Amount of Fill for a Frit Mould

There are several ways to determine the volume of a mould. 

Calculation of the weight of glass needed

Calculate the amount in the metric system of measures, as that gives much easier calculations. Cubic centimetres of volume times the specific gravity of glass (2.5) will give you the number of grams of glass required.

This works best on regular geometric forms.  Rectangles and parallelograms are easy to measure the length, width and depth in centimetres.  Multiply together and you obtain cubic centimetres.  That times the specific gravity – 2.5 – will give the number of grams to fill the mould.  The frit will of course be mounded above the levelled surface, because of the air spaces between the frit pieces.

Example of a small frit mould

Irregular shaped moulds

The moulds which are irregular in shape or depth are more difficult to calculate. 

You can determine the volume by starting with a measured amount of water.  Quickly fill the mould to the surface, so that no water is absorbed into the mould. Empty the water from the mould into the drain so it does not become soaked. The difference between the starting and finishing amount of water is the volume of glass required to fill the mould. 

You can use that volume in cubic centimetres times the specific gravity (2.5) to get the number of grams of glass required.

However, it is much easier to put the frit into the water until the measure shows the same amount as before the mould was filled. Then you only need pour off the water and allow glass and mould to dry.  No calculation required.

Glueing Glass Pieces

The best solution is to avoid the use of glue completely. If you cannot, use as little as possible and make sure it burns out cleanly.

The glues to which kiln workers have normal access, do not survive to tack fusing temperatures. Therefore they can only be considered as a means to get the glass assembly to the kiln. The glue will not hold the pieces in place until the glass begins to stick, so the pieces must have a stable placement. If not, the pieces will slip, roll and move once the glue has burned out.

The second requirement of glues is that they burn out without leaving a residue.

Glues that have been used with little or no residue include:

Powdered CMC that can be disolved in warm water

-CMC (carbylmethylcellulose). It is a cellulose based binder used in a wide variety of industries, including food. For our purposes, it is also used in the ceramics industry and is often called glaze binder. It is a main constituent of "glas tac" from Bullseye. This can be made up into a viscous solution to catch and hold frits and other sprinkled elements in place.

- PVA (Polyvinyl Acetate) is water-based glue. It is sometimes known as school glue. It can be diluted to about 10parts water to 1 part PVA. This is sufficient to hold the glass pieces together with only a drop for each piece of glass. It does not work so well for small sprinkled elements.

One of many brands of  Ethyl Cyanoacrylate
 glue

- Super glue burns off with no concerns about cyanide. It should be used sparingly and also works best for pieces of glass.

One of many hair laquers in pump spray bottles

- Hair lacquer is normally applied as drops at the edges of the assembled pieces and so can be used to hold pieces of glass as well as sprinkled elements.

In all uses of glue the principles to remember are:
- Use the minimum to hold pieces together while getting the work into the kiln.
- Put the glue at the edges of the glass or where its combustion gasses can escape easily.
- And in all cases, you need to test to see if a residue is left on the glass at full fuse when using a new glue.

An alternative to glue is frit as described here.

Devitrification

What is it? When does it happen? Why does it happen? These are frequent questions.

Dr. Jane Cook states that devitrification is not a category (noun), but a verb that describes a process. Glass wants to go toward devitrification; a movement toward crystallisation.*

Mild devitrification is the beginning of crystallisation on the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up or a crackling can appear on the granular surface.  This is distinct from the effects from an unstable glass or the crizzling as in a ceramic glaze. Divitrification can occur within the glass, but normally is a surface effect.

Differences in the surface of glass promotes precipitation of the crystal formation of silica molecules.  This fact means that two defences against the formation of crystals are smooth and clean surfaces. There are other factors at play also.  The composition of the glass has an effect on the probability of devitrification.  Opaque glass, lime, opalising agents, and certain colouring agents can create microcrystalline areas to "seed" the devitrification process.  One part of the composition of glass that resists devitrification is the inclusion of boron.

Devitrification generally occurs in the range of approximately 700°C – 840°C, depending to some extent on the type of glass.  This means that you need to cool the project as quickly as possible from the working (or top) temperature to the annealing point, which is, of course significantly below this range.

There is evidence to show that devitrification can occur on the heat up by spending too long in this devitrification range, and that it will be retained in the cooling. Normally this is not a problem as the practice in kilnforming is for a quick advance on the heat up through this range.  The quick advance does not (and should not for a variety of reasons) need to be as fast as possible.  A rate of 300°C per hour will be sufficient, as time is required for devitrification to occur.

