Slumping and Draping materials - Kiln Forming Myths 12

Always slump into ceramic, drape over steel

This myth is based on the fact that steel expands and contracts more than glass and ceramic expands and contracts less.

So, the myth goes, slumping into steel means the glass will be trapped or crushed by the contracting steel.  But draping over means the steel will contract more than the glass making the removal of the glass easy.

The reverse is the expectation for ceramic.  Slumping into the ceramic allows the greater contraction of the glass to be removed from the mould without sticking.  But draping over means the glass traps itself against the ceramic as a result of its greater contraction.

These things are true.  But….

The most important thing in considering a mould is the draft.  This not about cold air, but the angles of the mould. A mould with vertical sides will not release the casting or kiln formed object even if the expansion characteristics of the two materials are identical. To release, the mould must have a slight angle from the vertical away from the glass.  This applies whether a slump or a drape.  This is called a positive draft, as illustrated.

www afsinc org

 And here

If the draft is sufficient, it does not matter whether you are slumping or draping into steel.  In using a stainless steel mixing bowl for draping, you can only use the lower portion where the angle is shallow.  If you rest the glass on the rim, the draft will be too steep to allow the glass to slide upwards as the steel contracts on cooling.

www evetsourcesolutions com

Even when draping over steel, you need to have a draft to aid the easy removal of the glass, as in this example:

creativeglassguild co uk

When draping over ceramic, you need to be careful that you have sufficient draft over the whole of the mould. In the case of this ceramic draping mould you need to make sure the glass is not fully formed as the steep portion at the top will be where the glass grabs the mould.

glassartbymargo com

And if you were to use this casting mould as a slumping mould, the steep straight sides would make it difficult to get the glass out of the mould. 

sundanceglass.com

Although the facts behind the statement “slump into ceramic, drape over steel” are established, you need to understand that the draft of the mould is as important as the way in which you use the material.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

The 6mm Rule - Kiln Forming Myths 11

Glass always wants to be 6mm thick

This is true only at some temperatures.  

The surface tension or viscosity of the glass, together with gravity determines the extent to which the glass will thicken or thin.  The viscosity of glass is such that at high temperature tack and full fusing heats, the glass does tend to become 6mm - 7mm thick. This is taken advantage of in kiln forming to obtain rounded edges, and in making frit balls.  A single layer of frit up to about 10mm will become a round dome due the action of the viscosity and weakness of gravitational forces acting on a small mass. 

Larger pieces of single layer glass begin to shrink as the viscosity is great enough to overcome gravitational forces to allow thickening at the edges.  This causes dog-boning.   At the same time the glass is thickening at the edges, it is thinning in the interior allowing large bubble formation on thin pieces. It also is the cause of the needle points on thinner pieces at higher temperatures.  The glass is soft enough to conform to any imperfections in the surface and so be stretched thin as the main mass of the glass contracts. 

This contraction also applies to low mass items such as frit in casting moulds.  The glass particles contract to form a single mass of material, leaving some stuck to the mould. These pieces may be completely separate as tiny frit balls, or if attached to the main mass, a series of needle points on the edge of the finished piece.

However, the viscosity at full fuse temperatures is not great enough to keep thicker glass in its original shape.  So the effect of gravity on glass of 9mm or thicker overcomes the weakening viscosity force and the stack begins to expand. The extent of the expansion is the result of both viscosity (heat dependent) and gravity (mass dependent).

At lower temperatures, the viscosity is much greater.  This can be used for low temperature tack or laminating temperatures. The glass can be adhered with heat without distortion of the single layer, as the viscosity is so high the glass does not change shape, even retaining sharp edges, although stuck together.

At temperatures above full fuse the viscosity decreases further allowing the glass to flow.  This is used in casting, blowing, and various higher temperature processes, such as aperture melts and stringer formation.  Here the viscosity is low enough to allow gravity to make thin and elongated shapes.

There is a range of temperature above which glass will thin more than the 6mm – 7mm “rule”.  I do not know the exact correlation between temperature and thickness, but at around 1150°C  the glass will become only a little under one mm thick.  This can be seen from the results of kiln runaways. The glass that is melted onto the surface of the shelf is extremely thin, showing that the viscosity was so low that gravity was able to thin it to a fraction of what we think of as normal thicknesses.

