Ceramic vs. Glass Kilns

The purposes of these two types of kiln differ, so insulation properties differ too. Ceramic kilns have high density brick insulation to retain the heat and slowly cool the contents from the top temperature to avoid breakage. Glass kilns have light weight insulation – whether brick or fibre - to cool quickly from the top temperature to avoid devitrification.

Considerations

Controllers are necessary for controlled soaks and cooling on glass kilns. Much simpler controls are sufficient for ceramics firings. So a ceramics kiln needs to have a controller added. This is a significant cost. If buying a ceramics kiln new for glass work, ensure it has a controller that can be used for glass.

A bathtub or coffin type kiln with controller

Heat distribution is different in the two. Glass kilns are shallow to get even heat distribution to the surface of the glass. Ceramic kilns have elements around the sides and frequently in the top, but may not have any in the door, if it is not top loading. This means the heat distribution in a ceramic kiln is not as even as in a glass kiln, which is not a problem for ceramics as so much heat is retained at the target temperature, it equalises as the kiln cools.

Ceramic kilns with elements on sides rather than top

Loading

Ceramic kilns up to 450 mm deep tend to be top loading, the ones with greater depth tend to have doors. Glass kilns are shallower and tend to have top – just the lid opens - , clamshell – the lid is the whole chamber which opens giving direct access to the floor of the kiln - or bell type – where the lid also forming the chamber is lifted from the base of the kiln and often the base is on wheels so another base of prepared work can be wheeled into place for firing before the kiln has completely cooled.

Large top hat kiln showing direct access to the kiln floor

Baffles are required in ceramic kilns because the heat is greater nearer the elements on the initial advance in temperature. These baffles avoid premature sealing of the edges of pieces causing large bubbles.

Firing on multiple levels is possible in ceramic once you have built up the experience.

Schedules have to take into account the greater mass of insulation in ceramic kilns.

Kiln with fibre insulation all around

Annealing and cooling tends to require different strategies to encourage the ceramics kiln to cool fast enough in the devitrification range, but can have the power turned off earlier after the annealing soak, because of the slower cooling.

Large front loading ceramic kiln

Ceramic kilns are ideal for casting.

Cost – ceramic kilns tend to cost less than glass ones and second hand ones have been more widely available. That may be changing now with the increasing popularity of glass fusing. 

The electricity costs are marginally higher in ceramic kilns than glass, because the mass of brick to be heated up is greater..

Space - Ceramic kilns tend to take up less floor space because they are deep or tall rather than broad.

Small top loading kiln with combination brick and fibre insulation

Removing Kiln Wash

Kiln wash can get stuck on items for a variety of reasons. 

A variety of ways to remove the kiln wash are:

Grind the kiln wash off with diamond hand pads, or small rotary tool with wet sandpaper. You can then proceed to continue to grind with successively fine grits until a polish is achieved, or you can fire polish after a thorough cleaning.

Example of scrubbing kiln wash off

You can sandblast off the kiln wash and then proceed in either of the fashions above.

You can soak small pieces in tri-sodium citrate and then if necessary scrub with a wire brush – a brass wire brush is preferable to steel one to avoid scratching the glass.

Another solution is to place small items in an ultrasonic cleaner basket with water and a little soap or proprietary cleaner. Leave for an hour or two and they should be free of the kiln wash.

A link to some methods of cleaning shelves is here.

Grinder Chipping Glass

There are a number of reasons that may cause the grinder to chip the glass surface. Some of the things to check are:

Too much pressure

It may be that you are pressing the glass into the grinder head too hard. The grinder head should do the work. Firm rather than hard pressure should be applied. If the grinder slows, it is an indication that far too much pressure is being applied.

Insufficient water supply

There may be too little water reaching the head to lubricate the diamonds and keep the glass cool. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply.

Worn or damaged grinder bit/head

Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. Any dents or smooth places on the bit cause a vibration that is similar to a tiny hammer tapping the edge of the glass.

Grit size

It is possible that you may be using too coarse a grit on the grinder bit/head. The more coarse the grit is the larger the chips will be taken off the edge surfaces. Smaller grits take smaller chips off the edges, and so are less obvious.

New bits

Examples of the range and grit differences in grinding bits

If it is a new bit that is causing the chipping, consider dressing it. New bits often need to be dressed – removing protruding diamonds, or cleaning and exposing new ones on a worn bit. To dress the bit you can grind some scrap glass, brick, or use a dressing stone to lightly grind some of the abrasive material away. This most often settles the bit and avoids chipping.