The devitrification seen in typical studio practice results more often from inadequately cleaned glass than from excessive time at a particular temperature, up or down through the devitrification range.  

It is often seen as a result of grinding to fit shapes.  Even though the ground surface is cleaned, it may still be so rough as to promote devitrification.  The surface must be prepared for fusing by grinding to at least 400 grit (600 is better).  Alternatively, use fine frit of the same colour as the darkest glass to fill the gaps. This normally is applied in the kiln, so the pieces are not disturbed.

Dr. Cook suggests three approaches to devitrification:*
Resistance through:
 - Schedules
 - Flux
Dealing with it:
 - Cold work
 - Acids
Embrace it:
 - Allow it
 - Use it

Temperature range for devitrification
Homemade devitrification solution
Frit to fill gaps


* From a lecture given by Dr. Jane Cook at the 2017 BECON

[entry revised 25.9.19]

Bas Relief Moulds

Bas relief moulds that have an image carved into the surface are popular at the moment. They are most often called texture moulds.  The image is “carved” into the back of the glass, creating uneven thicknesses of glass that refract the light to show the image through the smooth plane of the front.

One of the problems with these kinds of moulds is that lots of bubbles are created, often very large ones.  This results from the many places where the air cannot escape from under the glass during the forming process.

Solutions

There are some strategies that can help avoid these bubbles.

Use the 6mm rule

Fuse the glass into a six-millimetre thickness first.  Two layers of glass give more weight to help the glass conform to the texture of the mould.  It also resists bubble formation more than a single layer.

Use the Low and Slow approach

It more important to have low and long bubble squeezes.  The most successful strategy will have a slow rise in temperature to put as much heat work into the glass as you can before the bubble squeeze.  The bubble squeeze is the most important part of firing these texture moulds.  It will start at about 600°C rising at only about 25°C/hr to around 680°C – that is, taking three to four hours. 

Use slow rates of advance

A third element is to rise slowly toward the forming temperature.  Possibly nothing faster than 75°C.  This enables you to keep the forming temperature much lower than a fast rise will.  The usual temperature recommended is about 780°C.

By using a slow rate of advance you can probably reduce the forming temperature by about 20°C.  You will need to peek at intervals to be sure the glass has taken up the required texture. Again, it is about putting as much heat into the glass at as low a temperature as possible.

Use Long soaks

An alternative to the slow rate of advance is to use a long soak at as low temperature as seems suitable.  You will need to peek at intervals to determine when the texture is achieved.  When the appropriate texture is imparted to the glass, you need to advance to the next segment.  This means that you need to know how to get your controller to skip the following segment.  Or, if the texture is not achieved before the end of the scheduled soak, how to extend the soak time.  If you are using 760°C as you target temperature with a rise of 150°C, you may wish to soak for about an hour or more.  Remember that this is in the devitrification range.

Alternative - Frit

A completely different approach is to use fine frit and powder to give a patè de verre appearance by sintering the frit.  This eliminates the bubble problem entirely.

You will need a lot of frit if you are trying to make a sheet of 6mm from the frit.  You could just take the sheets of glass cut to the size of the mould and smash them up to get the required amount of glass.  Or you can use your cullet, by weighing and smashing up enough glass. 

The calculations for weight are best done in the metric system (in cm) as there are easy conversions between volume and weight.  Assume your mould is 20cm square.  The area is 400cm².  The volume is that times 0.6cm or 240cm³.  The specific gravity of glass is approximately 2.5, so you multiply the volume by that and get 600gms of glass required to get a 6mm thick sheet. 

You could full fuse this into a clear sheet, although this would take a much higher temperature and longer soak that would be good for the mould. Better is to sinter the glass.

To sinter the glass, you need slow rises in temperature and long soaks.  A rise of about 75°C to the softening point of the glass (around 600°C) followed by a very slow rise (ca. 25°C per hour) to about 660°C is needed to allow the small grains of glass to settle together.   At the upper end of the bubble squeeze you need a three- to four-hour soak to sinter the glass. The thicker the layer of glass frit, the longer soak needed to ensure all the particles are heated.  The densest glass will be formed by a 50/50 combination of powder and fine frit.

