Tack Fused Drops

Description of the piece

The enquirer wants to cover some blemishes on the flat blank with clear powder and also tack fuse some additional pieces to a blank to be used for a vase drop.

Reactions

To avoid the grey appearance that often comes from clear powder at lower temperatures, you need to fire to contour fuse at minimum. 

Outside of the requirement for a contour fuse, my experience of making a drop vase with a tack fused blank shows disappointing results.  The temperature used in drops is not high enough to flatten the tack fused pieces.  During the drop formation, the space between the pieces stretches more than the thicker tack fused areas. The thinner glass becomes hot quicker than the thicker areas.

This leads to occasional stretched holes between the tack fused pieces.  The tack fused pieces appear as protrusions above the surface whether inside or outside.  Unless planned very carefully, these elements can be ugly. They will maintain much of their original shape, contrasting with the surrounding stretched imagery.

 

Recommendation

Put the piece back in the kiln and take to a full fuse, or at the very least a contour fuse. This will enable all the glass to stretch as one in the drop, because of nearly equal thickness.  Nearly even thickness is needed to avoid stretching some areas too thin in relation to the rest of the drop surfaces.

Drop Vase Design for Opalescent Glass

Credit: Missy Mac Glass on Folksy

When making a drop vase in opalescent glass, the design needs to be on the outside.  This will require ensuring the design will be on the bottom when suspended on the drop ring.

It is possible to build the whole piece as normal with the design on the top and fire it.  Then you can turn it over to make sure the design is facing downwards. 

To get a crisper design for the outside the flip and fire technique can be used.  Build from the outer layers to the inner layers.  You are building upside down. Place the design to be seen on the outside of the drop vase down on the prepared shelf first.  Follow this up by placing the inner layers in order from the most outside to the most inside layers.

These instructions rely upon firing the blank first rather than building on the drop ring.

However, you can build on the ring if you need to save one of the two long firings.  Only one modification is required.  Place a sheet of clear down first.  Assemble the design as for a flip and fire technique, i.e., outside layers first, inside layer last.  

This will require a slow heat up to ensure you have allowed enough time for the air to be squeezed from between all the layers and that all the glass at the same temperature before the drop begins.  Sprinkling a fine layer of clear powder over the clear is a good way to assist allowing the air out.  Place the design pieces down before applying the powder. 

This is not the best way to make drop vases, but it can work with care in placing the decorative pieces and applying the powder.

Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.

Smooth Surfaces on Drop Vessels

It is widely recognised that the usual results of kiln forming are one textured side and a smooth upper side. The common methods of having upper and lower surfaces both smooth is to blow the glass, avoid allowing the glass to touch the mould, and cold working the textured side to smooth.

The question arises about the possibility of getting smooth surfaces on the inside and outside of a drop vessel.  As the glass in a drop only touches the mould at the collar and edge, shouldn’t the glass be smooth on both sides?  The answer to that is in the temperatures and time used.

The temperatures used in a drop are not high enough to be certain of smoothing the outer surface.  But the soak times at drop temperatures are enough to create a fire polish on the upper/inside surface.  This indicates the blank in a drop should be placed with the texture up, facing the heating elements.  The smoother side facing the floor will be stretched and will remain smooth. 

The smoothing effect of firing with rough side up does depend a little on the depth of the drop.  Shallow drops will not have the same heat exposure that deeper drops do, assuming that a moderate heat is being used over three to four hours.

This implies that the design to show on the inside of the drop should be in contact with the separator when fusing the blank.

Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

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

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

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

Calculating the specific gravity of unknown glass.

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

Calculating the specific gravity of regularly shaped items

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

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume.

To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

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

Calculating specific gravity for irregularly shaped objects.

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

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

Credit: study.com

Application of specific gravity to casting and melts.

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

The formulas are:

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

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

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

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

For rectangles

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

For circles

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

For ovals

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

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

Calculating weight for irregularly shaped moulds.

