Colour Dilution of Powders

Sometimes you do not have a tone or shade of a colour you need for your project.  Other times you want to have a gradation of shade across a piece.  There is the obvious solution of mixing a colour with clear to produce lighter shades.  But there is a difficulty when mixing clear with powders to fuse. The result is often a pointillist effect with points of light coming through the colour. There are several approaches to this difficulty.

One way is to use a powder made from a tint of the colour.  But sometimes there is not a tint made. Sometimes you do not have that tint in stock. So, you must look to other solutions.

Credit: www.warm-glass.co.uk

An alternative is to use clear powder to mix with the intense colour you want to dilute.  You will need to test varying proportions of clear to colour to get the tone you need.  You may be surprised at the amount of clear needed.  And there still is the slight possibility of points of light coming through the clear.

Another possibility is to use one of the less dense white powders to mix with the colour.  White powders such as the Bullseye 000243, translucent white, or the 000113, dense white are possible.  The very dense or lacy whites are not as suitable. One is too opaque, the other is uneven in colour. Again, testing will be required, and you may be surprised at how little is required to alter the tone.

One other way I have used is to mix fine frit with the powder.  This has less control than the other methods but can provide significant dilution of the intense colours.  If you want to see if this is suitable, you can follow this process. 

Add a few drops of water to the clear frit in a small container. Close it and shake to get all the frit coated with a film of water. If after shaking the frit is not “clumping” you can add a little more. Too much water will create a slurry which is not suitable.  So, add only a small amount of water at a time until the frit is like damp sand on the beach. Any excess water must be poured off. 

Add powder to the damp frit, and shake well again to coat the frit with powder. If the frit does not seem to be fully coated, add a little more powder.  The film of water on the frit allows the powder to adhere temporarily to the frit.  

This mixture can then be applied to the surface and smoothed with a pallet knife. This will not guarantee there are no clear pinpoints, but it will reduce them to a minimum. You will not have the subtle differences in tone that sifting powder can give you, but it is a cost-effective way of diluting intense powder colours that can have advantages over mixing powders.

Of course, the various methods of diluting colour described here can be used to combine powders to produce new colours.

Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

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

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.

Firing Records

Bullseye Glass Company

To develop your fused glass practice, you need to record lots of information about your firings.  This tells you what has gone well and not so well.  It hones your expectations about how you should be preparing, scheduling, and analysing your experiences.  It becomes your detailed memory bank of results and gives directions for the future.  This should be done whether fired in your own kiln or someone else’s.

Categories of information for the record
There is quite a bit of information that needs to be included in such a record.  This is my view of what needs to be included  in your logbook for future reference.

Date
Record the date of the firing as that will give you historical information on similar projects.  It can show you what you have changed over time and the variations you have introduced.

Glass used
This is not only the type of glass (Bullseye, Float, Oceanside, Wissmach, Youghiogheny, etc), but the colours used.  This should include the manufacturer’s code numbers to enable you to replicate the glass used.

Lay up
This can be a description, a drawing or pictures of the set-up of the piece prior to firing.  This is vital to later understanding what you did in this firing.  Record any glues or stabilising elements you use. Any frits or powders used should be recorded. The placement in the kiln is important - centred, one corner or another, level/ height in kiln, etc., can affect the results.  You can make a sketch or take a photo to attach to the record rather than writing separate descriptons. How it comes out is recorded later.

Dimensions
The dimensions (h x w x d) including any variations in height are needed to compare with other projects.  This might be included in the lay-up diagrams or pictures, but it is most useful to have the dimensions and their variations recorded as numbers too.  You might think in terms of layers, but remember to record the thickness of each layer/piece (e.g., 2mm, 3mm, 4mm, 6mm, etc)

Kiln used
This is especially important if the kiln is not yours. Every kiln has variations and it is important to compensate for that in scheduling and placing of the piece in the kiln.

Process
This is essential in gaining an understanding for planning any modifications.  The process can be described by standard terms - e.g.,  sinter, slump, tack, contour, full fuse, casting, melt – or by your own terminology (if it is consistent).

Description
A statement of your project and aims is very useful for the future.  It is a reference point to use in comparing what you wanted with the results of the firing.

