release agent between
plaster sections (paint it on with a small brush, and thin it with
turpentine if necessary). Otherwise, simply remove the clay from the
mould, paint the petroleum jelly on the side of the plaster section, and
build the second section the same as the first.
Making a Piece Mould
·
If you have a complicated sculpture with too
many undercuts and deep areas to make a simple single skin mould, it
might be easier to do the silicone in parts.
·
This is done the same way as the plaster
parts: simply build a clay wall around the parting line of the sculpture
and paint on the layers of silicone in sections.
·
This method takes much longer; but when it
comes to taking apart the mould, it allows you to avoid the difficult
job of cutting the silicone off the sculpture without ruining it.
Cutting the Silicone and Removing the Mould
·
Take off the plaster parts of the mould. If
you have made it properly, the plaster sections should come off easily.
If any sections break, they can be rebuilt or it may be advantageous to
redo the plaster completely, using a different parting line.
·
You need to cut the single skin of silicone to
remove the sculpture, preferably without breaking it. You want to cut as
little silicone as possible. The beauty of silicone rubber is that it
can be stretched and pulled gently, almost peeled off the sculpture in
places. Therefore, try cutting as little as possible and then removing
the sculpture with a little manipulation. You will have to cut where
there are holes or gaps in the sculpture. Take this job slowly because
hasty cuts can ruin a mould.
Step Three: Pouring the Wax
The process of pouring the wax
into the mould is called “slushing.” The method is simple but will vary
from sculpture to sculpture, and it takes a small amount of practice to
figure out the best way for a specific piece. To pour any wax, you need
at least enough molten wax to fill the mould. Wax can be purchased at
foundries or sculpture suppliers. A traditional wax is Victory Brown,
which is a soft enough wax to sculpt in your hands, but hard enough to
hold it’s own weight for very small pieces.
·
The simplest way to pour into the mould is to
set it up so it won’t tip, and fill the mould with molten wax. This
often results in lots of bubbles throughout the wax, which is the reason
most waxes, solid and hollow, are poured in layers.
·
To pour a first layer of wax, use wax at a
temperature of 105-110° C, ladle wax into the mould until it is at least
2/3 full, and roll it around slowly, coating the entire inside of the
mould with wax. Slowly pour it out. This is the slushing method, which
is commonly recommended by literature on the subject because it is
believed to be the best way to avoid bubbles in the first layer.
However, another method is to completely ignore the slow slushing rule,
fill the mould 2/3 full of wax, and shake it vigorously until the
bubbles float to the surface.
·
Do the following layers by slushing the wax
slowly. The second layer should be wax at a temperature of approximately
90° C. The third layer should be 80° C, the fourth at 70°C and so on. If
you intend to fill the mould for a solid wax, you probably only need one
or two layers. However, the more layers, the less chance of defects and
bubbles, but if you pour the solid core too cold, it may not fill small
thin sections properly.
·
If you are pouring a hollow wax, try to get an
even thickness of wax throughout the piece. This will make the bronze
casting go more smoothly.
·
Let the wax cool before opening the mould.
Preparing the
Wax
Step One: Cleaning the
Wax Sculpture
What is in the wax will be in
the bronze, therefore, the sculpture should be cleaned to your liking.
·
Look for seam lines (where the cuts were made
in the silicone usually results in a line of wax that isn’t part of the
original; this is the seam line), bubbles, holes, miscasts (what happens
when the wax doesn’t flow into all areas of the mould), and anything
else that isn’t part of the original or to your liking.
·
To fill bubble holes in wax: Use an oil burner
that uses a candle to heat the oil: put wax where the oil should go. You
may want to melt the wax with a small torch, because it will take a
candle a long time to melt the wax. Take a small brush—the best ones for
the job have fiberglass bristles that can be lit to heat the brush and
smooth the wax with the warmed brush. You can also used a needle for
this job. Pick up a small amount of melted wax on the needle or brush
end and drop it over the bubble.
·
If you have a surface that is covered in
bubbles, try rubbing a wax paste onto the surface to smooth out the
texture: you can simply warm some wax and rub it over the surface, or
you can make a wax paste out of normal victory brown and beeswax.
·
To smooth seam lines: again, the best tool for
this job is a fiberglass brush. Simply warm the brush in a candle flame
and smooth the wax with the heated brush.
·
To imitate the original texture of the
sculpture try a variety of metal tools that can be heated in a flame,
including basic silverware. A butter-knife is ideal. Dentistry tools are
great.
·
There are some chemicals that soften wax.
