How to make vermiculite moulds?

 Vermiculite moulds have versatile applications as custom moulds both for draping and slumping. With care, they have a long life. They are relatively light for their size and strength. As the vermiculite is an insulator it does not store heat, making it useful for large moulds without the requirement for long cooling times.  The relatively low cost makes large moulds more affordable.

It is a wet moulding process, so the cast needs to be waterproof. In this example a body cast is going to be used as the form for the mould. The first thing to do is to calculate the volume of material required. For an irregular form such as a torso, I measure the surface distance of the largest part of the mould - the hips in a torso - and the length. This gives an approximation of the surface area of the mould. Then the depth of the mould material needs to be added. All these measurement should be taken in centimetres. The longer the mould and the fewer curved forms, the greater the depth of the mould material needs to be.

In the case of this tall cast, the height of the cast is 96cm and the distance from one side around to the other side of the hips is 45cm. This is an area of 4320 square cms. The depth of the cast was decided to be 5 cm. So the volume of material required is 21,600 cc, or 21.6 litres. This shows the advantage of using the metric system as fewer calculations are required to obtain volumes than when using the imperial measuring system.

Another bit of versatility of vermiculite moulds is that you can make them hard or soft. The softer ones are easier to carve and shape subsequent to the casting of the mould, but are easier to damage.

A soft mould will use a ratio of 6 parts vermiculite by volume to 1 part of cement fondu. A hard mould will use a ratio of 3 parts vermiculite to 1 part of cement fondu. You can of course vary your proportions as you wish within these limits. Any more cement fondu than 1:3 and you are both increasing the cost and the heat retention. Any less than 1:6 and there is the danger that the mould will crack easily and fall apart.

Measure out the vermiculite and cement fondu and mix them when dry. The first picture shows the beginning of the mixing.  During the dry mixing a dust mask should be worn, as the rising dust is irritating to nasle passages. 

The picture below shows the two ingredients fully mixed

You then mix in the water to give a stiff mix. For a hard mould start with water of about 3/8 the volume of the dry mix. For a soft mould start with about half the volume of water to the volume of the dry mix. You will need to test the stiffness of the mix before filling the cast. It should be easy to make a ball in two hands that will stick together, but when pulled at, breaks apart cleanly.

If you can squeeze water out of the ball, then the mixture is too wet. In this case you need to make up another portion of the dry mix and then incorporate that with the already wet mix.

The cast needs a separator to keep the vermiculite and cement fondu mix from grabbing the mould. Vaseline works well, as the mould mix is a wet process. The separator should be spread liberally and evenly over the mould taking area.

Pack the cast by pressing the wet mix firmly down into/onto the cast to get good compaction and conformation to the surface without surface pitting. Build the mould material up in thin layers to ensure even coverage.  You can incorporate some re-enforcement within the mould if you like, such as chicken wire.

The packed cast should be covered to keep the whole damp. Leave this for at least one day. The mould and its cast should be air dried for at least one more day before taking the cast from the mould.

By this time, if you are careful, the mould and cast can stand while air drying.

Once the mould has air dried for a while, you can take the cast off the mould. It is at this point that any gross defects to the mould can be repaired. Any protrusions can be taken away with a coarse or open rasp or other wood working tool. If there are areas to be filled, a small amount of the vermiculite and cement fondu mix can be added. The mould needs to be wetted and the repaired area covered for a day, and then air dried for another day.

The mould should be kiln cured to set the cement fondu. This needs to be done at at least 540C/1004F.

To avoid cracking the mould, you need to soak the mould at 90C/194F for a time - dependant on the size of the mould - before taking it up to the top temperature. I normally use 720C/1328F, which is beyond the slumping temperatures for any glass that I may be using in the future. Once the top temperature is achieved the kiln can be turned off and allowed to cool at its own rate.  This curing process is smelly and so should be done in a well ventilated area, or overnight so the smells do not become a nuisance.

This imshr shows the cured mould, which is much lighter with the free and chemical water evaporated.

After firing to cure, the detailed work can be done on the dry mould. This requires finer grained tools than were required for the un-cured mould. When the surface is as desired, the mould can be kiln washed. The mix that I use is twice as thick as used for shelves to reduce the amount of water that needs to be evaporated from the mould. The drying should occur at 90C/194F until no moisture comes from the kiln ports.