Support system
This includes essential information affecting the firing – shelf type (e.g., fibre, mullite, ceramic tile), mould type (e.g., ceramic, fibre, steel), and a description or sketch including any reference codes.

Kiln furniture. The kind and quantity of kiln furniture (dams, stilts, posts, etc) can affect the firing results, so need to be recorded.

Separators
This includes kiln wash (type, whether new or the number of uses), fibre paper type and amount, mould coatings, and anything else you may use to keep the glass from sticking.

Schedule
This is the thing most everyone remembers to record.  You need to record it each time you use it – even if you have used it many times before.  You need to record each step of the program.

So many times, people report that “it [the schedule] has always worked before”, only to discover that some element had been intentionally or accidentally altered from past firings.  I normally write the schedule in a logbook and then enter it into the programmer. I use the written record to check against what I have entered into the controller.  Then I know I have programmed what I intended.  I can also check on earlier, similar firings to see the variations I have used in the past.

Results
Drawings or pictures of the finished item are essential.  A description of the results is also needed as a picture does not tell the whole story.

Comments on results
You should also give a commentary on the results of the firing.  This should include successes as well as disappointments.  Thoughts for future similar firings should be written down.  They will be forgotten soon, if you don’t.

How to keep all this information
As you can see there are many elements that need to be recorded as they each can affect a firing. I see these as a minimum, and you will add elements important to you for this list.

It does not matter much in what form you keep the information.  It can be a ledger, spreadsheet, database or your phone or tablet that you carry with you always.  There are several apps for recording the kiln firings that can be used.  What is important is that you can record the information immediately, or as you prepare the work for the kiln, into the chosen form of recording.  I use a logbook and convert that in my leisure moments to a spreadsheet (usually at new years day).  This allows me to compare information over time and especially the kinds of firings that I rarely do.  It also allows me to search by various processes.

It is important that you back up any electronically held information to the cloud or other device to protect against loss or corruption. 

Forms
It is useful to have a form for compiling this record.  A number of elements of the records can be reduced to tick boxes to ease the recording.  It helps to remind you of the information you need to log for each firing.  Bullseye have an excellent form that you can use or adapt to your needs. There are a few apps that can be used on phones or tablets which are useful for those who record everything on their phone.  Remember to back it all up to the cloud for preservation in case of loss or damage.

Recovering from Devitrification

An explanation of what devitrification is, can be found in the link.

Mild devitrification is generally a smeary appearance on the surface.  Most often this can be corrected by either removing the surface, adding a flux or putting another surface over the piece.

mild devitrification
photo credit: Bullseye Glass Co.

Removing the devitrified surface

Sandblasting and grinding are two common methods of removing the surface. If you have access to a sandblaster, this is the easiest method of removing the surface.  You can remove the surface with manual methods too.  You can use wet and dry sandpapers, starting with coarse ones and proceed through grades to at least 400grit (0.037mm).  The flexibility of the sandpapers is that they can conform to uneven surfaces that tack fusing provides, to remove devitrification in depressions as well as the high bits. Diamond hand pads and sheets do the job more quickly, but are more expensive.

Acid etching is another surface removal method. There are various etching creams on the market which will remove the surface. You need to apply and leave for a long time to allow the acid to work on the glass surface.  It is best to keep the acid paste damp to enable the acid to work over a long period.  A piece of cling film will work well.

Making a new surface

You can provide a new surface by using devitrification sprays.  There are both commercial products and do it yourself ones that work.  The do it yourself product is a borax solution.  The method for making the solution is given here.

Borax powder

You also can give the devitrified surface a new one by covering it with clear powders.  Powders sifted evenly over the surface until there is a thin covering over all the piece will give a new surface concealing or covering the devitrification.  Fine frit does not work so well, as more needs to be sifted over the surface.  This will not be applicable to tack fused pieces, as the whole piece needs to be taken to a contour or full fuse to make sure the powder or frit is completely smooth.  This will make the tack fused areas flat.

Left to right - devitrified surface, powder covering, fired piece
Photo credit: Bullseye Glass Co.

When dealing with devitrification, the whole of the surface should be treated, not just isolated areas.  Treating isolated areas will most probably leave a difference in appearance between the treated and untreated areas.  It is not worth the risk of having to fire yet again.