Cutting thick glass

Use the correct angle of cutter wheel for the thickness. 

Use a similar pressure to cutting 3 or 6 mm glass. It is natural to think that as the glass is thicker, you need to use more pressure. The different angle of the cutter wheel is designed to transfer the standard pressure more directly downward.

Use cut runners made for thick glass to help break the glass. Run score from both ends of the score, especially on curved scores.

Example of cut runners for thick glass

Alternatively, turn the over and use hammer and rounded screw driver to run the score (similar to tapping method for thinner glass). Place the screw driver blade directly over the score line and tap it with a hammer. This will start the run. Continue it by placing the screw driver over the score at the end of the open score and tap again to continue the run.

Air Brushing on Glass

Raphael Schnepf Workshop

Air brushing onto glass is a little different than onto other slightly absorbent surfaces. As glass cannot absorb the moisture from the material being sprayed, the medium needs to be allowed evaporate. This means that each layer of paint must be allowed to dry before the next layer is applied. If too much liquid is applied to the glass, it will bead up giving a stippled appearance to the finished result.

There are some things that can help to give an even application of the paint or enamels to the glass.

Clean the glass very well. After thorough cleaning and drying, use some of the paint to rub the glass. As the paint is a slight abrasive, it cleans off anything the other cleaning methods could not get off.

Add a drop of washing up liquid to the mixture of paint and medium (liquid). This breaks the surface tension of the medium and reduces the tendency to bead up on the glass.

Use alcohol part or all of the medium. This reduces the evaporation time. Also apply in a warm rather than cold place. You can use a hair dryer on low speed and power to assist the drying.

Apply in thin even layers, allowing the paint to dry between applications.

Open the air brush trigger before reaching the edge of the area to be painted and close it after reaching the other edge. Any overspray can be cleaned up as in any other painting.

A slightly larger opening at the nozzle is required on the air brush than for other paints, but you have to be careful to avoid opening it so large that you get the spitting of large drops of paint onto your surface.

Because you are putting very small particles into the air you need to observe various precautions. You need to have a dust mask on at all times you are air brushing. You should do this in a spray booth with extraction if possible. If not, you need a well-ventilated area and very good clean up afterwards.

Avoiding Large Bubbles

“I tried small projects and they turned out fine. I have a 12" square with an emblem in the centre and a border set in slightly from the sides. Most of the glass is only the one layer. Both firings produced huge bubbles in the areas where the glass was only one thickness.”

Scale does matter. What can be done at a small scale does not always transfer to a larger scale without alteration.

 

The first problem this project created was using only one layer as the base. Glass has a surface tension which means that it tries to become 6-7 mm thick, which is twice the thickness of a single layer. As it thickens at the edges, it traps the air under other parts of the glass, and as the glass continues to soften the expanding air bubbles come up through the thin parts of the glass.  Using two layers of glass with the design on top will ease the problem.

 

The design is the second problem. The weight of the border makes it even more difficult for the air to get out from under the glass.  Although having two layers of glass will reduce the problem, think about ways to make the border incorporated with the second layer of glass, so the weight of the glass at the perimeter is not greater than the interior.

 

The third problem is that there is not a bubble squeeze in the schedule (indicated elsewhere in the query). The soak of 10 minutes at 538ºC/1000ºF is not necessary. You do need a soak at a point between 620ºC/1148ºF and 677ºC/1250ºF - this is the bubble squeeze temperature range. It is also the slump temperature, so you can determine what the bubble squeeze should be for your glass by what the upper slump temperature is.

 

The bubble squeeze can be accomplished by a half hour soak at the slump temperature, or by a slow rise from 50C below the slump temperature – taking an hour or so, depending on the size of the piece.

 

A fourth problem is the that the separator is kiln wash, and the edges of the glass conformed to the kiln wash, resisting the movement of air from under the glass.

 

You may need to change to fibre paper for single layer pieces, as that allows more air out. Shelf paper may be enough, but you can also put it over 0.5 mm fibre paper for greater air release. Alternatively, sprinkle powdered kiln wash over the fibre paper and smooth it if you don't want to use Thinfire.

 

Lastly, try to avoid the factory set schedules in your kiln's controller, as they are generally set for 6mm thick pieces.

 

Look at the glass manufacturer's website. Bullseye, Spectrum, Uroboros, and Wissmach give basic firing schedules that work with minimal adjustment. I don't understand why kin manufacturers don't simply refer to the manufacturers’ sites to give their customers good advice, instead of the pre-programmed stuff.