The 6mm myth arises from the behaviour of glass at a specific heat range and is the result of the combined forces of viscosity and gravity.  Knowledge of how these interact can enable you to understand the outcome of various projects.  This knowledge of the forces can be used to help create the effect you want.  It also enables you to employ various means to counteract the natural forces of gravity and viscosity. 

More information is in the e-book: Low Temperature Kilnforming.

Absolute Firing Temperatures - Kiln Forming Myths 9

There is a given temperature for each level of fusing – slump, tack, full, etc.

You will often see statements about the temperature for achieving a particular effect.  It is as if all glass under all circumstances does the same thing at a given temperature. These temperatures can only be understood in relation to several things.

• ·         Kiln characteristics
• ·         Speed of firing – i.e., heat work
• ·         Time at forming temperature

The relevant factors about the kiln are:

·         Insulation.  The two main types of insulation in kilns are fibre blanket and insulating brick.  Fibre blanket is often the main insulating element in kilns as it does not absorb a lot of heat. It of course loses heat more quickly than refractory brick.  Most often the floors of kilns are made of brick for rigidity and resistance to damage.  (They also can be replaced individually if one is damaged.)  Refractory brick comes in two densities.  The light weight one is not rated to such a high temperature and loses heat more quickly than the higher temperature rated dense brick.  Both lose heat much more slowly than fibre blanket.  This means the top temperature can be reached more quickly in a fibre insulted kiln than in brick insulated kilns. The brick insulated kilns radiate the heat back into the kiln upon cooling, making for long safe anneal cools without much effort in controlling the cooling rate. Thus the temperatures for an effect are different for kilns with bricks all around than with fibre blanket, and no comparison is easy between kilns with different insulations.

·         Size.  The size of the kiln has an effect on the temperature cited to achieve an effect.  A small kiln can heat up very rapidly, but the glass cannot heat evenly as quickly.  A large kiln takes more time to heat up, as there is more insulation absorbing the heat input.  So working temperatures for small and large kilns are different.  The size of the piece(s) of glass also have an effect.  Small pieces can be heated much more quickly than large or thick pieces, so the top temperature for an effect will be different for the two sizes.

·         Temperature variation across the kiln shelf affects the rate of firing possible and (as noted later) will affect the top temperature.  The more even the heat the faster it is possible to go and that affects the temperature chosen.

·         Element placement.  Some kilns have only side elements, some only top elements, and some have both.  All these variations affect the temperature required to obtain an effect.  In general, top fired kilns can be fired faster than side fired kilns.  Kilns with both, require an intermediate rate, unless the side and top elements can be fired independently.

Speed of firing, i.e., heat work

·         Heat work factors make the top temperature different in different circumstances.  This is mainly about the speed at which you fire the glass.  Generally, the slower you fire, the lower temperature you need.  Allowing the glass to absorb the heat gradually usually means that you can achieve a particular effect at a lower temperature.  A fast rise in temperature requires a higher temperature.

Soak times 

·         The amount of time you soak at the working temperature will also affect the temperature chosen.  A longer soak allows a lower temperature to be used (although that can get into the risk of devitrification from spending too long at the top temperature – it is a balancing act).  A higher temperature can be used to keep the soak time reduced. 

All these variables mean that without being given the kiln characteristics and a schedule, you cannot evaluate the temperatures and rates of firing that are given out by others.  You need to know how closely their kiln fits with your kiln in its characteristics as outlined above.  When asking for a temperature or a schedule, you should indicate what kind of kiln you are using.  You need to know in any schedule what the ramp speeds are and the soak times.  They can then, of course, form the basis for your experimentation.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Exceptions to Slow and Low - Kiln Forming Myths 8

The principle of slow and low always applies.

Although the principle of attempting to get the effect you want at the lowest possible temperature with the slowest practical rate of advance should always be considered, there are times when it is not wholly applicable.

Among these are when working with small scale pieces, such as jewellery, and in general pieces below 100mm that are at least 50mm from the side of the kiln.  In these cases you can fire much faster, as the heat has less distance to travel through the glass to maintain an even heat.  You still should be using two stages – the first and slower to rise to the strain point and the second much faster one to reach the top temperature.  In these cases the target may have to be a little higher than in a larger, slower firing.

