Showing posts with label Casting. Show all posts
Showing posts with label Casting. Show all posts

Wednesday 25 October 2023

Spikes on Frit Castings

Credit: The Crucible.com


It is frequent to have castings from frit with spikes, needles, or prickles around the edges. 

Causes

These spikes result from the glass touching the edge of the mould or separator during the hottest part of the firing. The glass particles first begin to compact as the glass rises toward the fusing temperatures. As the temperature increases toward the casting temperature it begins to expand both horizontally and vertically from that compact mass. As it cools, the glass sinks down and retreats from the edge. This movement leaves some small bits of glass stuck to the sides. The glass contracts as it cools, leaving the spikes as it contracts from its hottest state. 

Avoidance

The usual recommendation is to mound frit in the middle and let it flow to the outside. Still, the glass flows to the outside of the mould at casting temperature and it touches the sides. Leaving the risk of creating spikes. Accurate measuring of the amount of glass to charge the mould with is important. With the right amount of glass, the mould will not be overfilled and so, reduce the spiking. 

Measuring the weight of glass for the mould is not difficult. In many cases, the manufacturer of the mould has done the work for you. If you need to calculate the weight of glass required for the mould, it is not difficult. A method is given hereIn short, you use a dry fill of the mould. Measure the volume (using the metric system) and multiply by the specific gravity to get the weight in grams. 

Larger chunks of glass tend to produce fewer spikes than smaller frit. Usually longer soaks at top temperature are required to fully form the glass with smaller frit. It is also possible to drip glass into the mould from a pot suspended above the mould. Accurate measurement of the weight will still be important. But add 100gms/4oz. to the amount to allow for the glass that will stick to the pot.

My view is that with dams, it is better to use a straight sided shape with fibre cushioning around the outside. When annealed and cool, clean it well. Then fire polish with a slow ramp to 540°C/1000°F followed by a quick ramp to the fire polish temperature. This will polish the sides of the piece that were in contact with fibre paper.

Wednesday 7 December 2022

Fire Polishing of Frit Castings


Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

  • ·        They are relevant to particular kilns. 
  • ·        They are also dependent on the ramp rate. 
  • ·        The presence or absence of a bubble squeeze is important. 
  • ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

Wednesday 21 October 2020

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

Wednesday 29 January 2020

Amount of Fill for a Frit Mould



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

Calculation of the weight of glass needed

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

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

Example of a small frit mould


Irregular shaped moulds

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

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

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

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

Sunday 11 August 2019

Specific Gravity

This is an important concept in calculating the amount of glass needed to fill a pot melt, and in glass casting.  This will also help in the calculation of the amount of glass required to fill a given area to a defined thickness.

Specific gravity is the relative weight of a substance compared to water. For example, a cubic centimetre of water weighs 1 gram. A cubic centimetre of soda lime glass (includes most window and art glass) weighs approximately 2.5 grams. Therefore, the specific gravity of these types of glass is 2.5.  

If you use the imperial system of measurement the calculations are more difficult, so converting to cubic centimetres and grams makes the calculations easier. You can convert the results back to imperial weights at the end of the process if that is easier for you to deal with.

Irregular shapes

Water fill method
Specific gravity is a very useful concept for glass casting to determine how much glass is needed to fill an irregularly shaped mould. If the mould holds 100 grams of water then it will require 100 grams times the specific gravity of glass which equals 250 grams of glass to fill the mould.

Dry fill method
If filling the mould with water isn't practical (many moulds will absorb the water) then any material for which the specific gravity is known can be used. It should not contain a lot of air, meaning fine grains are required. You weigh the result and divide that by the difference of the specific gravity of the material divided by 2.5 (the specific gravity of soda lime glass). 

This means that if the s.g. of the mould filling material is 3.5, you divide that by 2.5 resulting in a relation of 1.4   Use this number to divide the weight of the fill to get the amount of glass required to fill the mould.   If the specific gravity of the filler is less than water, then the same process is applied.  if the specific gravity of the filler is 2, divide that by 2.5 and use the resulting 0.8 to divide the weight of the filler.  This only works in metric measurements.

Alternatively, when using the dry fill method, you can carefully measure the volume of the material.  Be careful to avoid compacting the dry material as that will reduce the volume.  Measure the volume in cubic centimetres.  Multiply the cc by the specific gravity of 2.5 for fusing glasses.  This will give the weight in grams required to fill the mould.  If you compact the measured material, you will underfill the mould. The smaller volume gives a calculation for less weight.