Orientation of Cutter Heads

In addition to the alignment of the cutter, you need to be sure you are moving the cutter forward in the way it was designed. If you look at the cutter from the side you will see two things. The most obvious one is that the angles of the cutaway on the head are shallower at the front and steeper at the back. In addition if you look carefully, the axel for the wheel is slightly forward of the centre line of the whole cutter.

Cutters aligned to score to the left

So it does not matter where the fixing screw on the head is. It may be at the front or at the back. What matters is the cutaway at the back which allows the cutter to be used with a shallower angle, than if reversed. This is most obvious on wide cutter heads.

Cutters arranged to score to the right

Repair of a Plastic Barrel Cutter

Generally, there are two parts that can break or be damaged – the wheel and the barrel.

Wheel

If the wheel is damaged or worn, you can replace the head. When considering that, look at the cost of the head and consider whether the small cost difference indicates you should buy a new cutter instead.

The most common damage is a flat spot on the wheel. You can check for this, immediately after dropping the cutter, by lightly moving the cutter across a smooth piece of glass. If you hear a regular ticking sound, the wheel is probably “flat spotted” and will need to be replaced.

A worn wheel is more difficult to detect and is also much less likely to occur. I have been using some of my cutters for 15 years without any sign of being worn.

Plastic barrel

Sometimes the plastic barrel cracks or breaks. This will mean that the oil in the cutter will leak out. So you can consider continuing to use the cutter without oil, or by dipping the head in an oil soaked bit of cotton wool or similar material before each cut.

If the break is at the threaded end and you want to continue to use the ball at the end for tapping purposes, you can glue the barrel back together with an epoxy resin. Make sure the pieces are free from oil. Then glue and allow to cure. After curing, sand down any excess resin to make the cutter comfortable to handle.

Then for additional strength you can wrap with dental floss or extremely fine wire. Start below the glued area and go to the end of the break and back again. You could coat this wrapping with epoxy again both to secure the wrapping and to further increase the strength.

Another solution is to cut the cracked end off and re-thread the remainder. It makes the cutter a little shorter, but will continue to hold oil and give good service.

Or, you could buy another cutter.

Gas Fired Kilns

Installing a Gas Fired Kiln

There are a number of considerations about the location of a gas kiln.

Air exchange

The kiln should be placed in an area where there is good air exchange. For a number of reasons, it is usually convenient to place the kiln near an outside wall – ventilation and canister storage are the two most important.

Ventilation

There should be a low level vent to the outside to allow air to rise for the use of the combusting gas. There needs to be high level vent for the gas to escape. Ideally a hood with powered extraction would be installed.

Space

As with any kiln, you need to have sufficient space around the kiln to avoid heating any flammable materials. In the case of a gas kiln you need to be careful to avoid storing any thing that might ignite above the kiln. You can put heat resistant materials around the kiln if desired.

Location and storage of gas canisters

The safest place to put the gas canisters is outside the building. This does mean going outside to turn the valve on the gas canister on and off. However this is the safest place, should any fire start in the building, because the fire brigade can find and remove it from the fire. This placing applies to both the in use canister and any other full or empty canisters.

Detectors

Finally, even with these precautions, you should install a carbon monoxide detector. Carbon monoxide has no smell and can overcome you very quickly. Even a few minutes of exposure can leave you feeling ill for days.  

Paint and Cold Working

One of the difficult things in cold working is determining when it is time to move on to the next grit size.  You really cannot tell while the piece is wet, because the water disguises the minute scratches put into the glass by the grinding process.

After you have dried the piece, you can coat it with a white paint pen. It is not necessary to cover the piece completely in white pen.  The white marks will be taken away by the grinding to give evidence of where you have already ground the piece.

Piece prepared for the next stage of grinding

You must make sure the paint has completely dried, or it will wash off with the water used in the grinding.

I find I get best results from this "witness" by making the paint pen marks at right angles to the grinding direction.  When all traces of the paint have disappeared, you have thoroughly covered the piece with that grit size.  Being ever cautious, I tend to dry, paint and do a second pass at the same grit before going to the next finer grit.

Cold Working Holes

If you have, or expose, holes while cold working a piece, you need to keep the glass waste from settling into them. Dry the piece and with a bar of hard soap rub over the area until the holes are filled with soap. Then polish off the excess with a cloth or paper towel. 