Turpentine will dissolve wax: dip a scouring pad into a little bit of
turpentine and rub the wax with it. Wipe away the dissolved
wax/turpentine with your finger or soft cloth. There is a product called
WaxKleen that has a similar effect.
·
Practice and patience are the best tools for
cleaning wax.
·
Many sculptors sign their work at this stage,
although it can wait. The signature might also be erased in the chasing
process.
Step
Two: Sprueing the Wax
Sprueing is the term used for
adding sprues, vents, and cups to the wax piece. Bronze is poured into
the cup, sprues are the channels through which the molten bronze will
flow, and vents are the channels through which trapped air can escape.
Therefore, sprueing the wax is
essentially the plan of how the metal will flow into the shell.
·
Sprues, vents, and cups can all be made in
wax: To make a cup, take a plaster mould of any cup shape around the
outside of the cup. Once the plaster has set, put it in water for twenty
minutes (otherwise, the wax will not come off the plaster). Pour wax
into the cup and let it cool around the edges (you can see the molten
wax dull slightly as it cools), then pour it out, leaving a layer of wax
on the inside of the mould. Let this cool and then take it out. This is
the cup. Methods for Modern Sculptors, by…, recommends making
square cups and square sprues, because it may reduce turbulence in the
metal as it flows from the cup into the sprue. Round cups also work
well, however.
·
Sprues can be made from rectangular or
cylindrical shapes, but should be made as thick as the thickest part of
your sculpture. Again, Methods recommends rectangular sprues to
reduce turbulence. Vents can be any kind of shape; they do not need to
be very thick. Make the sprues and vents using plaster moulds of these
shapes. Let the plaster moulds sit in water for twenty minutes. Pour wax
into the moulds, and let them cool. You’ll probably need several vents
for each sculpture, so make a lot.
·
You want to attach the sprue to the thickest
part of your sculpture, where the most bronze will be. This is done for
multiple reasons; mainly, you want the metal to flow to all parts of the
sculpture, so if you attach the main sprue to a big area, more metal can
flow into the sculpture all at once and out into the smaller areas more
easily. If you attached the main sprue to a thin area, the bronze might
freeze before filling the entire sculpture, resulting in a miscast.
·
Attach a wax sprue by welding the end of the
sprue to the sculpture. Welding wax is simple, although delicate: take a
metal knife and heat it with a small torch. The knife is hot enough when
it melts the wax instantly upon coming into contact with it. Take the
hot knife between the two wax ends that you want to weld. Put the ends
together on the knife and gently slide the knife out, holding the two
waxes together. Test the bond once it has cooled to make sure it is
strong enough to hold the sculpture with the sprue.
·
You want to attach the sprue at an angle that
will allow the bronze to flow gently into the sculpture. It is often
recommended to attach the sprue at such an angle that the metal flows up
from the ground to reduce turbulence and encourage a smoother flow of
metal. This method of sprueing from the bottom works best when the
heaviest part of the sculpture is at the bottom and it might be too
complicated for other pieces.
·
Weld the bottom of the cup to the end of the
main sprue.
·
If you have two thick sections on the
sculpture, connected together by a thinner section, called a neck, you
may want to add a feed, which is simply a secondary sprue into those
other large sections. Weld all feeds to the main sprue, not the cup, to
ensure that the metal flows smoothly. Feeds should also be welded at an
angle so that the metal flows up into the feed as it is flowing into the
sculpture. This ensures that two flows of metal aren’t created, which
sometimes results in miscasts.
·
Attach vents at all appendages where air might
get trapped. It is good to attach vents from the appendage to the cup to
allow easier dewaxing, but it doesn’t usually make a difference how long
the vent is when pouring the bronze.
·
Weigh the final piece and mark the number down
for reference when pouring the metal.
The
Investment Process
"Investment Casting" is another name for Lost Wax
Casting. Investment is the term used for coating the wax in a refractory
ceramic shell. Once the shell has dried, the wax is then melted out in a
process called dewaxing.
The
Primary Layer(s)
·
Before shelling up, clean the wax with a
mixture of dish soap and isopropyl alcohol at a ratio of 50:50. You can
also use shellac over the wax. Brush on the mixture and let the wax dry.
·
Throughout the entire investment process, you
will make a slurry: a mixture of molochite flour and colloidal silica.
For the first layer, you want a thin creamy slurry.
·
Paint the slurry onto the wax. You can also
dip the wax into a bucket of the slurry if you have made a lot. Make
sure the entire wax is covered, including vents, cups, and sprues.
·
Apply fine sand over the wax. For the first
layer, use zircon sand. The thin slurry and very fine sand will catch
the details of the piece.
·
Let the wax dry. Set it up in front of a fan
to accelerate drying times. Drying times vary depending on the
temperature and humidity of the area.
