Firing Metal Clay Tips and Info

Minimum vs Ideal
The variety of times and temperatures for each clay type makes it possible to co-fire embedded items along with the clay, and it also allows for a variety of firing methods. What the guides don't mention is that there is a trade-off in the strength of the final product when firing at anything less than the optimum schedule of 2 hours at 1650°F / 899°C in a digitally controlled kiln.

A common misconception about silver metal clay is that after firing it's solid metal, just like a casting, but it's not. Castings are made by pouring molten metal into a mold and allowing the metal to cool in the mold. The result is a very strong, solid metal object with very little porosity (more air spaces). But silver clay is not at all like a casting. Silver clay is sintered, and sintered objects are very different.

The Sintering Process
Sintering is a process where tiny particles of metal are brought to a temperature just below their melting point, a point where the metal is able to bond to itself. Sintered objects are always porous because the metal particles are not like blocks that stack together neatly, they are various sizes and shapes, and they don't fit together perfectly.

When a sintered object breaks, the break is irregular and the inside appears to be grainy and is a lighter color than the outside. Silver clay objects have the same look when broken. Many mistake this typical breakage pattern for "under-fired" silver clay when what they are actually seeing is the telltale sign of sintered metal.

Time, Temperature & Strength
Sintering does not happen instantly, which is why it's called a process. It takes a while for the metal particles to form a strong bond. The longer silver clay is held at its sintering temperature, the more fully the individual particles are able to bond to each other. When fired at the lowest temperature for the shortest time, the bond that is created is not nearly as strong as when fired at the ideal temperature and time.

When to Lower the Temperature
Lower the temperature and shorten the firing time only when you have to. If you want to fire glass, heat-sensitive gemstones with silver clay, the temperature and time will need to be reduced for these items to survive. This is where the various firing schedules come in handy. Use the firing guides to find the clay that is compatible with the item you want to co-fire, and use the highest temperature and longest time you can within that range.

Teachers will often fire at less than ideal temperatures in a workshop or classroom situation because they have to budget their time to be sure your work gets in and out of the kiln and still leaves time to finish. When they have the luxury of time, however, they will fire at the optimum schedule. It's a good idea to re-fire at optimum anything that was quickly fired in a classroom as long as there are no heat-sensitive inclusions (glass and heat-sensitive stones).

When firing at less than the optimum schedule, be mindful of the trade-off. Avoid delicate tendrils and lacy details that are not supported. These can be easily broken during finishing, tumbling or in daily wear. Beef up ring shanks and bracelet links. Reinforce connections so you have a larger area of contact on components such as jump rings, bails, and pasted on elements. Remember that thicker pieces are less prone to breakage. Consider multiple firings if you need to include something that requires low firing, especially in the case of rings or bracelet parts.

 

Maximum Firing Temperature
The melting point of fine silver is 1761°F / 961°C. You may find slightly conflicting numbers, but the fact is, at this temperature, give or take a degree, fine silver will completely melt. Obviously, in order to transform our carefully crafted treasures into fine silver works of art, we need to avoid anything close to 1761°F / 961°C or our hard work will be just a forlorn memory.

At 1110°F / 599°C fine silver becomes "open" to fusing, but is cool enough to maintain its form. Up to 1650°F / 899°C fine silver will fuse and still hold its form, so this is why the range of temperatures for sintering all forms of silver metal clay is between 1110°F / 599°C and 1650°F / 899°C.

Digitally controlled kilns will sometimes creep up above their target temperature and a rise of a few degrees above or below your target temperature is nothing to worry about. Kilns usually go just a little over their target temperature, but will fall more than that. If your kiln overshoots your target temperature by more than 20 degrees, check the thermocouple to be sure it's functioning properly because temperatures just a few degrees above this will show the early stages of melting and will be detrimental to your silver clay objects.

Maximum Firing Time
Each type of silver clay has a minimum firing time and temperature, but what's the maximum? We know the maximum temperature for all forms and types of silver metal clay is 1650°F / 899°C, but what's the maximum time? Technically, there is no maximum firing time. As long as you do not exceed 1650°F / 899°C.