Dealing with devitrification usually involves removing the devitrified surface or making a new one.

Annealing Bullseye and Oceanside Together

Credit: Bullseye FAQ_kilnforming_annealing

The question sometimes arises as to whether Bullseye and Oceanside can be annealed in the same firing, since the two glasses cannot be combined in the same piece.  They also have different published annealing soak temperatures (also known as the annealing point).  The explanation requires some knowledge of annealing.

Annealing can be done at other than the annealing point. This is because annealing can be done over a range rather than being a single magic figure. Bullseye did not change their glass when they altered the recommended anneal temperature.  This means that the annealing point is still at 516°C. Their research has shown that good annealing results are obtained by doing the temperature equalisation soak at the lower end of the range.  Temperature equalisation throughout the piece is what happens during the annealing soak. Therefore, it is a descriptive term for what happens at the annealing temperature.

Bullseye's previous annealing temperature was 516°C and Spectrum's was/is 510°C. These are very close, and in the past, many chose to anneal at either - or in most cases, both - of these temperatures. Bullseye's research has shown doing the temperature equalisation at the lower end of the annealing range provides good results and ones that are more reliable than the higher temperature.  This research is applicable to all soda lime glasses, not just Bullseye. Therefore, the same principles can be applied to Oceanside fusing compatible glass, or any other fusing compatible glass. This further indicates that you can anneal both Bullseye and Oceanside fusing compatible glasses at the same temperature. 

Further support to this view of the possibility of annealing the two glasses at the same time and temperature is given by Wissmach.  Wissmach W90 and W96 now are both given the annealing temperature soak as 482°C.  Previously they both had been at 510°C.

If you feel the need to compensate for the annealing point differences, you can increase the 482°C for Bullseye by 6°C to 488°C for both. Although I don't think it is necessary, 488°C will be fine for Bullseye and safe for Oceanside.

Float annealing

As a result of various memory failures, I've done a bit of searching on the annealing of float glass.  There are now various compositions of float glass and with different coatings for various applications.

This leads to a variety of annealing points for Pilkington float glasses. The search led to various hard to find documents, which indicate a range of annealing temperatures between 548°C and 559°C. This is not a huge range, so anywhere between 548°C and 560°C can be taken as the annealing point. Pilkington indicate that optifloat has an annealing point of 548°C

The strain point seems to be mostly between 525°C and 530°C for all the varieties.  This indicates the temperature equalisation soak should not be less than 535°C.

The conclusion seems to be that annealing should have a temperature equalisation soak between 550°C and 535°C. It will not matter much where you choose, but remember that the closer to the strain point you do the temperature equalisation, the longer the soak should be.  The length of soak at 535°C can be determined by use of the Bullseye chart for Annealing Thick Slabs. This gives the times and rates for the anneal cooling of glass by thickness.  The temperatures need to be changed, but otherwise the information can be applied.

The softening point seems to be 725°C for all the glasses. This is a good low temperature for slumping.

Mould repairs with ciment fondue

Ciment Fondue

Ciment fondue was a French discovery and so the French name has become common in Europe. The name ciment fondu is used for the formal name Calcium aluminate cement which is also called high alumina cement and aluminous cement.  It is composed mainly of Aluminium oxide (alumina) and calcium oxide (quicklime) with varying amounts of ferric oxide. The Aluminium Oxide varies from 40% to 80% for various applications. The calcium oxide content varies from 40% to 20% and the ferric oxide varies from 16% to none for refractory applications.  For kilnformers, the general purpose composition of 40% aluminium oxide, 40% calcium oxide and 16% ferric oxide is sufficient (the rest is made up of minor amounts of incidental minerals and metals).

It is costly in relation to Portland cement and is used mainly where quick curing strength is required and at low temperatures; in refractory concretes where strength at high temperatures is needed; and in sewer piping and other applications to provide protection against biological attack of the concrete.

It is also used in sculptural applications, both as the casting material, and as a strengthening element in a non-metallic structure.

It is mixed with water to form a paste.  The proportions are not required to be exact, as the ciment fondu separates out of the water due to its weight and very low water absorption.  Slightly different methods are needed to repair breaks, and to fill divots in the surface.