These images show the kiln washed mould ready for placing the glass

Custom Moulds

What can I make custom kilnforming moulds from?

There are a wide variety of moulds available to purchase, but sometimes they are too expensive, or a special, distinctive form is wanted and needed.  There a number of ways to make a custom mould with a variety of materials.

Fibre Paper (including SilkeMat)

These are the most flexible in terms of shape and ease of use.  Most fibre papers range from 1mm to 6mm. Fibre blankets range from 12m to 50mm. They can be shaped directly or pressed into or onto existing forms.   Often, the fibre papers become more flexible after firing, although more delicate.  When combined with a hardener, they can provide nearly permanent moulds if treated with care.

Fibre board

Fibre boards are semi-rigid pressed refractory fibres which will maintain their form but become fragile after the binders are burned out.  Common thicknesses for fibre board are 15 to 25mm (0.625 to 1.0”).  They can be used as simple moulds, bottomless bowls, single drop out rings, and multiple drop outs without needing any separators.  The advantage of these is that when the base of the formed glass becomes larger than the hole, the board can be broken away from the glass without damaging the glass.  The mould is destroyed, of course.  However, when rigidised the fibre board becomes a strong, permanent mould.  It will need to be coated with a separator to prevent glass sticking.

It is difficult to obtain much detail with fibre board as the mould, and any glass resting on the board will take up the texture.  Once rigidised, it is possible to sand it to a smoother surface, but the surface will always have a texture.

Vermiculite

Vermiculite board is much firmer than fibre board, and is available in 25mm and 50mm (1” and 2”) thick boards.  It must always be kiln washed before any use because of the mica content.  The vermiculite board can be cut and shaped with wood working tools, but makes the tools unsuitable for woodwork afterwards, so the tools should be kept separately.  It can be worked and carved to obtain a lot of detail.  It is reasonably robust even though it does not require rigidisation.  It can also be machined with CNC tools.

Investments

Investment moulds are usually a combination of plaster of paris, silica, and strengthening materials such as chopped fibreglass or grog.  Generally they are one-use moulds, although there are a few formulations that will last a number of firings.  The instructions for mixing and moulding are widely available.

Found moulds

The most common found moulds are ceramic or stainless steel kitchen or table ware, but they can be of any material that will withstand the temperatures required for the glass. Glazed ware will normally need roughening or removing the glaze before adding a separator.  However boron nitride (Zyp is a trade name for it) often can be applied directly over the glaze or metal.  Casts can be taken from other less robust materials to form investment or other mould materials around them.  Room temperature vulcanising (RTV) rubber is a good material to obtain a lot of detail.  If less detail is required, fibre papers can be formed around the object with a separator of some sort to prevent sticking, and rigidised.

Customising existing moulds

Moulds can be modified in a number of temporary ways.   Adding a fibre paper design to the sides and edges can emboss it into the glass.  A mould with a round bottom can have a disc of fibre paper or a flat layer of kiln wash powder placed to give a flat bottom.  Square glass can be put into a round bowl to give a dramatic appearance, and round glass can be put into a square mould for a different look than in a round mould.  The variety of modification is limited only by the imagination.

Carbon for moulds

Recently there has been a flurry of interest in “carbon” moulds. These are actually graphite moulds, a particular form of crystalline carbon. Whether these are as good materials for kilnforming as they are for glass blowing can be understood by considering some of the characteristics of graphite.

Graphite is crystaline carbon and it has a planar structure that allows slipping between planes just as kaolin and boron nitride do. Graphite and boron nitride share similar physical structures and continue to be slippery at high temperatures.

Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite readily oxidises to form carbon dioxide at temperatures of 700 °C and above. https://en.wikipedia.org/wiki/Graphite

The consequence of this instability in the presence of heat an air is the creation of large bubbles from the decomposition of graphite into carbon dioxide when used above that decomposition temperature of 700°C/1292°F. Therefore, big bubbles are created in graphite texture moulds. They may not occur on the first firing because the exposure time was not long enough. By the second use, the graphite has already begun the decompose, and suffers large enough decomposition to produce the carbon dioxide.