Support
Remember that silver clay is just below its melting point when it's sintering. At sintering temperatures, silver clay is vulnerable to the forces of gravity because the binder has long since burned off and the particles are all by themselves with no support. Gravity will push against a silver clay structure as it's firing and slump it if it can. Give your pieces support with a firing media like alumina hydrate while they fire so they can't slump or deform.

To fire flat pieces, gravity will work in your favor, but sometimes you'll find that a piece that was fired flat ends up slightly lumpy and not flat after firing. This can result from "surface drag." Placing a flat piece of unfired, dried silver clay onto a firing shelf that has been sprinkled a thin layer of alumina hydrate will allow the clay to move as it shrinks during the firing process.

Puckering
When a stone or some other object is co-fired with the silver clay, the embedded object is not going to shrink, but the silver clay shrinks from all directions (length, width, thickness), pulling itself toward the center. If a small stone is embedded (3mm or less), it's not that noticeable. But larger stones will show strain around objects, depending on the size and how they were embedded. The clay will often begin to move upward as if trying to eject the embedded object. It is best to cut a small hole on the back of the piece to allow the clay to shrink around the stone.

Dimensional Objects
Anything that is domed, hollow, rounded, or has pieces that are hanging out in space, needs support during firing. A firing dish filled with vermiculite or alumina hydrate can be used to nestle pieces for support during firing. Vermiculite is a mineral used industrially as insulation, packing material, and even as a hydroponic growing medium, among many other uses. Vermiculite is not recommended with EZ960. Alumina hydrate is a powdered ceramic material used in refractory products and glazes. Alumina hydrate has a very nice texture and provides superior support. Be careful when working with alumina hydrate that you do not kick up dust. Alumina hydrate can be an irritant to the lungs.

Another good support is fiber blanket. You can use tufts of the fiber blanket like cotton and make custom supports for cantilevered items or pieces that need some support during firing, but do not lend themselves to being buried in vermiculite. Fiber blankets are made of spun ceramic fibers and the airborne fibers should be avoided as they can irritate your lungs if inhaled. Fiber blankets are not recommended for use with EZ960 Sterling.

Look at your piece and decide the best way to fire it to avoid flat spots, deformation, etc. A ring should be placed onto a ring pellet of the desired size, placed in a firing dish filled with alumina hydrate and fired at the appropriate temperature. Round objects need to be placed on a fiber blanket or nestled into a dish of vermiculite or alumina hydrate. Place dimensional objects so that gravity will work for you rather than against you. Domes should be placed face down during firing. Lentils can be placed sideways. Be careful when "nestling" the unfired pieces into vermiculite to avoid scratching the soft clay surface.

Re-Firing
Anything made of fine silver can go into the kiln more than once. Follow the original process and fire according to package directions. Keep in mind that you don't have to fire silver clay just once. Fire as many times as you need to accommodate low-fire inclusions, to add on to previously fired pieces, make repairs, or add findings. Re-firing does not hurt fine silver.

If the piece you wish to re-fire was supported in a firing dish with vermiculite, alumina hydrate or on a fiber blanket, you'll have to support it the same way re-firing.

Adding Unfired Silver Clay to Fired Silver Clay
To successfully add new clay to pre-fired silver clay, the new clay needs to get a grip on the old in some way. If you know you will add to a piece as you are making it, you can prepare the area in advance to get the strongest connection. Score grooves or lines in the wet clay or scratch into the dried clay where you know you'll add on. Once the main piece has been fired, the grooves will give the new clay a hand-hold and make the connection much more reliable after firing.

Adding Ring Tops and Stacking Components
Where you want to stack a component for a ring top, score the area to be added to when the clay is wet or after its dry. Then you can fire that portion of the project. After firing, added clay can be pressed into the grooves where it can get a hold on the existing metal, making for a more secure connection of the new piece.