Breaks

To repair breaks or cracks in ceramic moulds the ciment fondu needs to be used on its own.  Mix the dry particles with water until a stiff slurry is formed.  Thoroughly wet the edges of the broken pieces or the cracked area.  Then apply the ciment fondu slurry to both edges.  Press the pieces together and bind them if they would otherwise separate.  This can be with elastic bands or tape or any material that will withstand moisture.

The internal surface must have all the ciment fondue cleaned from it.  It cures so hard that it is not practical to sand it smooth without damaging the ceramic surface.  This clean up can be with a lot of water and paper towels. Any tools you use need to be immediately cleaned with water.  Do not dispose of this clean up water down your drains. It will harden and narrow your drains, potentially blocking them so firmly that whole sections of the drain will need to be replaced.

When fixed together put the mould in plastic or other waterproof material for at least 24 hours to give a wet cure.  The ciment fondu is not completely cured until it is given a heat cure.  This should be above the expected operating temperature.  Although I have never fired any of my ceramic moulds above 680°C, I fire my repairs to 800°C.  The firing is smelly, so ventilate the kiln and room well.  Try to do the heat curing when the smell will not disturb you or your neighbours.

Divots

This mould had glass stuck to it and was damaged in removing the glass.

If there are scratches or divots in the mould surface, you need to add some material that will absorb water into the ciment fondue mix.  Cured ciment fondue rejects water and so does not get as well coated as the rest of the mould when kiln wash is applied.  

To prevent this rejection of water, I add finely ground vermiculite to the mix.  I use 3 parts or less vermiculite to 1 part ciment fondue (measured by volume).  This provides a firm surface that absorbs some water. Although the absorption of moisture is not as good as the ceramic, it is sufficient to get the kiln wash coverage required.

Once the mix is prepared, you need to thoroughly wet the area to be fixed. This prevents the ceramic absorbing the water from the ciment fondue too quickly. Apply the ciment fondue mix with whatever tools seem appropriate.  

You must smooth the applied mixture before it dries, as it is so hard when cured that it is not possible to sand it smooth without damaging the ceramic surrounding the repair.  Smoothing can be done with significant amounts of water and a smoothing tool such as a ceramicist’s kidney or a palette knife. 

Once smoothed to achieve the surface required, pour off the excess water.  Enclose the mould in a plastic bag for 24 hours for a wet cure. Once out of the bag and dry you can further smooth with very fine sandpaper.

Then fire to 700°C to 800°C to complete the cure.  When cool it is ready to kiln wash.  If you warm the mould to around 100°C, the kiln wash will adhere to the repaired areas a little better than the cold mould.  Once the first kiln washing of the mould is complete, further applications of kiln wash will be easier. Of course, if you use boron nitride to coat the mould, there will be no difficulty with the repaired areas.

All tools need to be cleaned immediately of the ciment fondue and the cleaning water disposed of on the garden or waste ground.  It should never be put down domestic or public drains.  It does no harm to the soil or plants, but it will certainly harm your plumbing.

Clumping Kiln Wash

There are some reports of properly prepared kiln wash (1 part powder to 5 parts water by volume) clumping or going onto the shelf or mould unevenly.

My experience is that this happens on shelves that have been kiln washed and fired several times.  The dry kiln wash that has already been fired absorbs the water quickly leaving unevenly applied kiln wash.  The water is absorbed so quickly that it leaves unevenly distributed kiln wash over the existing, already fired kiln wash.

The immediate response of diluting the kiln wash even further leads to a lot of water being absorbed into the shelf leading to longer air-drying times.  It also risks getting insufficient kiln wash over the existing kiln wash. This risks the kiln wash sticking to the fired glass, which is the opposite of the intention of using fresh separator.

When the new kiln wash solution begins to clump, it is time to stop adding more over the top of the old.  It is time to remove the old, clean the shelf and start with a new smooth kiln washed shelf.  It does not take long and gives the satisfaction on knowing the bottom of your pieces will be flat.

Applying new kiln wash repeatedly over old leads to uneven application and clumping of the new.

Consignment Rates

The most common comments about the rates for consignment of pieces to a gallery or gift shop are that they are not fair. They are too high. The gallery is greedy. And so on. How do you judge whether the commission rates are fair?  What are the factors that should be considered?