Graphite might be useful as low temperature slumping moulds. The surface is easy to carve and shape to make smooth mould surfaces that do not require a separator. The disadvantages as a low temperature mould are the surface will erode quickly and mark easily, so detail will deteriorate and become damaged. Care is required to preserve it from marks and sharp materials. In addition, the slow, unnoticed decomposition will produce a rough surface over a number of uses.

The rapid deterioration of graphite moulds at low temperatures, make them unsuitable even for low temperature texture mould firings.

The expense of a thick slab of carbon thick enough to carve a decent depth and diameter of bowl needs to considered against a short life span. Small lamp working graphite moulds work well because the short times they are exposed to the heat do not give time for the decomposition to begin. And in any case, most often the moulds are open face, allowing any gases to escape without any distortion to the glass. 

How can I Release Glass Trapped in Casting Moulds?

How to get stuck glass out of a reusable mould?

The material is important to the method of removing stuck glass.

• Metal expands and contracts more than glass.

• Ceramic expands and contracts less than glass.

Mechanical methods

• Metal moulds can be hit relatively hard to break the contact between the mould and glass.

• Ceramic moulds should have only gentle taps, as they are more fragile than metal.

• If the glass is stuck, but moveable within the mould. It may be possible to wiggle the glass and mould against each other, which after a time may wear away the contact points and release the glass.

• Do not try to pry the glass from the mould. It is likely one or the other will break.

• Destructive method is to break the mould or the glass, which ever is the least important.

Contrasting temperature methods

Drape over metal –

• The metal contracts more than the glass, so placing the two in the freezer is one possible approach.

• Alternatively, heat the glass with hot water

• A third method is to place the drape upside down in the kiln and take it up to slumping temperature. Peek to determine when the glass has relaxed enough to be free from the mould. If the release temperature is above the annealing point, anneal again as before.

Drape over ceramic -

• The glass contracts more than the ceramic, so heating the glass with hot water may provide enough expansion to release from the mould.

• Or place the drape upside down in the kiln and take it up toward the slumping temperature. Peek to determine when the glass is released and skip to the anneal and cool process.

Slump into metal -

• The metal contracts more than the metal, so heat treatments will work best.

• Apply hot water to the metal until the glass is freed.

• Place the mould upside down on short posts and fire until the glass drops out. If the annealing temperature is exceeded, anneal again.

• Bang the metal mould with a rubber mallet. This risks breaking the glass, of course.

• Freezing only tightens the hold of the metal to the glass.

Slump into ceramic -

• The glass contracts more than the ceramic, so cold can work.

• Usually, glass sticking to a ceramic mould is a result of insufficient coverage of the mould with the separator.

• Placing the mould and glass in the freezer for a few hours may allow the glass to contract enough to be freed when taken out.

• Place the mould upside down supported on short posts. Set the firing to go to fusing temperature. Monitor with quick peeks from the slump temperature at regular intervals. When it drops, skip to the anneal process.

• Firing to a high temperature does not always release all of the glass.

Using adequate and appropriate separators to avoid trapping the mould or the glass need to be used to prevent the need to employ these release methods.

Found Moulds

 I want to try thrifted [charity shop] dishes to slump into. What should I know and look out for?

Material

• Ensure they are ceramic or stainless steel.

• Plastic and glass will not work. Differences in weight will help determine the material, as plastic is light and ceramic is heavy.

• Cookware, almost certainly, is stainless, but not all woks or metal serving ware are.

• If the potential mould is not one of these heat resistant materials, a cast can be taken to make a refractory mould.

Shape

• The items should have a shallow draft. 

• The sides of the proposed mould should not be vertical. The more gentle the slope the better. 

• A rim is better than none, as the glass can rest on a rim, but not so easily on an edge. 

Testing

• The items need to be tested for their resistance to the heat of forming to determine how they will perform.

• Take the proposed moulds slowly to about 50 degrees above the working temperature that they will be subject to.

Preparation

• A separator will be required to keep the glass from sticking to the mould.

• The surface of the mould has to be altered to accept the separator. This varies according to material.