Regular shapes

If you want to determine how much glass is required for a circle or rectangle, use measurements in centimetres.  

Rectangles
An example is a square of 20cm.  Find the area (20*20 =) 400 square cm. If you want the final piece to be 6mm thick, multiply 400 by 0.6cm to get 240 cubic centimetres, which is the same as 240 grams. Multiply this weight by 2.5 to get 600gms required to fill the area to a depth of 6mm.

Circles
For circles you find the area by multiplying the radius times itself, giving you the radius squared.  You multiply this by the constant 3.14 to give you the area.  The depth in centimetres times the area times the specific gravity gives you the weight of glass needed.

The formula is radius squared times 3.14 times depth times specific gravity.   R*R*3.14*Depth*2.5
E.g. 25cm diameter circle:
Radius: 12.5, radius squared = 156.25 
Area: 156.25 * 3.14 = 490.625 square cm.
Volume: 490.625 * 0.6 cm deep =294.375 cubic cm.
Weight: 294.375* 2.5 (s.g.) = 735.9375 gms of glass required.  
You can round this up to 740 gms for ease of weighing the glass.

Wednesday 26 September 2018

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C


These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.


In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.


This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

Thursday 13 July 2017

Quartz Inversions and Conversions

You need to know about this in both casting and when using ceramic pots in the kiln.

Quartz
Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.

Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.

Inversions
Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.




Quartz
The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.




Crystobalite
The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.

Wednesday 28 September 2016

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.

Wednesday 18 November 2015

The 6mm Rule - Kiln Forming Myths 11

Glass always wants to be 6mm thick


This is true only at some temperatures.  

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

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

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

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

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

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

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

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

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

Wednesday 16 April 2014

Making Billets





One of the uses of cullet (small pieces of glass) is in casting. However, simply placing the glass into a mould and firing, leaves many bubbles and often shows the edges of the original pieces of glass. Billets (ingots of glass) are more useful because they have fewer of the small bubbles and fewer edges than cullet.

It is possible to make your own billets. This can be done in a fashion similar to pot melts, although the temperature does not have to be so high. And the results are easy to store, if the dimensions are kept regular.


You need to have a mould for the melting glass to be contained within. These moulds can be made from plaster. A simple way is to use old margarine tubs placed upside down and fastened to the base within a dammed area. Pour the plaster of paris over the tubs to make the moulds. An alternative is to use strips of refractory material (fibre board or cut up kiln shelves) surrounded by heavy bricks to stop any movement due to the weight of the glass.



The glass to be formed is put into ceramic flower pots and can be directly onto the plaster of paris or dammed areas. You should put at least one piece of glass to cover the hole at the bottom of the pot. All this glass must be clean. Calculate the amount of glass required by determining the volume of the containment area (in cubic centimetres) and multiply by the specific gravity to give the number of grams required.



Don't get too ambitious about size, as these billets need to be fitted into the mould reservoir for filling the mould. A small margarine tub is approximately 12 cm wide, 7 cm deep and 7 cm high. This is as large as required, and smaller may be better. If you are making your own from dams, something like 4 cm by 8cm by 2cm may be better. This size is convenient for filling a reservoir, and has the advantage of being able to compare the intensity of colour the different thicknesses will give to the casting.


Remember that the thicker you make the billets, the longer you have to anneal. So the annealing time of the billet may be the factor that determines time. A 2 cm billet will take at least 9 hours of annealing time; one of 4 cm will take 28 hours of annealing.


When setting up the kiln for making the billets, remember that in general the higher the reservoir above the billet mould, the fewer bubbles you will get in the billet, although you are confined by the height of the kiln. Although there still will be some bubbles, these will further reduce by the second flow of the glass during the casting process.


To fire the set up, you can advance the temperature rapidly to 650/670ºC with a long soak there (possibly 3 hours). The final temperature can be below pot melt temperatures, so a casting temperature of 830ºC with a long soak (possibly 6 hours) will be sufficient. Take note of your final thickness – including any containment material – to determine the annealing soak and schedule.


Sunday 15 December 2013

Pot Melts – Weight of Glass Required

Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.