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

Wet fill

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

Dry fill

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

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

Irregular mould frames

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

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

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

Regular mould frames

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

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

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

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

Volume of a rectangle = thickness*length*width

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

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

Specific gravity = volume/ weight

Convex Shapes for Wall Hangings

The most common shapes for wall hangings seem to be the “S” or wave form in various sizes, and flat pieces of what ever outline supported by stand-offs.

There is another possibility.  You can produce a shallow domed shape which can work well for either landscapes or abstract pieces.  They will be best if circular, although rectangular forms can be used.

The usual resistance to doing this is that the surface will be marked, or that the tack fused surface will be flattened.

There is a way to do this without either effect.  Place the work upside down on a mould of appropriate diameter or dimensions and fire the piece slowly to a low temperature. 

Raise the temperature more slowly than you usually would for a slump in the normal way – top side up.  This allows both surfaces of the glass to be at the same temperature at the same time.  This equalisation of heat throughout the piece will protect against any breaks or splits on the underside of the glass – which will become the top surface.

Set the temperature for about 620C, depending on the span of the piece.  This temperature will be suitable for pieces of 300mm to 400mm and 6mm to 9mm thick.  Pieces with a smaller span will require higher temperatures or longer soaks.  Larger pieces will need a lower temperature.

You should set the soak at about 45 minutes. You will need to observe at intervals until you have the amount of depression you wish.  You will also need to know how to advance to the next segment of the schedule when that point is reached, so that you do not over slump the piece.

Since the piece only touches the mould at the rim, and you are not allowing much movement in the glass, you will not mark the glass with the mould. 

This process of making a domed wall piece will be unusual, although it will not be appropriate in all circumstances.

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

Pot Melt Saucers as Dams for Melts

Preparation

Many ceramic plant pot saucers can be used as circular moulds.  Most are unglazed and will accept kiln wash easily.  Some are unglazed, but polished to such an extent they are no longer porous.  These and glazed flower pot saucers need some preparation before applying kiln wash.

Plant pot with saucer

Polished and glazed saucers require roughing to provide a key for the kiln wash solution to settle into.  This can be done with normal wood working sand papers.  You may want to wear a dust mask during this process, but not a lot of dust is created.  You could also use wet and dry sandpaper or diamond handpads with some water to reduce the dust further.

If the sanding of the surface does not allow the kiln wash to adhere to the saucer, you can heat it.  Soak it at about 125C for 15 minutes before removing it from the kiln to get the heat distributed throughout the ceramic body.  One advantage to the ceramic is that it holds the heat, because of its mass, for longer than steel.  Apply kiln wash with a brush or spray it onto the warm saucer.  As it dries, apply another layer of kiln wash.  Two or three applications should be enough to completely cover the surface.  If not, then you probably will need to heat up again before repeating the process.

Alternatives to plant pot saucers

There are alternatives to the saucer approach to getting thick circles from a pot melt.

 

Fibre paper

You can cut a circle from fibre paper and melt into that.  The advantage of fibre paper is that it requires little preparation other than cutting and fixing.  You may have only 3mm fibre paper and want a 9mm thick disc.  Simply fix the required number of layers together with the circle cut from each square.  The fixing can be as simple as sewing pins, copper wire, or high temperature wire.  Then place some kiln furniture on top of the surrounding fibre paper to keep it in place on the shelf during the melt.  This furniture can often be the supports for the melt.

Fibre board

If you find cutting multiple circles of the same size a nuisance, you can use fibre board.  Simply cut the circle from the board with a craft knife.  You will probably want to line the circle with fibre paper, as the cut edge of fibre board can be rough.  Alternatively, you can lightly sand the edge.  Wear a dust mask and do this outside, if possible, to keep the irritating fibres away from the studio. If you want a thicker melt than one layer of board can give, just add another in the same way as for fibre paper.

In both these cases, you may wish to put down a layer of 1mm fibre paper to ensure the glass does not stick to the shelf and does not require sandblasting.  

The advantage of the fibre paper or board alternative to flower pot saucers is that you do not need to kiln wash anything unless you want to. If you do not harden the fibre paper or board, it will not stick to the glass.