Pieces that have been burnished and finished will have a smooth, hardened surface which makes it more difficult to get a good bond. A finished piece may also have a patina. A patina is an oxidized surface that will interfere with bonding, so it must be cleaned off before adding to it. To clean and prepare the surface, the piece can be pickled or re-fired. Pickle in Silver Prep and then rinse well, or fire at 1400°F / 760°C for 20 minutes. Rough up the area to be bonded. Drill holes, file grooves, or use a separating disc or diamond cut-off wheel to score up the area to be bonded before adding fresh clay. The idea is to give the new clay something to hold on to.

Components can be pasted on with slip, syringe or lump clay directly to the freshly fired surface. Allow additions to dry completely and then fire at the highest temperature and longest time you can for the items you are adding. Art Clay Oil Paste makes a very strong bond for connecting 2 previously fired pieces, adding silver components to fired pieces, and for making repairs. Lavender oil can be added to PMC paste to make an oil paste (add 15 drops of pure lavender oil to a container of PMC3 paste to make your own oil paste, stir it in and let it sit overnight to assure complete absorption before using). You can use Art Clay Oil Paste with PMC plus or PMC3 as well as all Art Clay products.

Repairing Broken Areas
Most pieces that fail break because they were not fired at optimum and/or the piece was too thin or delicate for the application. Often times the repair that is needed is a butt joint. A butt joint is where 2 pieces butt up together, head to head, but don't overlap. This is usually a very small area of contact that can be easily re-broken if the rest of the piece is too weak to support the structure. Repaired joints are not as strong as a virgin piece of silver clay, so you may want to add additional thickness to the piece as well as repairing the joint.

Consider the durability of the design compared to how it's used. If your piece is a ring, maybe it's just too thin to survive the trials and tribulations of a ring. You may be better off making a new shank out of fresh silver clay and adding the broken pieces of your original ring to the design and create something new and really cool. This is one of the ways you'll learn what's possible and not possible with this medium.

Where you do want to make a repair, it's best to use oil paste for the strongest bond, and if you can, add clay to the area to support it. If a bail breaks off, adding a tab of clay to the back to bridge the broken pieces is better than trying to make a bond with just a little paste added to the break.

GEMSTONES

Co-Firing Natural and Man-Made Gemstones
Our gemstone firing guide, Gemstones in Metal Clay, lists safe times and temperatures for many gemstones, both man-made and natural, that can be successfully fired in place. Our guide lists the highest safe temperature and time for each gemstone and teaches you how to determine which stones will be safe in the kiln. To avoid burning your stone, do not exceed the safe firing time or temperature.

Most cubic zirconia stones can be fired at the ideal schedule with no change in color. There are some colors that are very heat sensitive and have special firing schedules. In these cases, we recommend firing in a firing container with carbon.
Sensitive CZs include:
Blue Topaz
Green Apple
Green Emerald
Tanzanite
These CZs are very heat sensitive and are probably best to set after firing, but can be fired in place at very low temperatures. Never quench or crash cool pieces with embedded stones of any kind.

Burnt Stones
Have you ever pulled a piece from the kiln to find what was once a beautiful, brilliant gemstone has turned green or muddy brown? What happened is very simple...the temperature and/or time were too long for the stone and it burned. It might look awful now, but there is a way to save it.

Stones go through a series of colors as they burn. Peridot, for instance, when overheated or kept too long at its safe temperature will turn a slight brownish green, then an olive green color and eventually a reddish color. To salvage a piece with a burned stone, just re-fire it at 1650°F / 899°C for 1 or 2 hours to get a deep cognac red color. It may not be the color you were after, but it beats scrapping the whole project. In some cases this process may bring back the original color.

Co-Firing Glass
Whether you want to fuse your own glass cabs or co-fire pre-made glass, co-firing in silver clay is very easy.

There are many types of glass, and each brand and type has a different COE (coefficient of expansion), which is the rate at which it expands and contracts as it's heated and cooled. The most common glass used for fused cabs has a COE of 90. We'll focus on firing with this type of glass however, just about any glass can be successfully fired in silver clay.

Co-firing glass and silver clay presents a few technical issues that you should know about. Glass will stick to an untreated ceramic kiln shelf, so you'll need to prepare your shelf with kiln wash, or protect it with ceramic fiber paper. The ceramic fiber paper is easier to use, but should be pre-fired to burn out the binders before use. Fire to 1400°F / 760°C before use. There is no need to hold the temperature, just heat to 1400°F / 760°C and turn the kiln off. Handle with care after firing as the paper will be fragile.