The
Secondary Layer(s)
·
Make a thin slurry; more milky than creamy.
·
Apply the slurry to the piece: if the previous
layer comes away, it is not dry enough.
·
Apply molochite 30-80, which is heavier than
zircon sand, to the entire piece and set it aside to dry.
The Final Layers
·
These layers are for adding thickness to the
ceramic shell. Therefore, make thicker creamier slurry.
·
Apply heavier molochite sand.
·
Depending on the size of the piece and amount
of metal to be poured, the number of layers needed will vary. For
smaller pieces, a minimum of five layers is sufficient. For larger
pieces, up to eight layers is standard. If the shell is too thick, the
metal will not “breathe” and trapped air won’t be able to escape through
the shell. If the shell is too thin, it may crack or break during the
dewaxing.
·
Let the shell dry at least 24 hours before
attempting to dewax.
Dewaxing
·
In a foundry, dewaxing is done in a furnace at
such high temperature that the wax burns out, leaving the empty shell.
This is done in a matter of a few minutes. You can build a furnace for
dewaxing, leaving the shelled piece standing on a grate, with a tray a
little below to catch the wax as it drains from the shell. Another easy
way of dewaxing is standing the pieces on a grate with a tray or metal
pan below to catch the wax, and using a large torch to heat the piece.
An advantage of draining the wax from the piece and catching it in a
metal pan is that the wax can then be reused.
·
Take care to set the piece up (cup-side down)
so that it won’t fall over and break. Once the shell is empty and before
it is baked, it is at its most fragile stage.
·
If you are using a torch to melt the wax, it
is best to heat from the bottom and move the torch slowly to the top of
the piece. This ensures that the wax has somewhere to go when it starts
to melt. The wax, when heated, expands. If it has no space to breathe,
it expands against the rigid shell, which cracks. Most shell failures
occur at this stage.
·
Continue to torch the shell until there is no
more wax melting out of the bottom of the piece, and no more flames
indicating burning wax.
·
Gently set the shell up somewhere safe to
cool.
Reinforcing the Shell
·
Often during dewaxing, the shell cracks. These
areas must be reinforced so that the metal doesn’t leak out of the
shell. There are also areas on the shell that will experience more shock
as the metal pours into the shell. These areas should also be reinforced
to ensure a strong shell capable of withstanding the pour.
·
Mix a very thick slurry.
·
To fix visible cracks, take a small torch and
heat the slurry as you paint a little bit at a time over the crack. This
will make sure the slurry dries before dripping into the shell.
·
To reinforce a general area, paint on slurry.
Over this slurry, lay strips of fiberglass cloth and paint slurry over
the fiberglass. Sprues should be reinforced as a rule, as well as the
areas where the metal will fall first.
·
To fix clean breaks where the shell hasn’t
shattered: put the two pieces together at the break and paint slurry as
you heat it, taking care not to drip slurry into the shell. Once the
break line is covered with dry slurry, it should hold enough to where
you can reinforce it with fiberglass.
·
Let the reinforced shell dry for two or more
hours.
The Pour
Baking the Shell
The shell should be baked in a
furnace. A simple furnace can be made from refractory bricks lined with
"k" wool. K wool is toxic to the lungs, so it's best to avoid it.
·
The shell must be baked at high enough
temperature so that the shell remains red hot for at least twenty
minutes. This process strengthens the shell enough to withstand the
contact with molten metal, and it ensures that no moisture remains in
the shell. You can tell that the shell is hot enough when it has a red
almost transparent appearance.
·
Let the shell cool down slowly. Leave it in
the furnace until the metal is almost ready to pour.
·
Set up the shell to pour. It must be placed in
sand, with the cups up. The sand is a safety measure against spills or
leaks. Take care not to get any sand or other material into the open
shell. Set the shell up so that it is easily accessible when you are
ready to pour. When you pour, you should take the crucible from the
furnace and easily pour into the shell without having to walk backwards
and without having to move around too much. Think about how you will
have to pour the metal, and set up the shells to allow for the smoothest
possible operation.
·
Another option is to bake the shells until the
metal is ready to pour, and then to pour the metal into red hot shells.
This method captures very fine details but often results in miscasts.
Most literature on the subject says to pour into very hot shells, but
past experience has proved more luck with cooler shells. It is something
to experiment with.