• Ceramics most often must have the glaze removed, although occasionally unglazed forms can be found. At the least, the glaze must be roughened to allow the liquid kiln wash adhere. There is a formulation of emulsion paint and kiln wash that can be painted onto the glazed surface, and sometimes boron nitride can be used with out altering the surface texture. On some shapes it is possible to lay shelf paper on the glazed mould without further preparation before slumping.

• Steel moulds need to have the protective oils removed, which can be done at the test firing. Boron nitride (a high temperature lubricant) is the popular choice of separator by many. It is possible to use standard kiln wash as the separator by heating the form to a little above boiling temperature and spray or brush the kiln wash onto the hot metal. This may need repeated applications, although the kiln wash needs to be only a film. This heating and coating method can be used for glazed ceramics too. On nearly flat steel forms, it is possible to place shelf paper over the steel to form the separator as in ceramic moulds.

• The interior of ceramic bowls is not likely to be perfectly flat unless it has a broad base like a soup bowl. A disc of fibre paper that has a smaller diameter than the interior base can be placed so the glass will form a flat base, or with a smaller diameter disc, a foot.

Use

• Ceramic forms are most often used as slumping forms because the glass contracts more than ceramic. Draping over ceramic shapes can be difficult because of the different amounts of contraction on cooling can cause the ceramic mould to become trapped. It is possible to wrap steep sided moulds with 3mm fibre paper to provide a cushion between the glass and the ceramic.

• Steel forms are most often used for draping, because the steel contracts more than the glass on cooling. It is possible to slump in shallow forms or those with a shallow draft. It is also possible to place glass into a steep sided bowl below the height at which the sides become too steep. Careful levelling of of both mould and glass is required to be successful.

Can I Get Texture on Front and Back?

There are occasions when it may be desired to have a double bas relief, or texture on both sides of the glass. There is one way, among others, to do this.

• Fire the glass on the texture mould (6mm thick is best).  Clean it well.
• Put a layer of kiln wash or whiting powder that is thicker than the profile depth of the texture you want to preserve. 
•  Smooth it over the shelf, but do not compact it. 
• Then firmly press the textured side into the powdered kiln wash, to maintain the existing profile.
• Then add the embellishments to the top and tack fuse with a slow ramp up to a low temperature.

Plan carefully, because the side fired down will not be as shiny as the top, unless low temperatures and long soaks are used.

There are, of course, other ways to achieve this.  The main one is to cast the piece.  But this procedure avoids the mould making process.

What are the Risks of Cast Iron Moulds?

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Using Ceramic to Drape

Characteristics

Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials.  This is one circumstance where CoE is actually useful. 

The expansion of the two materials is different. 

• Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104.  
• Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64.  

This is important in the final cooling of the project.  As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool.  This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much. 

Shape

The shape of the ceramic form will have a big effect on the usability of it as a mould.  Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning.  The forms suitable for draping need to have a significant draft to work well.

Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.  

A cube shape unsuitable for draping

Ceramic cylinders with straight sides

Although rounded at the base, the sides are too straight to be a draping mould

The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.

You can experience this trapping effect in a stack of drinking glasses.  Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one.  On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one. 

Effect of Shape

The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.

Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.

However, if you choose ceramic forms with significant draft, you can drape over ceramic.  This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required. 

 

These pyramid shapes have sufficient draft to allow the glass to move up the mould during cooling.

Compensation for Lack of Draft

You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form.  The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling.  This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper.  It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.

Kiln post wrapped in 3mm fibre paper with cap over the post's hole.

The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper.  You can tie off the wire with a couple of twists.  Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire.  If you want further assurance, you can put a bit of kiln wash onto the wire.

Conclusion

The choice of ceramic shapes to drape glass over is very important.  It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling.  You often can use items with no draft if you wrap fibre paper around the sides of the form.

Reversibility of Boron Nitride

After using Zyp/MR97, can I sand it off and use kiln wash?

Some people are applying boron nitride to ceramic moulds for the "smoother" surface.  Boron nitride is an excellent separator for metal moulds and casting moulds whether metal or ceramic. But it has limitations, including the price and requirement for a repeated application at each firing.  Some are beginning to wonder if they can go back to kiln wash after having used the boron nitride. Some say you cannot unless you sand off the separator.