Another issue with glass and silver clay is fuming. As silver clay fires, silver fumes can build up in the kiln and cause a smokey-metallic discoloration called fuming. This is simple to avoid by venting the kiln. When co-firing glass and silver, remove the ceramic plug from the top of your kiln, or use the vent prop on the kiln lid to allow the silver fumes to escape as the piece is fired.

Glass must be annealed after heating. Annealing is a slow cooling process that allows the glass to return to room temperature without cracking. Heating creates stresses in the glass that must be relieved as it cools, and this process is called annealing. If glass is not properly annealed, it will crack or shatter. If a piece is improperly annealed (cooled too quickly), it might break.

Make sure glass pieces are clean before firing. Every speck of silver clay dust left on the glass will fuse to the surface when it's fired. If this is not part of the plan, be sure to clean the surface with alcohol before firing. Use a pointed cotton swab dipped in alcohol to remove silver bits from the surface and edges where the silver clay and glass meet. Don't use water on your swab to pick up the silver dust. Water will soften the clay.

Never quench any piece with embedded glass.

Creating a Setting
Glass cabochons are easy to fire in place. Most of the glass cabs available are COE 90. Whatever your design, you'll need to make sure the glass is trapped in place so it cannot fall out of its setting after firing. One option it to make a "seat" for the stone by placing it on the wet clay and tracing around it with a needle tool, then removing the excess clay below the cab and setting the cab in the resulting hole. When cutting out around the cab, hold the needle tool straight up and down so you have a little extra space around it to account for shrinkage of the clay. By giving a little extra space, you'll relieve the stress that can break the glass or tear the silver clay as it shrinks.

Depending on the size of the cab and the thickness of the clay, this may be all that is needed. However, if your cab is large, or to add visual interest, you might want to add a rope of clay or syringe around the stone to form a "bezel" or set the stone in a bezel made from bezel wire or silver clay.

Co-Firing Schedules
Most COE 90 glass is fused at temperatures around 1480° - 1550°F / 804° - 843°C. At this temperature range, the glass is molten and will slump, rounding the edges. To preserve the shape of a cabochon, stay below the fusing temperature.


COE 90 Firing Chart

 

Technique	Schedule	When to use
Tack Fuse	1200°F - 1350°F 649°C - 732°C soak for 30 minutes	To co-fire in silver clay with no change to glass
Full Fuse	1400°F - 1500°F 760°C - 815°C soak for 30 minutes	To fuse glass and silver clay together
Fire Polish	1100°F - 1200°F 593°C - 648°C soak for 5 minutes	To make cut or ground glass edges shiny, without changing shape
Crack Repair	1400°F - 1500°F 760°C - 815°C soak for 30 minutes	To repair a cracked piece of glass
Bubble Repair	1200°F to 1250°F 649°C - 676°C soak for 30 minutes	To squeeze out bubbles from glass

Annealing Glass
Annealing is a critical step. Glass annealing with metal annealing are two different processes that have two different outcomes. Glass annealing, is simply a controlled cooling. Glass expands as it is heated and contracts as it cools. If glass is cooled too quickly, it will thermally shock and crack. Annealing metal is basically making the silver softer in order to rework it.

Crash-Cool Annealing
If the glass reached the melting point (full fuse temperature), crash cool the kiln. Crash cooling brings the interior temperature down quickly so the glass stops moving.

To crash cool, open the kiln door about 2 inches and watch as the temperature falls. When it reaches 1100°F / 593°C, close the door. The temperature will now begin to creep back up. When the temperature stops rising, open the door again and allow the temperature to fall to 1100°F / 593°C. Repeat this until the kiln stays right about 1100°F. Then do not open the door again until the temperature is below 200°F / 93°C.

No-Peak Annealing
If you have fired to 1400°F / 760°C or less, just leave the kiln undisturbed with the door open until the interior temperature is below 200°F / 93°C.