The filled hole in this piece is the white dot half way up on the right side

When finished cold working, you only need to wash out the soap, which is much easier than trying to get rid of powdered glass.

Based on comments from Cynthia Morgan, posting as Morganica

Diamond Quarries

Diamond quarry glazing appears to be simple. But because if its simplicity it must be both accurate and aesthetically pleasing. Diamond quarried windows look best if the individual pieces are taller than they are wide. The window also looks better if the lines forming the diamonds end in the corners of the window as it gives a pleasing wholeness.

Simple sketches to show the relative clutter of a 6x6 and a 4x4 pattern

You can try making a sketch and trying various options of numbers and angles. By this simple exercise, you can determine the amount of clutter (how many lines vs. how much glass shows) produced by various combinations. It is important that the proportions of your sketch should be the same as the that of the window for which you are designing. This sketching process lets you try out various numbers of quarries along the bottom and sides quickly as the lines do not have to be accurate. Once you have decided on a size that looks good on the sketch you can count up the number of divisions on the horizontal and vertical lines. This can then be translated into approximate sizes on the full sized cartoon.

Begin with the cut line cartoon. This seems obvious, but until you have measured the window and given the glazing and border leads allowance you are unable to design a damond quarry glazed window that will have its lines meeting the corners in the finished window.

Drawing the diagonals

Begin the designing process by drawing two diagonal lines from the corners of the panel to ensure the lines will finish in the corners. This will also determine the centre of the panel and show you the slope of the diagonals. If you have not already determined the size of the diamond quarries, you can try out different sizes of diamonds by drawing parallel lines to the diagonals. 

Centre point determined, trial sizes of quarry, and the sides divided

When you have a size you like, you can determine the number of diamonds horizontally and vertically across the panel. You need to measure the width and height – point to point – of the test diamond and divide the lengths of sides to determine the number of quarries up and across the window. 

In this example of a small window (380 mm x 280 mm), the quarry is about 50mm wide and 65 mm high.  The exact sizes are not too important.  Divide the length of the base and side lines to determine the number of divisions.  In this example it is five on the base and six on the side. To get the exact dimensions for the quarries, you can use the no-calculation method of dividing a line. [link]

The divisions are extended to the opposite vertical or horizontal line to provide points from which to draw the diagonals.

Lef to right diagonal lines drawn

In this example feint lines were drawn up and down across the panel.  This provides a check that the diagonals remain accurate.

Remaining diagonals drawn  and confirmed by horizontal and vertical check lines

Sometimes, however, the corner to corner diagonals are too steep or too shallow to make pleasing diamonds. Diamond quarries generally have slopes that fall in the range of 50 – 60 degrees (45 degrees gives square quarries and anything over 60 degrees gives such a narrow quarry as to be unplesant). In these cases you need to determine the approximate angle for the diamond shape before proceeding from the centre point. 

Centre found and test diagonal drawn

In this landscape format example the diamond is horizontal, but otherwise a suitable size, so the dimensions were taken of it to determine the number of quarries on each side.  In this case it was seven along the bottom and four along the side.

Normally a 55 degree angle is a suitable middle range slope for diamonds, so you can start with that to determine what number of pieces vertically and horizontally are appropriate.  

After marking off the equal number of spaces, you can determine which points will give you appropriately angled diagonals. 

Number of quarries determined and first diagonal drawn

It is important to count the same number of marks along on the base line as there are marks on the side.  You will see in the above picture - if you look closely - the effects of narrowing diamonds if you do not.

The second set of diagonals being added

After setting the diagonals on the first side and part of the base, continue with the parallel lines to the end.

Completed set of diagonals for a landscape window

This photo shows the completion of the diagonals.  It also shows the need for concentration.  I was interrupted and when I came back I drew a line from centre top to the next to last mark on the cartoon.  However, as you can see from the erasing, it should have been the corner mark that I drew to.  I point this out to emphasise how important it is to concentrate while doing this apparently simple task of joining the marks.

Dividing a Line

Dividing a Line into Equal Parts

There is a no-calculations method of dividing a line into equal parts. This describes how to divide a line into any number of equal parts without calculating and measuring the divisions.

The steps are:

Determine the number of divisions required across the base or perimeter line. Then draw a long line at a shallow angle to the line to be divided.

Angled line and dividers

Take a pair of dividers and open them to the approximate width of the individual division or piece.  In this example it was decided to have five divisions of about 50 mm.  The dividers are opened to 50 mm and marked off on the angled line from the corner the required number of times - in this case five.  