Vermiculite board

Another alternative is vermiculite board.  The advantage of this is that it comes in 25 and 50 mm thicknesses, so you can make the melt as thick as you like without having to add layers.  You can cut the vermiculite board with wood working tools.  Knives will not be strong enough to cut through the vermiculite board. You will need to kiln wash or line the vermiculite with fibre paper, as the board will stick to the glass without a separator.

Damless circles

Of course, if you want a circle without concern over the thickness, you can do the melt without any dams. You need to ensure that the shelf is level.  Any supports for the pot will need to be both kiln washed and far away enough that the moving glass does not touch the supports and distort the circle.  In general, one kilogramme of glass will give a 300mm circle, so your supports need to be further apart than the calculated diameter of the circle.  An undammed circle will vary from 6mm at the edge to as much as 12mm at the centre, depending on temperatures and lengths of soaks.

Pot Melt Temperature Effects

When firing a pot melt, you have to consider how high a temperature you wish to use.

Viscosity is reduced with higher temperatures so increasing the flow and reducing the length of soak, although there are often some undesirable opacifying effects.

The size of the hole is also relevant to the temp chosen. The smaller the hole, the higher the temperature will have to be to empty the pot in the same amount of time. Of course, you can just soak for longer at a lower temperature to achieve the desired object of emptying of the pot without changing the temperature.

Using the same principle, the larger the hole the lower the temperature required to empty the pot in a given amount of time.

The temperature used to empty the pot will need to be between 840C and 925C. The problem with temperatures in the 900C to 925C range is that the hot colours tend to change, e.g., red opal tends to turn dark and sometimes become brown. Some transparent glasses also opacify. There is also the possibility that some of the glasses will change their compatibility.

So the best results seem to come from temperatures in the 840 to 850C range with longer soaks than would be required at 925C - possibly 4 or more hours.

Also remember to give melts a longer than usual anneal as they will be thicker than 6mm at the centre - somtimes as much as twice the edge thickness.

Drop Rings

Mould

It is possible to purchase drop rings of various sizes. It is also easy to construct one from vermiculite board or ceramic fibre board. Merely cut a circle of the desired radius from the board. Leave at least 50mm of board outside the circle, and more for thinner boards.

Kiln wash the top and inner sides of the drop ring

Glass

The glass should be larger than the hole in the ring. This will vary by radius of the hole. The glass will need to be from 50mm larger diameter than the hole for smaller holes to 100mm larger diameter for holes over 300mm.

Glass should be at least 6mm thick for the first 100mm of drop and an additional 3mm for each 50mm more. So, a drop of 200mm would require glass of 12mm thick


Temperatures

The temperature rise should be no more than 150C per hour to about 675C for 6mm glass and less for thicker glass. Remember the glass is much closer to the elements than normal and it is easy to thermal shock the glass.

With close inspection you can see that the edge of the glass rises from the mould as it sinks in the middle.

The outside edges of the glass rise from the mould as the centre begins to drop in the centre.  As the glass gets hotter, this raised edge settles back on to the mould.  If the glass is really near the elements, there is a small risk the glass will touch the elements.  No harm will be done to the kiln, but the glass edge may have some needles.

The rate and amount of slumping is controlled by temperature, span (the width of unsupported glass on the mould) and time. The higher the temperature the faster a piece will slump and the thinner the walls will be. However you can slump at lower temperatures by holding the temperature for a longer time to reduce the thinning of the sides.

Also note that the wider the span, the faster the glass slumps.

If you slump at high temperatures with a drop ring the sides of the bowl tend to be straight and steep. The strain is limited to the region immediately inside the rim. Therefore the glass tends to thin next to the rim and the colours are diluted. If you slump at a lower temperature for a longer period of time the strain is distributed over the entire unsupported area. This results in a more rounded shape for the bowl and even thickness of the glass across the bottom of the bowl.