Pouring the
Metal
To pour molten metal into the baked shells, you need a
furnace, a crucible, a pair of tongs for lifting the crucible, and
something to hold the crucible when you pour. This is typically a long
metal handle with a metal ring on one end that holds a certain sized
crucible. A manageable size for a single person is a 10K crucible. Above
that you may need another pair of hands to help carry the weight. Then
you would also have a long metal handle with a ring in the center and a
person at each end lifting the molten metal. Also, each person involved
needs a pair of heavy gloves, a face shield, long sleeves and pants, and
appropriate footwear. No skin should show when you pour hot metal. It is
a good idea to have a bucket of cold water nearby.
Gas furnaces are more highly recommended because they melt metal
quicker, although there are charcoal furnaces that work. If you are
building a furnace, there are some things to keep in mind: It should be
cylindrical to allow for even heating throughout, with the torch
entering the furnace near the bottom. Use a propane torch. See
Methods for Modern Sculptors for some good tips and ideas. Also
there is a good reference online at
http://www.artmetal.com/project/TOC/proces/cast/ag_cast.html
·
Load a crucible to the desired amount of
metal. If you weighed the piece before investing, then you know about
how much you need, but it is wise to have a little extra.
·
Place the crucible in the furnace
·
Safety check: make sure there is nothing in or
around the furnace, including the area above, that might catch fire.
Check the gas connectors and regulators against gas leaks. Make sure
safety equipment is nearby. If you happen to get burned during the
process, immediately immerse the burn in cold water.
·
Begin melting the metal.
·
If you are using a gas powered furnace, it
will take approximately twenty minutes to melt the metal. This time
varies from furnace to furnace, and takes longer for charcoal furnaces.
For bronze to melt, it needs to reach a temperature of approximately
1300°C.
·
Check to see that the metal is the correct
temperature: there are expensive electronic devices called pyrometers
that can tell you to the exact degree the temperature of the bronze,
but, lacking this, you can check it with a steel rod. Dip the rod into
the crucible and wait five seconds. If the rod comes out clean, the
metal is ready. If the rod has metal stuck to it when you pull it out,
it needs more time.
·
When you are ready to pour, turn off the gas,
and put on gloves and face shield.
·
Pull the crucible out of the furnace with
heavy tongs, and either place the crucible on the ground if you will be
using the tongs to pour, or place the crucible into the metal handle.
Remove the "slag" with a metal scoop. The slag is the term for the
impurities in the metal that float to the top when the bronze has
melted.
Jillian Varani pouring molten bronze under
guidance of Olivier Duhamel
(Photo Maeva Bodin - © 2008)
·
Pour the bronze and pour any extra into a
ready made refractory cup. Lay the crucible on its side to make cleaning
the crucible easier.
·
If there is a leak in a shell while pouring,
leave it and move to the next one. You can try to salvage the leak, but
it helps to have someone else do this while you pour: take a wet cloth
and press it against the metal leaking from the shell. This will freeze
the metal coming from the shell and allow you to continue pouring into
the shell until the metal fills the cup. This is very dangerous.
·
Let the bronze cool fifteen minutes or more.
Many experts say to let the shell cool down slowly over a matter of
hours or even days. However, after fifteen minutes the metal has
hardened, even though it is still hot. Take a pair of tongs and drop the
piece into a bucket of cold water. Again, most experts say not to do
this, but nobody seems to know why not. If you know why, please send me
an email.
·
To get the ceramic shell off the bronze, knock
the sprues with a hammer. The vibration should be enough to get most of
the shell off the bronze. In deep areas, it may be necessary to take a
chisel and carefully knock away at it. As you get into smaller crevices,
smaller files, picks, and even needles come in handy.
Chasing
and Patina
The final steps in the bronze process are cleaning the metal
(chasing) and coloring the bronze with oxides (patination). Often the
bronze comes out of the shell full of defects, effects in the bronze
that weren't in the original wax. These include everything from surface
problems to holes and miscasts, where the metal hasn't run fully into
the piece. Most of these problems can be fixed with a bit of chasing.
Once the piece is chased to an acceptable level, the sculptor either
does a patina or sends it to a patina master.
Chasing
Tools for chasing
include sandpaper or a course pad such as scotch brite, files of various
sizes, and chisels and punches. A very handy tool is a dremmel or
handheld rotari tool. These typically come with many useful attachments
for cleaning very small places. You will also need a saw to cut the
sprues and vents, a sander to flatten the bottom of the piece, and
grinding wheels of various size and grit. The best tool to have, if you
can afford it, is a sandblaster.
·
Cut the sprues and vents with the saw. If the
sprue was attached to the sculpture at a flat area such as the bottom,
then simply sand it flat. If it was in a rounded area you will need to
reshape it with grinding belts and then sandpaper and a dremmel.