The general experience has been that you can't apply kiln wash on top of the boron nitride. It just beads up and flows off, because the boron nitride creates a non-wetting surface that survives relatively high temperatures.  The water in the kiln wash mixture merely beads up or washes away. This means the kiln wash in suspension has no opportunity to adhere to the mould.

The most accepted way to get rid of the boron nitride is by sandblasting. Then apply kiln wash as normal. The sandblasted ceramic mould previously coated accepts kiln wash with no difficulty. In the absence of a sandblaster, you can use a sanding pad. You do need to be cautious about taking the surface of the mould when using abrasive removal methods, as the ceramic is relatively soft in relation to the abrasive materials.

However, boron nitride is soluble in various alkaline chemicals, such as potassium hydroxide (caustic potash), sodium hydroxide (caustic soda or lye), and sodium nitrate (Chile saltpeter).  Soaking or washing the surface with one of these will dissolve the boron nitride, and should return a surface that will accept kiln wash.  Be cautious in the use of these chemicals as they are dangerous to the skin.

The difficulty of removal of the boron nitride means that you have to think carefully about which moulds you put it onto.  If the mould has delicate or fine detail, removing the boron nitride risks the removal of some of the detail.  This indicates that this kind of mould, once coated, should not be taken back to the bare mould to change the kind of separator.

Another use of boron nitride is to spray a very small amount on a fiber strip to be used for damming. This will give you fewer needles as it provides a non-wetting surface at relatively high temperatures. This allows the glass to slide down the fibre paper without hanging up and creating the needles.

One advantage of kiln wash over boron nitride is that you do not have to reapply every firing as with boron nitride. With the boron nitride it is recommended to apply before every firing.  It is best to use a paint brush to dispose of any lose material before giving a light re-coating. Not a whole lot is required on subsequent coatings.

If you are using boron nitride to get a smoother surface to the object, also consider using a lower slumping or draping temperature, as this will also minimise mould marks.  

Hard Spots in Moulds

Hand pouring of slip into a mould

Some ceramic moulds have small areas where the kiln wash does not seem to adhere as well as on the rest of the mould.  This comes from the manufacturing of these slip cast moulds and this blog post explains how it occurs.  The question is what to do to make the mould separate from the glass after firing.

Coat the mould as usual, which shows up the area where it seems no kiln wash is sticking.  There is some coating the area, but not in the same amount as the rest of the mould.  You can add a little extra kiln wash to the area once first layer has dried, but be careful to avoid creating a ridge against the rest of the kiln wash. If one does appear gentle smoothing with a finger can disguise the transition.

Another approach is to abrade the spot a little to make a more textured surface for the kiln wash to attach.  This needs to be done carefully and by hand to avoid creating a shallow divot in the mould.

The safe approach is to coat as usual and slump a sacrificial piece of glass to ensure the glass does not stick to the hard spot.  If it does not, the spot has enough separator to be useable, although I would continue to add kiln wash to that spot for several firings.

 

Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

Knowing the specific gravity of a glass can be useful in calculating the required amount of glass needed, e.g., for casting, and screen and pot melts, where a specific volume needs to be filled.

Most soda lime glass – the stuff kilnformers normally use – is known to have a specific gravity of approximately 2.5.  That is, one cubic centimetre of glass weighs 2.5 grams. 

If you have glass that is of unknown composition for your casting, you will need to calculate it.

Calculating the specific gravity of unknown glass.

Specific gravity is defined as the ratio of the weight of a substance to the weight of water (in simple terms).  This means first weighing the item in grams.  Then you need to find the volume.

Calculating the specific gravity of regularly shaped items

For regularly shaped item this is a matter of measuring length, width and depth in centimetres and multiplying them together. This gives you the volume in cubic centimetres (cc).

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume. To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

To find the specific gravity of a piece of glass 30cm square and 6mm thick, multiply 30 x 30 x 0.6 = 540cc.  Next weigh the piece of glass. Say it is 1355 grams, so divide 1355gm by 540cc = s.g. of 2.509, but 2.5 is close enough.