Experiment

Finding the right combination of time and temperature requires a bit of experience and guess work. If you want a rounded bottom, heat the glass to the point that it starts to bend on the mould and wait for 30 minutes. If it has slumped about 1 inch in that time wait another 30 minutes. You are looking for a slumping rate that is acceptable. If it hasn't moved very much then increase the temperature 15C and check again in 15 minutes. Keep moving temp up and waiting for 15 minutes until the piece has completely slumped. This might take several hours.

If you want straight sides keep heating the piece rapidly.

Stopping
When the piece has slumped to the desired shape, flash cool the kiln to about 30C above the annealing point to stop movement in the glass. Extend the annealing soak and increase the length of the annealing cool time (reduce the rate of temperature fall) over normal slump firings of the same thickness.

Glass falls through drop rings in relation to the size of the glass on the drop ring, the size of the opening, the temperature rise rate and to some extent the colours and amount of opalescent glass used. 

Melts, Apertures and Height Effects

The effects on the pattern of melts are a combination of several factors. The normal pattern is of spirals as a thread of glass moves down to the shelf and begins to spiral just as any other viscous fluid around the high spot of the drip.  The specific effects centre around three main elements.

Aperture size

The size of the holes determines the diameter of the thread of flowing glass.  Also, the larger the diameter, the quicker the flow.

relatively small apertures

large, long apertures

Height from shelf

The height from the shelf has the effect of determining both the thickness of the thread at touch down and the degree of spiralling.

Relatively low screen

Relatively high screen

Heat

The temperature and time determine the heat work.  The amount of heat (as well as top temperature) influence the flow of the glass.

These three elements interact

Aperture

Aperture size determines the maximum diameter of the thread.  You can thin the threads by having smaller grids or holes. 

The height affects two things. 

Height affects the relative thickness of the flowing thread. Higher makes for thinner strands. The reduction in size can be lessened by placing the apertures closer to the shelf.

Height also affects how the thread behaves on touching the shelf.  More spiralling occurs with height.  A low height will reduce the spiralling to just moving outwards.

Note that when talking about height, it is relative to the aperture sizes.

Heat affects how the glass flows

The higher the heat or the greater the heat work, the faster the glass flows.  Lower heat gives slow moving threads.  Faster flowing glass promotes thicker threads.  Slower moving threads can take up patterns other than spirals.

These factors give you three interacting elements

You could have, for example, a high screen with large openings and low heat to give thin threads with eccentric spiralling.

You could have low height with small apertures and high heat to give thick threads with minimum spiralling.

In theory, you could have at least twelve main combinations by using the extremes of each element, with multiple variations of dimensions in each case.

Experimentation Required

This is to illustrate the interactions are complex and require significant experimentation to be able to predict the probable outcome.  The outcomes will always be only probable, even though you can come to control more aspects of the process and you develop experience.

Glass on Drop Rings

When glass drops through a ring, you need to check on some things relating to the placement and firing.

When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.

Glass behaviour

The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff.  The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring.  This is the source of a lot of the stretch marks at the shoulder of drops.

Rim width

To avoid the glass dropping through, you need to have an adequately sized rim.  The width of the rim sitting on the ring, needs to be related to the size of the hole.  

The consequence of an inadequate rim

I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim.  The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter.  For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole.  I do not have the experience to say how much more is required for larger diameter drop rings.  There is more discussion on blank sizes here. 

Heat

The rate at which you heat the glass and the top temperature both have effects on the possible drop through.  

High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities.  It also promotes excessive thinning below the shoulder. 

Fast rates. The surface will become hotter than the bottom, but at different rates.  The glass over the hole is heating from both top and (to a lesser extent) bottom.  The rim is sitting on the ring and so heats only from the top.  The differential in heat may cause a break.

Weight. The thickness of the glass effects when the drop will begin.  The heavier the glass and larger the hole, the effective weight will be greater.  In these cases, you can use a lower temperature for the drop.

Additional methods.  You can use other methods to reduce the chance of a drop through.  Two of them are:

Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop.  These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter.  More would be required for larger apertures.  Of course, these will mark the rim, meaning that it must be cut off.

Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.

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.