Another case is in fire polishing.  Fire polishing can often have a fast segment to avoid distorting the piece.  In this case you fire appropriately slowly for the thickness of the piece until you are past the upper strain point.  This can usually be taken as 540°C.  (For float and bottle glass the temperature is around 690°C).  As you have passed the brittle phase of glass by this time, you can advance the temperature quickly.  The objective is to achieve enough heat to change the surface, but avoid heating the interior to the softening point.  You may want to observe the finish of the surface, so that you can switch to the cool down phase of the firing as soon as the polish is achieved.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Soak ca 550C - Kiln Forming Myths 7

A soak at mid-500's °C can soften the glass to enable sealing against the mould.

The only glass that this might apply to is lead crystal.

The idea seems to be that it is possible to fire the glass in such a way that it conforms to the edge of the mould, so trapping air inside before the glass begins to slump. It goes on to indicate this is a cause of large bubbles from under the glass. This trapping of air is possible of course.  But not at a temperature of ca. 550°C.  Although this is above the strain point of fusing compatible glasses, it is not in the range where most glass begins to soften.

Bullseye, Spectrum, and Uroboros all only begin to bend near 600°C.  This means there is no possibility of the glass conforming to the edge of the mould at 550°C before it begins to slump.  The softening point of Float glass is around 720°C, and although with a large span, a 4mm piece will begin to bend at about 610°C (thicker glass will begin to bend earlier), it takes up to an hour for the glass to take up a gentle bend at that temperature.

It is possible that you can seal the edge of the glass against the mould in a side-fired kiln that is quickly fired. This is where the edge receives greater radiant heat than the top surface.  However, I doubt that this will happen even in a side fired kiln until nearer 600°C.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Bubbles are Art - Kiln Forming Myths 6

Big thin bubbles are art

Unless you have designed the bubbles, they are mistakes, not art.  Even when designed, they are delicate and when broken are very sharp.  So, they cannot be sold or used as they are even by yourself. 

Bubbles within the glass in a plate.

fusing101.files.wordpress.com

People frequently make the suggestion that the bubble should be broken and the cavity filled with frit.  Of course this can be done, but almost always appears a fix rather than a design choice.

The more important thing is to learn the cause so it can be prevented in the future.  Bubbles can be between layers or from underneath the whole piece.

Bubbles between the layers of glass are usually the result of inclusions or layup and firing rates.  Anything which holds the upper layer above the lower one has the potential to induce bubbles.  Most often, with a bubble squeeze, these are relatively small and are 2mm or more thick.  These may be acceptable or seen to be unsightly, but are not dangerous.  The bubbles can become large and/or thin with high temperatures or fast rises in temperature.  Be sure to have a good bubble squeeze, and a moderate (ca. 300°C) rise in temperature from there.

Bubbles can also rise between the shelf and the glass.  This happens most often when firing single layers above a low temperature tack fuse.

A single layer piece with large, burst, healed and emerging bubbles.

www.warm-glass.co.uk

It can also occur when there is either debris between the glass and shelf, or when there is a depression in the shelf.  Both these cases allow air to remain trapped between the shelf and the glass.  Slower rates of advance and bubble squeezes can help reduce these, but the shelf needs to be checked for debris and high or low spots.

The piece below is disfigured by the random bubbles at the left and in the centre of an otherwise acceptable platter.

fusedglass.files.wordpress.com

Evaluate your pieces before you declare a single or series of large thin bubbles art.  Of course, you should play around with the piece to learn from the mishap.  You can use the pieces of it in other projects.  But unless it is truly exceptional, it is a mistake, not art.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Short Holds in Schedules - Kiln Forming Myths 5

Frequent short soaks on the way up will make a schedule safer

Safer in this context usually means less subject to thermal shock.  To determine the validity of this requires a bit of understanding on how the glass takes up heat, and as effected the lay-up.

Glass is a good insulator, both of heat and electricity, although we are only concerned about heat here.  This means that glass transmits heat poorly or, as it may be thought of, slowly.  A steady input of heat at an appropriate rate is less likely to shock the glass than quick rises with (catch up) soaks. 

In general, there is not much change in the rate required when you go over to a single rate without soaks.  For example, a ramp rate of 200°C from 20°C to 400°C with a 20 min soak, then 300°C to 540°C with another 20 minute soak could also be written as 193°C/hr to 540°C - both take 2.8 hours to achieve the same temperature. So the rate is not very different, but the way the heat is put into the glass is.

The glass is subject to heat shock below its softening point, and so rapid increases in temperature at the start of the schedule increase the risk of thermal shock below the 540C region.