Time

How much is your time worth?  

Think about the amount of time used to prepare, promote and attend craft fairs, pop up shops, or prepare for and administer online selling. Could you be using that time to make more things, or be with your family?  How much would it improve your quality of life to have to do less selling?

Costs

What are the costs of attending craft fairs?  

    You have to acquire display materials, whether you make or buy them.  You must travel to the event.  You have to be prepared to accept breakage risks from repeated movement of the pieces.  You must pay for the space at the craft fair.

Customer base

Is the shop’s market different than yours at craft fairs or online marketplaces?  

    Shops have a different clientele than craft fairs or online shops.  They spend effort in attracting customers.  They know their clientele and what kind of things will sell to them.  They are aware of the pricing levels needed for their visitors.

Decision

Answering these questions about time, costs and customer base will give you an assessment of whether consignment commission rates being offered are fair. 

Other posts on consignment:

Consignment

Consignment Agreements

Consignment Rates

Consignment Venues

Creating Flat Bottoms by Hand

No jokes please!

Often the moulds we use do not have a suitably flat bottom to them, making the resulting item wobble when set on a flat surface.  There are several ways to create a flat spot in the mould, reaching in to re-set the glass while firing, putting the glass in at a complimentary angle for a second firing - but they are not always successful.  

Of course, if you have the money you can use a flat lap or a linisher with a back plate to grind a flat spot on these bowls and other unstable pieces.

But,

You can still make a flat spot on your piece without machine tools.  Use a piece of float glass larger than your piece as your grinding base.  Put a slurry of 100 grit sand on the base and put your piece over.  Holding it level, make circular motions with firm downward pressure.  In only a few minutes you will have produced a large enough flat spot to stabilise your piece.

If you do not like the mess of the slurry, fasten a 100-grit sandpaper onto float glass, add water and do the same as you would with a slurry of grit.

Annealing at the Lower End of the Range

Annealing can be done at other than the defined glass transition temperature - also known as the annealing point. Annealing occurs over a range rather than a single magic temperature. Bullseye did not change their glass when they altered the recommended annealing temperature.  Their research has shown that good results are obtained by annealing at the lower end of the range.  

A graph of some aspects of a specific and stiff soda lime glass illustrates this.

Annealing can be between the glass transition (annealing) point and the strain point
credit: Lehigh University

Bullseye's previous annealing temperature was 516C and Spectrum's was/is 510C. These are very close. Bullseye's research is applicable to all soda lime glasses. Therefore, the same principles can be applied to Oceanside fusing compatible glass.  It has already been applied to the Wissmach fusing lines.  This means that you can anneal both glasses at the same temperature.  If you feel the need, you can increase the 482C by 6C to 488 for both, but I don't think it is necessary.

The purpose of the annealing soak is to equalise the temperature within the glass to vary less than 5°C (i.e., +/- 2.5C).  If this is done at the lower end of the annealing range, there is less difficulty of maintaining that small difference throughout the cooling stages. 

Sticking Fiber Paper

People are reporting different behaviours of their thicker fibre papers.  Mainly the difference is that small fibres stick to the glass after a full fuse or kiln carving firing.  There also seems to be a different smell from the burning binders.  This is most likely to be a body soluble refractory fibre paper that is being used.

It seems more suppliers are selling the body soluble versions of fibre paper. It sticks and it gives off a smell of volatile chemicals. I don't like it, but I may have to use it due to the unavailability of that more health risky stuff that worked very well.

There are several ways to minimise the fibres sticking to the glass.  They all relate to adding a separate coating of separator to the fibre paper before firing.  Among the coatings that can be used are kiln wash brushed on or powder dusted over, alumina hydrate, and boron nitride (Zyp is one brand name).  Some cut out Thinfire or Papyros to the shapes required.  (It seems to me that a single sheet of either of these placed over the whole area would do the job, as they turn mostly to dust upon firing).

Others have found that simply soaking in water overnight allows the fibres to be brushed off with stiff brushes.

Body soluble refractory fibre papers tend to stick to the glass at anything over low temperature tack fuses.  This requires an additional layer of separator to be applied over the paper.

It is each person’s choice, of course, but I will continue to attempt to get the older version of fibre paper.

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