Then make a line from the last mark to the angled line to the end of the cartoon line with a right angle drawing square. 

Setting the angle

Place a long straight edge on the bottom of the drawing square and then mark the remaining divisions by maintaining the angle of the long straight edge while moving the drawing square from one mark to another. 

Moving the drawing square along the line and marking the base line

This process allows you to make equal divisions of a line without worrying about fractions of a centimetre or of an inch. It is simple and just as accurate as your skill in holding the straight edge steady while you run the drawing square along the edge and mark the base or vertical line.  In general it is more accurate than any calculations and measurements on the base or vertical lines.

Storing Moulds

Requirements

• protection – some form of container is required to maintain the life of the mould. A container also prevents the kiln wash or other separator from being rubbed off.
• ease of access – It should be easy to get to the moulds, especially as some can be heavy.
• flat vs vertical – some like to store moulds in boxes vertically rather than horizontal with them stacked one above another.
• In either case you need shelves of the right depth and height to store and support the moulds.
• custom made vs bought in – Moulds you have purchased normally come in their own boxes which can be retained for storage for a number of years. You can also make light weight boxes either from old boxes or card cut to size and taped and glued together. More robust boxes can be made from thin plywood or thin pressed board.

Quarry Glazing

For some reason that I have never learned, the regularly shaped pieces of glass that are leaded into a window are called quarries.  The derivation of the word relating to excavation comes from its middle English variant of medieval Latin quareria, from Old French quarriere, based on Latin quadrum 'a square'.   How it came to be applied to pieces of glass is unknown apparently.  However, in stained glass, "quarries" can relate to rectangles, diamonds, circles, etc., not only squares

These kinds of glazing are weak as there are long straight lines created with these designs. The best way of counteracting this weakness is to “weave” the lead joints within the panel combined with more frequent than usual glazing bars.

It may be wise to use a glazing bar at every second set of joints rather than merely consider the distance between bars.

Soldering corroded metals

Some investigation has shown that zinc corrodes more easily than lead (lead being a medium anodic and zinc a high anodic metal). Bronze, Copper and Brass are among the least easily corroded materials other than precious metals. Lead comes immediately after copper in the ease of corrosion.

The implication is that more easily corroded materials need more aggressive/abrasive cleaning before soldering and that their longevity in the weather will be shorter than the less easily corroded metals.

Slumping Tack Fused Stringers

With a piece made up of tack fused stringers it is important to anneal carefully. There are a multiplicity of small contact points where the glass is little more than laminated. This means that each piece of glass will try to contract into itself rather than acting as a single unit. An annealing soak and cool should be chosen to be several times longer than its nominal thickness.

Slumping of this piece will also need to be done carefully. First, to ensure all the contact points can relax as soon as the thinner parts of the stringers. Second, the weight of the piece is not as great as one of the same size and so will not fall into the mould in the same way – it will take longer. Third, the movement of the glass will be within the fragile stringers so it is important to take the heat up slowly so they can bend rather than fracture . So, fourthly, it is important to establish a schedule that will have a slow rate of advance to the lowest possible temperature with a long soak for a slump of the required depth.

Rods in Glass

Inserting large rods into glass

Normally large diameter rods are incorporated into a glass piece for display supports. However, rods over about 2mm will break the glass when incorporated in the fusing. This means that rods to support the glass that will be of sufficient diameter for the strength required cannot be fused into the glass.

You can use a rod of the same - or slightly smaller - size if you wrap it with thin fibre paper to cushion the differentials in expansion and contraction. When the glass has cooled, pull the rod out and clean out remaining fibre paper with water and a pick. 

Alternatively, you can use fibre paper to create a void of similar size to the wire or other metal support that will be inserted.

If you find that you do not like the hump that the rod or fibre paper create, you can use a third layer between the top and bottom.  Cut the glass to leave a channel of the length you want.  This means that the intermediate glass will need to be in at least two vertical parts, with half the channel width taken out of each side.  You can cut the glass in three parts horizontally.  The main piece is cut to a length minus the depth of the channel.  Then two strips are added to the bottom which leave the desired width. Finally, put the top layer on.  

Alternatively, the middle layer can be the design, with the bottom and top being clear or any other colour desired.

Once fired, clean out the channel and glue the rods into the glass. Silicone is a good all purpose adhesive that will allow a bit of movement.