When you have uneven coverage of the base glass, as most of us do, more care is required than when we have evenly thick glass.  This relates to the poor heat conductivity of glass.  The need is to have all the glass heat up at the same rate.  This is relatively simple when there are no partial layers on top as when doing a decorative tack fusing.  The pieces on top insulate the heat from the glass immediately below.  This gives a cool spot under the top glass, in relation the uncovered glass. To avoid this difference in temperature, which causes stress, becoming too great you need to slow the rate of advance as well as keeping it a steady increase.  This indicates you should be scheduling the rate of increase as though there were two more layers over the base glass.

The steady input of heat also becomes more important with thicker glass or more than two layers of glass.  The rate of heat input needs to decrease rapidly with increasing thickness – there is not a linear relationship.  For example, doubling the thickness from 6 to 12mm requires a reduction of 2.3 times the rate of advance.  Increasing the thickness by 4 times to 25mm requires a reduction of 10 times the 6mm rate of advance.

Other factors that require slower and steady increases in temperature are where you have dark and light glasses next to one another.  The same applies where you have a viscous and a less viscous glass together.  The classic is black, the least viscous of the glasses, and white, the most viscous.

However there is at least one circumstance where soaks are useful.  When draping over steel or ceramic, the free hanging glass heats up more than the centre where it is resting on the mould.  In this case, the mould forms a heat sink, drawing the heat away from the glass into itself.  You need to go very slowly or insert a few soaks to allow the supporting mould to heat up. More information here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Relationships of bubbles to more holes in moulds - Kiln Forming Myths 4

More vent holes reduce the possibility of big bubbles

The position of the vent holes is more likely to prevent bubbles than simply the number.  A ball mould only requires one at the centre bottom.  A rectangular bowl with sharp curves needs the holes in the corners, not the centre.

The holes in a mould that are intended to allow air to escape should be at the places where the glass will last touch down on the mould.  When placing the holes, you need to think where the glass will last conform to the shape of the mould.

In a square or rectangle mould, the corners are the last places the glass will stretch into.  So the vent holes in the mould need to be there rather than in the centre, or along the straight edge of the bottom.  If it is a square slumper, it may be that there is no actual need for a vent hole, as the curve is gentle, but it is safest to have one at the centre. 

If the firing is too hot or too long in any but gently sloping moulds, large bubbles will be created even though there are adequate or multiple vent holes, as explained by the glass slipping down the mould and pushing the bottom up.


More information on big bubbles is available here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Vent Holes - Kiln Forming Myths 3

Vent holes in moulds will prevent big bubbles

The big bubbles found in slumped pieces are normally between the bottom of the glass and the mould at some place near the lowest part of the mould.  The idea behind the myth seems to be that the air space between the mould and the suspended glass will be trapped and so cause the bubble.

In thinking about how likely this is, look at when bubbles are formed in flat pieces.  This occurs at tack fusing temperatures and above.  Applied to slumping which occurs at lower temperatures, it shows that the glass is unlikely to be plastic enough to allow large bubble formation from heat alone.

Of course, this assertion assumes some things. 

• You need have vent holes in the area where the glass will last touch down.  Their placing will depend on the shape of the mould. 
• The vent holes in the bottom of the mould should be clear, with holes in the side or supported on pieces of 1mm or thicker fibre paper to allow the air from under the mould.
• You should be advancing in temperature at a moderate and steady rate.  Fast rates are likely to cause the edge to conform to the mould and close any air escape through the perimeter.

Large bubbles at the bottom of slumps are most often the result of a too high a temperature or too long a soak or a combination of the two.  A high temperature will allow the glass to continue to move.  As the glass is not plastic enough to thicken, the weight of glass higher in the mould causes the glass at the bottom to rise up in a bubble-like form.  If the slump is at a moderate temperature, but with a very long soak, the same result will be observed.

This means that prevention of large bubbles is by observation.  When using new moulds or new layups for the glass, you should observe the progress of the slump from the softening point upwards and through the soak.  This observation should be by quick peeks at regular intervals and recording the results at each peek.  This will tell you the temperature at which the slump is complete. 

If you find that the slump is complete before the set top temperature, or in the early part of the slump, the target temperature is too high.  In a subsequent firing, reduce the temperature while keeping a half hour soak.  Repeat this until you have a complete slump in that time.  If the reverse is true, increase the temperature until the slump is complete in the half hour.