Much better is to have a much thinner sheet formed from the frit.  This will be about two to three millimetres thick.  The weight of powder and or frit can be determined by the formula above, substituting 0.2 or 0.3 for the thickness.  This frit mixture needs to be evenly spread over the mould, with as much on the high points of the mould as the low ones.

If the mould has a lot of variation in height, you can sinter the frit mixture as a flat sheet first.  Then place it over the texture mould and give it a slow rate of advance to the to the top end of the bubble squeeze and soak for an hour or more, as required.  This will ensure you get the same thickness across the whole piece even though there differences in height.

The resulting piece will be very light and translucent.  It will have a fine granular feel to the touch.  It will have the same shape on both sides of the piece, with the upper surface having a slightly more shiny appearance than the bottom.

Further information is available in the ebook Low Temperature Kiln Forming.

Frit by thermal shock

Frit can be created by thermal shock.  You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces.

Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. Once the glass is cool, pour off the water and dry the glass.  When dry, you can break the crazed glass into smaller bits just as you would with other glass.  Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.

The advantages of this quenching method of obtaining frit are that you can create frit with less effort.  You also get less fines and powder with this method. And less effort is required to smash up the glass.

Some indicate that ice cold water to quench the glass is a good idea.  This is because warm water will not provide enough of a shock to the glass to craze it throughout.  But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough.  Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.

You can, in part, control the size of the resulting frit.  Firing at 300C results in larger frit than firing at 400C.  However, firing at 500C does not provide even smaller frit.  The best results are between 300-400C, although frit can be made at 200C as well.  Experiment with temperatures to get the frit you want.

Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential.  The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.

Preventing Devitrification on Cut Edges

“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”

Devitrification occurs where there are differences in the surface.  This means that the surfaces exposed to the heat must be both clean and smooth.  It is not enough for only one of these to be the case, both are required.

First, the sawn edges need to be clean.  A good scrub with a stiff bristle brush is essential.

Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.

Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.

1 – Grind

This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap.  The grinding should go down to at least 400 grit before cleaning and arranging to fire.

2 – Clear glass

This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface.  You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.

Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead.  In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.

Capping with Frit

Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.

Capping inherently has bubble creation potential.  The development of a bubble squeeze helps prevent the largest of bubbles.  It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.

An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion.  In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles.  However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.

The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece.  This means that you need to use medium sized frit for cast work.  Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications.  Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform.  This means that medium frit is a good compromise between large and small bubbles in capping. 

The layer of frit should be at least 2mm thick.  This means a lot of frit is required to do the job.  To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required.  Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area. 

Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top.  If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base.  If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.

Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.

Stencils vs. Saw

Saw

Frequently when people want to make a complicated shape they resort to a saw to create the shape.  This is used in both stained glass and fused glass work.  Although it may be necessary in stained glass applications, it is not as necessary in fusing.

One of a variety of saws

Stencils

There is an alternative to an expensive saw – stencils and frits.  You can make a stencil from stiff card. Place the stencil in the appropriate place. Then sift powder or sprinkle frit over the stencil.  Lift carefully and the shape is there ready for fusing.

Example of sifting powder over a complicated stencil

To get the depth of colour obtained from sheet glass, you need to apply the powder or frit to at least the thickness of sheet glass. This also means that you need to go to a full fuse with the powder or frit on the top surface.  You can, of course, later cap and fire again.

Example of the cutting of a stencil

More guidance on stencils is available here: 

Filling Gaps in Fused Pieces

Often your cutting is not as accurate as you would like so there are small gaps between the pieces as you assemble the piece.

Tammyhudgeon.com

One solution that is often used is to grind the edge of the too large piece to get the fit desired.  The problem with this is that thorough cleaning is required to avoid devitrification lines appearing on the final piece.  Also, even with extensive grinding, the fit is not perfect.

The alternative is to fill the gaps with fine frit or powder. 

artistryinglass.com

Assemble the whole piece and assess the gaps.  If they are very large, you need to adjust the glass. If they are only millimetres wide, powder and frit can fill the gap to disguise the join.  I generally use powder for almost perfect joints, and fine frit for anything larger.

I first cover the gap with powder or frit and with a soft brush work at right angles to the line of the join.  This ensures that I have filled the gap to the height of the glass.  However, the frit and powder have air spaces, and so will fuse to a lower level than the height of the glass.  So, once gap is filled, I build a small ridge over the gap trying not to extend beyond the gap.  This mound compensates for the lack of density of the frits.