List of Common
Problems/Defects
·
Miscast: Often unfixable, this happens
when bronze doesn't fully flow into the empty shell. Possible reasons
include gas trapped in the shell, or more likely, a poorly sprued
system. Try recasting with a different sprue system. If the missing area
is very simple in shape and small enough in size, if may be possible to
weld a lump to the sculpture and shape it back in the metal. The problem
with welding/braising is that it requires another whole set of skills
and knowledge, and the metal won't be quite the same alloy as the
bronze, which will result in different color effects during patination.
·
Fin/"Flashing": This occurs when there
is a crack in the shell before the pour. It is easily fixed by carefully
using a hammer and chisel or using the grinder wheel on the rotary tool.
·
Protruding Rough Surface: Often this
happens because either the shell is too hot at the time of the pour, or
too cold. It's one of life's mysteries, but fortunately also easily
fixed with sand paper or again, the rotary tool.
·
Bubbles/depressions: These can appear
as a result of gas or other organic material trapped in the shell.
Sometimes, these defects can be ignored or hidden under a dark patina,
or if they're really obnoxious, then braising is an option. Braising
bronze takes a little practice but the technique can be learned quite
easily: heat the entire sculpture: put a little flux on the hole. Then
take a bronze rod, of thickness depending on the size of the hole you
are filling, and melt a little of the end into the hole/bubble, until it
is filled. Let the sculpture cool and chase back the excess braised
metal until you have the surface you want. It may take a few tries. Like
welds, braised spots will also show a different color to most patinas
than the bronze sculpture.
·
It's difficult to pinpoint the cause of most
defects. As with all skills, practice much, have patience, and soon
you'll know the best techniques for bronze casting.
Patina
Patination is the chemical process that oxidizes the bronze to a
specific color. Many colors and effects can be achieved with a few
chemical mixtures and a little practice. The basic technique for any
patina involves a propane torch, a brush, the chemical, water, and a
soft cloth.
List of Chemicals,
Techniques, and Effects:
·
Liver of Sulfur: Used for the base coat
and dark browns to black, liver of sulfur is usually purchased in its
unmixed form, as small rocks. The mixture ratio is approximately 5mL
liver of sulfur to 1L water. To make a good light grey base coat,
submerge the bronze in the liver of sulfur mixture for half a minute, or
paint it on until the color change is apparent all over. Rinse the
sculpture in water. Use the torch to evenly heat the entire piece. You
will notice the moisture evaporate first, then the color should go
almost bright blue. Using a clean brush, polish the blue while heating
to make a beautiful silvery grey base coat. To make a dark black, keep
applying layers of liver of sulfur, although you don't need to keep
brushing the heated sculpture.
·
Ferric Nitrate: Used for browns,
oranges, reds, ferric nitrate is usually purchased in its liquid form
and applied diluted with water. It is applied hot or cold. To apply it
cold, brush it on all over the sculpture and let it dry or dry it by
heating. To apply it hot, heat the sculpture and use a brush or a spray
bottle to apply it. It should sizzle as it touches the sculpture. If it
burns the brush, the sculpture is too hot, if the liquid runs without
sizzling, the sculpture is too cold. To achieve red and dark browns, you
need more layers. Experiment to achieve different colors.
·
Cupric Nitrate: Used for blues and
greens, it is usually purchased liquid and applied hot or cold.
·
Bismuth Nitrate: Used for tan and
white. For tan, mix with ferric nitrate. You need very little bismuth to
have a noticeable effect.
·
Experiment to achieve different colors. There
are a huge amount of other chemicals, some are specific brands sold as
"blue" or "tan," and some you can find pure chemicals. With just liver
of sulfur and ferric nitrate, you can achieve a spectacular range.
·
For spots, dab the chemical onto a hot
sculpture.
·
For rings, use a circular shaped brush.
·
For a granite effect with many little spots,
try using a spray bottle onto a hot sculpture.
·
Try fading from one color to another.
·
Safety: For any hot application, use a mask.
To throw away waste chemical, mix with plaster before dumping.
Finishing Touches:
Signing and Sealing
Once you have achieved a patina to your liking, sign the piece if
you haven't already.
·
To sign the metal, you can use a little
engraving attachment for the rotary tool. If you are making a series,
decide beforehand how many are to be made, and make sure you put the
correct number onto the piece. It is helpful to make a log of all
previous sculptures to keep track. Series are signed "number of
edition/number in the series." If you don't limit the number in the
series, put UL for unlimited.
·
Usually artists use a hard wax or neutral shoe
polish to seal the patina. Remember that a wax sealant may slightly
change the appearance of the patina. Heat the sculpture evenly and brush
on melted shoe polish over the entire sculpture, even the bottom. Let it
dry before touching it.
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