Slumping Single Layers

Slumping small single-layer pieces

A different set of conditions are formed when slumping small pieces. It takes more time or heat to slump small pieces, because there is less weight to pull the glass down into the mould. Because there is less weight in a single layer than in a two layer fused piece, more time is required for the glass to conform to the mould. The exact amount of time and heat will have to be observed for each kiln and kind of layup.

The alternative strategy is to increase the heat. This can work, but leaves more mould marks on the finished piece. The increased temperature causes the forming to occur much more rapidly and may lead – if not closely observed – to over stretching or “bubbles” at the bottom of the mould.

If you are not concerned about the exact nature of the shape of the piece you can fire pretty fast and high. This will produce a curve, but probably not good conformation to the mould. Even if there is good conformation, there will be significant marking of the back of the glass.

If you want a relatively unmarked and slumped piece, you need to change strategy. You can take the glass up in temperature rapidly to ca. 600C and then use about 40C/hour to 677C. Begin watching from about 635C. This slower advance in the slumping range gives better control of the slump. Slower rates of advance give less distortion and less movement within the mould. It also provides a less marked piece, because the slumping is done at a relatively low temperature.

When the piece achieves the shape you want, record the temperature and advance to the next – cooling - segment. The recording of the temperature will give you information for future slumps of this nature.

On any slump that is new to you, either in terms of the mould or the thickness and size of the glass, you need to make records to help determine the optimum combination of conditions. You should of course, record each firing anyway.

You will find more information on factors affecting slumping at these locations:

Weight/thickness

Mould sizes

Mould shape and detail

Uneven slumps

Firing speeds

Rounded edges on slumped pieces

Often people want to know how to get rounded edges during slumping of a single layer piece, especially when trying out techniques with their spare art glass.

Achieving a rounded edge on a slumped piece is a combination of temperature, thickness, larger top layer and cold working.
Rounding of the edges of a piece of glass occurs at tack fusing temperatures, which are beyond the slumping temperature. It is possible to take the glass to a tack fuse within the mould as long as you are prepared for some consequences.

More mould marks are evident on the bottom of the vessel.
Mould life is reduced.  You get a lot more mould marks on the bottom of the glass because the bottom of the glass is softer than in a standard slump. These marks will be directly related to the surface texture of your mould.

You need to re-coat the mould before the next slumping to avoid the kiln wash sticking to the glass.  Ceramic based moulds last a long time if fired below 680C. But numerous firings at tack fusing and higher temperatures increase the possibilities of glass sticking to the mould and occasionally, thermal shock. If you insist on tack fusing in your mould, you need to renew the separator each time, as the kiln wash breaks down at tack fusing temperatures leading to it sticking to the bottom of the next piece you fire in that mould.

You may get an uprising at the bottom, as the glass slowly sinks down the mould and pushes the glass up at the bottom in any mould other than a simple, shallow shape.  

These things indicate that it is best to tack fuse first and then slump at the lower temperature.

Of course the best result can come from using 6 mm of glass, with the top layer 6 mm larger than the bottom layer. This allows the upper layer to sink over the outer edge of the lower one, giving a rounded edge with no sign of any differences between the two layers.

Cold working solution 
If however, you want to work with a single layer, you need to realise that the edge will be the same when it comes out as when it went in. So you need to cold work the edge before slumping. You do not need sophisticated machinery to do this. A few diamond grit hand pads will do the job. Start with one at about 100 grit to shape the edge. Make sure you keep the pad and working surface damp. If you begin to get a white paste appearing, you need more water.

After shaping the edge satisfactorily, take a pad of about half the grit size (twice the number) and begin the smoothing of the scratches created by the shaping. When finished with one grit move on to the next. You can use a paint marker to help tell when one grit is finished. 

This process will give a sheen that will change to shiny during the slumping.
  

  

Mould preparation

The preparation of a mould is more than adding a separator to it, although that is essential.

The glass will always take up the mould texture if it touches. So start with smoothing the mould itself. You can do this by wrapping very fine wet and dry sandpaper round a flexible sanding block. You can use this wet or dry, although wet is more healthy. When you get the desired surface texture you can let the mould dry or immeidately apply kiln wash. When this is dry you can again sand the kiln wash but with dry paper only this time and very lightly. Alternatively you can ball up nylon stockings to provide a soft abrasive action. If you sand through to the mould surface, simply apply the kiln wash again.

You can minimise mould marks by slumping at the lowest temperature which can achieve the slump you want.

If you want to avoid mould marks altogether, do an aperture drop and cut off the rim. There is no mould contact with this technique, so the glass retains the surface it had in fusing.