The frit and powder colour must match the glass exactly to become invisible.  It can be made from your scraps or purchased at the same time as the glass. I find it more successful to do these fills with the darker glass.  It provides a more distinct edge to the joint.  It also conceals the base glass better.

It can also be used to conceal the joint in a single colour where the piece cannot be cut as one and needs several pieces to make the whole.  This is more simple as any overspill will not be noticed when fused.

This method only works with full fusings.  At tack fuse temperatures the frit will not fully combine with the sheet glass to form a smooth invisible join surface.

Bubbles in Casting Mould Firings

There seems to be an increasing popularity for re-useable ceramic casting moulds.  One of the common problems with these moulds is bubbles.  

Frit size 

It rather depends on the sizes of the frit and cullet used as to how many and what kind of bubbles are created. The converse of expectations is what happens.  You get more small bubbles with powders and fine frits than with coarser frits.  The small bubbles rise and coalesce to form larger bubbles which rise more slowly as they have to push through a greater mass of material (just as in a liquid). Since glass is viscous, these little bubbles usually do not have time to push their way through the glass at fusing temperatures.  But at casting temperatures, there is less resistance from the glass, as it is less viscous, and so the bubbles can clump together and form the larger bubbles that burst through the surface.

Temperature range and rate of advance

The amount and kind of bubble also depends on the speed of the ramp and the bubble squeeze you give it. If you proceed rapidly to top temperature, you will have to go to a higher temperature, allowing the surface to become more plastic and be pushed out of the way by the expanding air that almost certainly is in the mix. A slow rise will allow all the glass to become the same temperature throughout without using a high top temperature, so reducing the risk of the bubbles pushing through the more viscous glass to the surface.

Vents

All these problems would be reduced by having a vent or sprue to allow the air out from the bottom. Almost all purpose made casting moulds have these things. Sometimes they are as thin as a few hairs (from somebody with long hair) to as thick as a toothpick. As you have to do some cold work on the results from these moulds anyway, a few little strands of glass should be no problem to clean up. If the manufacturers won't do it, it is possible to take your Dremel or similar drilling tool and with a fine drill bit and make these tiny holes in appropriate places.  

I do not understand why these casting moulds do not have tiny air vents at the bottom of the depressions. Yes, there would be a tiny pimple on the surface of the final piece, but this can be cleaned away easily. The holes could be really small diameter ones. They just need to be opened after each coating of separator with a fine wire. I'd be sending the ones without vents back to the manufacturer as not fit for purpose. If these moulds had vent holes, they would be a lot less bubble prone. 

Master moulds

If the mould continues to give trouble with bubbles, it might be best to take a negative of the mould that you can keep as a master.  Then make one-use investment moulds from this master positive as you need. Investment moulds usually allow air to move through the material pretty well, but you can add sprues if you want.

Reservoirs

A further possibility is to drip the glass into the mould.  To do this you need to place a ceramic pot, supported by kiln furniture, above the mould with the glass for the casting in it.  Take to a temperature between 850°C and 900°C, depending on how long you wish to wait for the glass to flow out of the pot and into the casting mould.  The action of the glass forming in the pot eliminates many of the bubbles caused by frits and powders.  A further advantage is that this forming in the pot eliminates the possibility of the edges of the original glass pieces being seen. It would also allow you to add a different colour causing swirls or wisps of colour to move through the main colour.

The main effort is to eliminate the bubble formation.  This can be done with vents, adjusting the schedule, modifying the method by melting the glass into the mould, or making a master and individual investment moulds.  You can also combine several of these methods in one firing if you wish.

Powders Burn Away - Kiln Forming Myths 21

Glass powders will burn off at high temperatures.

No.  The powder is glass.  Glass does not evaporate or otherwise combust at kiln forming temperatures.

The appearance of glass powders fading at fusing temperatures is related to the different appearance before and after firing.  Before firing, the powder looks both denser and paler than the final colour.  The initial experience with glass powder always is to put less on than needed. 

You need to remember that a thin film of powder is a tiny fraction of the thickness of the glass it is made from, so the colour will be much fainter.  A considerable amount of powder is required to give the colour shown by the colour charts – as much as 2mm for paler and transparent colours.  Opalescent colours show a little better with thin applications, but still require significant amounts.

This shows the application of powder on a piece where the powder provides almost all the colour for the piece.

The best procedure is to make test tiles with varying amounts of the powder to determine the thickness required for your desired result.  This gives a visual reference and experience in laying down the powder in appropriate thicknesses.