Calculating specific gravity for irregularly shaped objects.

The unknown glass is not always regular in dimensions, so another method is required to find the volume.  You still need to weigh the object in grams.

Then put enough water in a measuring vessel, that is marked in cubic centimetres, to cover the object.  Record the volume of water before putting the glass in.  Place the object into the water and record the new volume.  The difference between the two measurements is the volume of the submerged object.  Proceed to divide the weight by the volume as for regularly shaped objects.

Credit: study.com

Application of specific gravity to casting and melts.

To find the amount of glass needed to fill a regularly shaped area to a pre-determined depth, you reverse the formula.  Instead of volume/weight=specific gravity, you multiply the calculated volume of the space by the specific gravity.

The formulas are:

v/w = sg to determine the specific gravity of the glass;

v*sg = w to determine the weight required to fill a volume with the glass.

Where v = volume; w = weight; sg = specific gravity.

You determine the volume or regular shapes by deciding how thick you want the glass to be (in cm) and multiply that by the volume (in cc). 

For rectangles

volume = thickness * length * depth (all in cm)

For circles

Volume = radius * radius * 3.14 (ϖ) * thickness (all in cm)

For ovals

Volume = major radius * minor radius * 3.14 (ϖ) * thickness (all in cm)

Once you have the volume you multiply by the specific gravity to get the weight of glass to be added.

Calculating weight for irregularly shaped moulds.

If the volume to be filled is irregular, you need to find another way to determine the volume.  If your mould will hold water without absorbing it, you can fill the mould using the following method.

Wet fill

Fill the measuring vessel marked in cc to a determined level.  Record that measurement.  Then carefully pour water into the mould until it is full.  Record the resulting amount of water. Subtract the new amount from the starting amount and you have the volume in cubic centimetres which can then be plugged into the formula.

Dry fill

If the mould absorbs water or simply won’t contain it, then you need something that is dry.  Using fine glass frit will give an approximation of the volume.  Fill the mould to the height you want it to be.  Carefully pour, or in some other way move the frit, to a finely graduated measuring vessel that gives cc measurements.  Note the volume and multiply by the specific gravity.  Using the weight of the frit will not give you an accurate measurement of the weight required because of all the air between the particles.

An alternative is to use your powdered kiln wash and proceed in the same way as with frit.  Scrape any excess powder off the mould.  Do not compact the powder. And be careful to avoid compacting the powder as you pour it into the measuring vessel.  If you compact it, it will not have the same volume as when it was in the mould.  It will be less, and so you will underestimate the volume and therefore the weight of glass required.

Irregular mould frames

If you have an irregular mould frame such as those used for pot and screen melts that you do not want to completely fill, you need to do an additional calculation.  First measure the height of the frame and record it.  Fill and level the frame with kiln wash or fine frit.  Do not compact it.  Carefully transfer the material to the measuring vessel and record the volume in cc.

Calculate the weight in grams required to fill the mould to the top using the specific gravity.  Determine what thickness you want the glass to be.  Divide that by the total height of the mould frame (all in cm) to give the proportion of the frame you want to fill.  Multiply that fraction times the weight required to fill the whole frame to the top.

E.g. The filled frame would require 2500 gms of glass.  The frame is 2 cm high, but you want the glass to be 0.6 high.  Divide 0.6 by 2 to get 0.3.  Multiply that by 2500 to get 750 grams required.

Regular mould frames

For a regular shaped mould, you can do the whole process by calculations.  Find the volume, multiply by specific gravity to get the weight for a full mould.  Measure the height (in cm) of the mould frame and use that to divide into the desired level of fill (in cm).

E.g. The weight required is volume * specific gravity * final height/ height of the mould.

The maths required is simple once you have the formulae in mind.  All measured in centimetres and cubic centimetres

Essential formulae for calculating the weight of glass required to fill moulds (all measurements in cm.):

Volume of a rectangle = thickness*length*width

Volume of a circle = radius squared (radius*radius) * ϖ (3.14) * thickness

Volume of an oval = long radius * short radius * ϖ (3.14) * thickness

Specific gravity = volume/ weight

Revised 18.1.25