In conclusion, prevention of big bubbles is a combination of elements.

·         Make sure there are vent holes in the mould,

·         Make sure the air can get from under the mould,

·         Use a moderate rate of advance, and finally

·         Use the minimum temperature possible to achieve a complete slump in half an hour or a little more.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Wet Moulds and Bubbles - Kiln Forming Myths 2

Kiln Forming Myths 2

Wet moulds cause bubbles in glass 

This is difficult, as with most myths it is true in some cases and not in others.  The case where it is true is that casting with wet plaster/silica moulds causes water vapour to move toward the glass.  Casting practice has alleviated some of the problem, by having an extended steam out at about 200°C, or pouring the glass into the hot dry mould from a reservoir.

In pate de verre, the mould is most often packed while wet. The small particles allow any steaming of moisture to pass through, and so be dry at forming temperatures without blowing any bubbles.

In kiln forming, the moisture resulting from recently applied kiln wash is considered by some to be a cause of bubbles.  The water in the mould will be evaporated by around 250°C in any sensible slumping programme.  At this temperature the glass will not have begun to move, so the moisture can move out of the mould through any vent holes at the bottom of the mould, or past the glass as it rests on the edge of the mould.

The circumstance when a damp slumping mould could cause difficulties is when using an extremely fast rise of temperature. This is detrimental to the mould, as the rapid formation of steam is more likely to break the mould rather than the glass.  It is also unlikely to result in a good slump conforming to the mould.

Bubbles at the bottom of the glass are much more likely to be the result of too high a process temperature. This allows the glass to slide down the mould.  The glass is not plastic enough to thicken and form a puddle at the bottom at slumping temperatures.  Instead, it begins to be pushed up from the lowest point due to the weight of the glass sliding down the sides.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Bubble Formation - Kiln Forming Myths - 1

This short series on the so-called rules of kiln forming looks into their accuracy.

Wet shelves cause bubbles

The common sense observation that water turns to steam as the temperature rises above 100C and so could affect the glass, does not apply in kiln forming (in most cases).

Firing at around 200°C per hour will give approximately 2.5 hours for any moisture to evaporate.  This is because the glass only begins to move after about 540°C. So, unless you have a lot of water in your mould or shelf, the vapour will have disappeared some time before the glass begins to move and conform to the shelf, or round up at the edges.  You may wish to leave the plugs out or crack the lid a little to allow the moisture to escape more easily.

  3.bp.blogspot.com

The holes in this piece result from a combination of factors, not all of which might apply – layup, contaminants on shelf, firing too high or fast, low spots in shelf – but not moisture on shelf.  Note that some of the holes have been filled with frit and refired.

bstiverson.files.wordpress.com

  

These holes are more clearly the result of the layup, top temperature and speed of firing. The three strips did not allow air to move out before the edges conformed to the shelf.  The bubbles at the joints of the dark and yellow green seem to be the result of a poor fit, so having a thin area where the air could push through the softened glass at top temperature.

Two circumstances where this moderate rate of advance does not apply are 

- where extremely fast initial rates of advance, such as in small jewellery scale firings are going to be used. Here the distance for the air or steam to escape is very small so there is little concern about causing bubbles from any cause.

- The other case is in casting, where there is a lot of both free water and chemically bound water in the moulds. Special considerations are required for investment moulds.  In summary, the requirement is to dry the mould in some manner before the firing of the glass on top of, or in it. Plaster moulds require two kinds of water removal.  One is to remove the moisture by air drying in a warm area for a week or longer.  The second is to remove the chemically bound water which is usually done at about 200°C for a couple of hours before proceeding up in temperature.  The length of time required for these two dryings relate to the size of the mould.

For shelves, you can air dry on top of a firing kiln, or fire in the kiln at a rate of about 200°C per hour to 200°C and soak for 10 minutes, if you decide the shelf must be dry before firing.  The plugs should be out, or the door cracked a little to allow the moisture to escape easily. After the soak, just turn the kiln off.  You can open the lid or door if you need a quick cool down.  The shelf will then be ready for a rapid rate of advance firing as there is no moisture to be trapped by the glass conforming to the shelf.  This of course, is rarely necessary although you may be more comfortable in using a pre-fired shelf. 

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

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

360fusionglass.blogspot.co.uk

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

360fusionglass.blogspot.co.uk

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.