The appearance of the glass powder burning off, is merely the application of too little powder.

Frit Making with Shock Treatment

Among the many ways of making frit, using thermal shock can be a simple way of producing significant quantities of frit.

The process:

Clean the cullet well.  It is not important to dry it as that will happen in the kiln.  Place the cleaned glass in a stainless steel container.  Take the temperature up to at least 300C as fast as you like – the glass is going to be fractured anyway.

The cullet in stainless steel bowl

360fusionglass.blogspot.co.uk

While the temperature is rising get a bucket or basin of cold water to place very near the opening of the kiln.  When the kiln has reached the temperature, switch off and open the kiln.   Reach in with heat resistant gloves and pull out the container.  Tip all the glass into the water.  It will steam and crackle, but no damage will occur, even to a plastic container.

The heated frit in water

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When the glass is cool (a few minutes) drain the water off and dry it, either in the kiln or on top or spread out on newspaper.  After the glass has dried you can break it up further with your hands, or any of the other ways of smashing glass into frit.

The fractured glass after drying and before breaking

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Cleaning:

My practice is to discard all of the very fine frit and powder resulting from this smashing process, as it is likely to be contaminated with other things, which can give a grey appearance to the work.  

However, you can use strong magnets to remove steel particles from the glass frit and powder.  Some have recommended the use of the magnetic trays used by car mechanics to help remove the steel contaminants. In both cases, the magnets should be covered in plastic to make cleaning of the magnets easier.  You simply take the plastic off the magnet or tray and shake the residue off the plastic, leaving an uncontaminated magnetic surface.

The magnets will not remove non magnetic materials such as a range of stainless steels, and other non ferrous metals. This requires you to use metals that can be magnetised as your breaking implements.  Also, magnets will not remove other non metal contaminants. This means it is important to clean the glass well at the start of the process and keep it clean throughout the breaking process.

Making Frit

There are many ways of making frit. I have used a variety of instruments to make frit.  I am sure there are many more ways, but these are the ones with which I have experience.

The frit maker can be any of a number of things.

The mortar and pestle is a very good implement for small quantities of frit.  It produces a variety of sizes with little contamination, especially if you use a ceramic set.  You use a grinding motion mainly with occasional thumps.  Cover the open mortar with a cloth to keep the bouncing glass pieces from escaping.

A coffee grinder whether hand cranked or electric is easier.  But it tends to produce one size of frit in small quantities unless you control the grinding to short pulses and frequent sieving.  The other drawback is that the blades of the electric grinders tend to be stainless steel which cannot be removed by magnets. Really thorough cleaning of frit is required.

I have used an adapted kitchen sink waste disposal unit. It produces frit of two sizes – powder and whatever the grid opening is.  The results contain quite a bit of metal and need to be cleaned very well.   The advantage is that it can produce large quantities of frit quickly.

The closed end pipe and plunger is much more work but can produce quantities of varied sizes of frit. It is better to use mild steel rather than stainless steel, as much of stainless is not magnetic, and so the metal fragments cannot be removed in the simplest way.

Shock treatment.  This uses thermal shock of the glass to assist in the frit making process.  It is described elsewhere.

Grading and Cleaning

Sieve all the fines and powder out of the frit and discard.  The metal fragments mixed in these fines can be removed by multiple passes of a strong magnet.  

See here for a method of keeping the magnet clean. However, there are often a number of contaminants that cannot be removed with a magnet.  Washing powdered glass, is a good way to block drains. So the best and safest thing to do is to remove these fines from your frit. A kitchen sieve works well to this.  You can dig this into your garden, as it makes a good soil conditioner, or bag it and put it into the municipal waste.

Grade the remaining frit with a garden riddle or other graduated set of screens.  At this point you may wish to wash your frit to remove any powders from the glass.  Do this in a basin to avoid eventually blocking your drains.  The resulting water can be poured on the garden, or indoor plants.  Alternatively, decant the water from the heavy glass powder and put the damp powder into a sealed bag to protect the refuse workers if you dispose of it in public waste.

You can dry the frit by leaving it in the open air and stirring it occasionally.  You may wish to place some newspaper over the open tray of frit to keep it free of contaminants.  This still has the risk of dust settling onto the frit as this method takes days to dry.  The quicker alternative is to put the glass in flat open metal containers such as baking trays on top of the kiln.  A bit of aluminium foil placed on top of the tray will reduce the dust that can get onto the frit.