Co-Firing Dichroic Glass
Most dichroic glass cabochons are made by fusing layers of glass at around 1500F / 815°C. To maintain the shape of the cabochon, fire at a temperature below 1450°F / 787°C. If you fire at 1470°F / 798°C (a typical temperature used), the glass will fuse with the silver clay and slump somewhat. The longer it is fired, the more it will tend to slump. If you want the glass to remain unchanged, fire at a temperature well below the softening point so the glass remains solid. A good choice is 1300°F / 704°C.

If your glass develops a crack, re-fire it at 1450°F / 787°C for 30 minutes to allow the glass to fuse. Then anneal.

Co-Firing Lamp worked Glass
Lamp worked glass pieces can be fired in place with silver clay. It's best to keep the temperature low since Lamp workers often use a COE 33 glass.

Co-Firing Beach Glass
Some beach glass can be safely co-fired with silver clay. Beach glass is usually a soda-lime composition with a fairly low melting point. If it's fired at too high a temperature, this type of glass can slump and lose its frosted surface. To assure the glass remains unchanged, fire at a temperature well below the softening point so the glass remains solid. A safe temperature is approximately 1250°F / 677°C for 30 minutes. Another issue with beach glass is the color. Since you probably don't know exactly what the glass is, there's no telling what might happen to the color in the kiln. You''ll have to test fire a sample alone in the kiln to find out how hardy the color is. Place a sample of your glass on a kiln shelf lined with kiln shelf paper (thin fire paper) and fire at 1250°F / 677°C for 30 minutes, then anneal to see what happens to the color.

Repairing Cracked Glass
If you know what kind of glass you have, you can easily repair a cracked glass stone in your kiln. To repair a crack, you just need to re-fuse the glass. See the glass firing chart above and use the full fuse temperature.

Co-Firing Fine Silver Findings
Fine silver wire, bezel wire, bezel cups, findings, or anything made of purely of fine silver can be co-fired with silver clay without any change in firing time or temperature. Anything that is made of fine silver can be fired at 1650°F / 899°C for 2 hours. Remember, most silver clay is made of fine silver, so anything that applies to silver clay also applies to fine silver findings, wire, etc.

Co-Firing Sterling Silver
Sterling silver findings can be co-fired with silver clay. Sterling silver will need to be properly prepared to fire in place, and care must be taken in the firing. Do not fire sterling silver findings over 1200°F / 649°C and do not fire for more than 30 minutes.

Depletion gild sterling silver before firing in place. Depletion gilding will bring up a skin of fine silver on the surface of the metal by oxidizing and then dissolving the copper near the surface. To depletion gild, first heat the sterling silver with a torch or in a kiln until it turns a dark charcoal gray. Then soak in Silver Prep to dissolve the oxidized copper from the surface (the surface will appear white). Repeat this process until you cannot darken the surface of the metal with the heat source. For small items such as wires, findings, and stone settings, 2 rounds is usually enough. For larger pieces, 3 or 4 rounds may be required. Rinse the pickled item and embed or paste on the sterling component. When the added silver clay is completely dry, fire at no more than 1200°F / 677°C for 30 minutes.

Co-Firing Porcelain
Porcelain is high-fire ceramic and can handle temperatures in excess of 1650°F / 899°C. Co-fire porcelain at optimum temperatures. Do no quench porcelain.

Co-Firing Bisque Ware
Bisque ware is a low-fire ceramic and can handle temperature in excess of 1650°F / 899°C. Co-fire bisque at optimum temperatures. Do not quench bisque ware.

Burning Out Organics
Natural organics are sticks, twigs, leaves, pods, seeds and anything else that was created by nature, not man. For the most part, these items burn away completely in the kiln. Some items are dense and take more time to burn out. For these types of organics, such as twigs and cork clay, fire to 800°F / 427°C and hold for 30 minutes before progressing to your ultimate firing temperature. Firing at a lower temperature gives the organic material time to combust and burn completely before the silver clay begins to sinter. Slowing down the firing schedule gives dense organics a chance to burn away without endangering the silver clay object.