Put resulting dried and graded frit in labelled containers for future use.

Alternative to Glue

Frit

Yes, clear fine frit can be a risk free alternative to glue.  Glue is to keep things in place while moving to the kiln.  Fine to medium grit can do that too.

Glass is pretty heavy and if you move the piece carefully the frit will not move and its weight will keep the other glass pieces in position.  It is better, of course, if you can build in the kiln.  Then you add the frit at the last moment to keep everything in place during the firing.  At rounded tack fuse the clear glass frit will become part of the surface. 

This method will not work for everything.  If you are using laminating or only softened edges for your tack fusing, the glass frit will remain granular.  If you have to tip or manipulate your shelf into your kiln, this will not work either.

This note indicates the use of frit to stablise stringers and rods.

Using frit to stabilise your pieces is yet another way of reducing the amount of glue usage in your work and so reduce the risk of bubbles and marks, even though the practice is not always applicable.

Frit Making

Start with clean cullet. Rinse or wash off all the dust and felt tip marks. Spread out to dry, or put the collection in the kiln at about 200C to dry.

  

An example of clear glass cullet

It is possible to start with the dry glass, or you can heat the glass to a temperature in the 300C to 450C range in a stainless steel container.  Then take the hot container with heat resistant gloves and pour the hot glass into a bucket of water (do not pour the water onto the glass, as a great deal of steam will be produced burning you).  The fractured glass can then be further broken down in size after being dried.

Smashing the glass can be done in a number of ways.  A small amount can be made in a mortar and pestle.  Larger amounts can be put between sheets of newspaper, in plastic bags or any other container that will keep the glass from shooting all over.  Hit the glass package with a hammer multiple times.

You can build a frit maker from pipes with end caps.  Fit one pipe inside the other with the glass between the ends and pound the glass between the two surfaces.  There are a number of variations on this method of production.

An example of  a commercially available frit maker

Large scale frit production can use coffee grinders, or adapted waste disposal units. Waste disposal units tend to produce a lot of frit of the same size, while coffee grinders with blades produce frit sizes related to the time the glass is ground.

Example of a suitable coffee grinder

Example of a garbage disposal unit

When you have created a pile of frit, sieve it into various sizes with screens.  It is best to discard the fine frit and powder, as they contain contaminants that are difficult to clean out and will discolour the finished product.

Example of a commercially available set of seives

You also need to ensure the metal is removed from the glass by using a strong magnet.  Fridge magnets will not do.  Put the magnet in a plastic bag and run it over the frit several times.  When finished take the bagged magnet to the waste bin and remove the magnet.  The metal will fall into the bin.

Example of  a pair of strong magnets

It is a good idea to rinse the remaining frit to remove dusts and ensure the frit is clean.  Again you can put the frit on a metal tray in the kiln to dry.  Put the frit into a closed container to keep it clean until needed for use.

Cleaning Frit and Powder

If you make your own fine frit and powder, make sure it is clean to avoid black specks, or a grey appearance caused by metal dust and fragments.

Clean the glass you are going to break up before you start the process.

Use mild steel or other magnetic metal to break up the glass, or protect the glass from the breaking tools with layers of paper, plastic, cloth or combinations of these materials.

Then with a powerful magnet remove any metal residue from the frit and powder. The magnet will need to be passed over and through the glass particles a number of times, cleaning the magnet after each pass. To ease the cleaning you may wish to put the magnet in a plastic bag. Then move the bag over the waste bin and remove the magnet. The particles fall into the bin.

Do not use stainless steel to break up the glass as it will not be attracted to the magnet. Stainless steel particles will result in the same discolouration as if you left the glass uncleaned.

Using Space on Shelves

Often there is unused space on the kiln shelves when you are firing a project. With a bit of planning, you can make use of the spaces for a variety of things.

Frits fired on fibre paper

Bowl made from frit balls

You can place piece of frit in the clear areas to make frit balls.

You can make colour tests on plaques of glass to see the results of strikers, powder combinations or results of various depths of colour.

Compatibility tests can be done with pieces of glass of which you are not certain.

simple stress testing set-up

Strip of fired glass samples for testing

Results - those with halo are stressed

In the same way, annealing tests can be conducted.

You can fire small pieces of jewellery at the same time as your larger pieces.

You can also prepare elements for incorporation into other fusing projects and lay them out in the open spaces on the shelf.  Your use of the spare space is related both to your imagination and to your future projects.