50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass

Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.

Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.

For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.


Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).  

If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres.  As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.

If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.

To make a 1 cm slab of the same size you need 250 grams of glass.

To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).

To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.

A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.

Wednesday 18 September 2013

Bubble Reduction in Casting


There are several things that can be done to reduce the number and size of bubbles in casting.

  • Fire higher - to 830ºC instead of 815ºC - and soak for at least four hours. This allows more bubbles to rise to the top and burst. If there are still more bubbles than wanted, increase the soak time.

  • Stack the glass in the centre of the mould, allowing a few centimetres from the mould walls. This allows the glass to spread and flow from the bottom and up the sides, reducing the likelihood of trapping air. If you have more than one stack, keep the same space between the stacks as the mould walls.

  • Make sure that the way you stack the billets or casting plates so there is a smaller space at the bottom of any cavity than at the top. The reverse allows the glass to soften and seal in the air in the space.

  • You can construct a mould to make billets of the general shape of the final object. This of course, is much more work, needing two moulds.

  • A major thing to avoid is the use of frit, especially at the bottom or deep in the mould as bubbles will collect around each piece and lead to a multiplicity of bubbles throughout the casting.

Sunday 30 September 2012

Freeze and Fuse


The object of this technique is to make a shaped piece without use of a refractory mould. It is applicable to small items.

You can use jelly, soap, candle, etc. moulds. They can be rigid or flexible. They should be without undercuts and have a draft, which is why jelly, soap, and candle moulds are so suitable. Be careful of the size, as a large amount of frit can be required even for a small mould.

Some people use only powder for this process. I use a 50/50 combination of powder and fine frit. You can use clear frit with powder. If you do so, you need to measure out the appropriate amounts.  Then put the frit and some water into a container with a closure. Close and shake to wet the frit. Then add the powder and shake again to ensure the powder adheres to the frit. Once thoroughly mixed, add more water to make a thick slurry.

Pack the mould with the mixture. Then using absorbent paper towels firmly pat the contents of the mould as dry as you can. Place the mould in the freezer for at least a couple of hours, or for large ones overnight.

When frozen, remove from the mould and place on the kiln shelf. The shelf needs a separator which can be kiln wash or fibre paper. Some leave the piece to thaw out and some more of the water to evaporate.

Whether you fire immediately or let the piece thaw and evaporate, you need to fire slowly to 100C and soak there until no more moisture is evident to avoid creating pockets of steam that will blow the piece apart. After that you can fire as normal for an initial firing of a two layer piece.

The piece will shrink a bit during the firing, but it is safe to anneal for the original thickness of the frozen piece. If you have a large piece or one with lots of variation in thickness, you should use at least the next thickness up from the Bullseye tables for annealing thick pieces. Sometimes you should use two steps up.

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

Sunday 25 March 2012

Home Made Billets

You do not always have to buy cullet for casting or billets - you can make your own. Billets lead to less veiling and bubbles than just putting in your old cullet into the mould.
Create a mould by using dams or pouring investment materials around something like a plastic salad tub to make a billet – the shape is not usually critical. Place a reservoir such as a terracotta flower post above. Take the temperature to the 650C – 670C region for a 1- 2 hour soak followed by a long soak at 830C.
Normally, the higher you allow the glass fall, the fewer bubbles, but you're usually limited as to how high you can go in the average glass fusing kiln. You'll get some bubbles, but if you then put your new billet as a single piece in a reservoir for your casting you'll get the second flow that removes more of the bubbles.

All the glass must be thoroughly clean before being put into the pot for making the billet. Do not use iridised glass as it reduces the clarity of the billet. Do not use glass that has been ground as that will cause hazing in the billet. Instead, cut off the ground sides before washing the remainder and including in the melting process. Do not include the ground off-cuts.

Based on information from Cynthia Morgan (Morganica)

More information here

Tuesday 2 March 2010

Effect of Plaster-Water Ratio on Some Properties

Plaster-water ratio (by weight) 100/30

Setting time (min) 1.75

Compression strength (kg/sq.cm) 808
Dry Density (kg/cu metre) 1806

Plaster-water ratio (by weight) 100/40

Setting time (min) 3.25

Compression strength (kg/sq.cm)474
Dry Density (kg/cu metre) 1548

Plaster-water ratio (by weight) 100/50

Setting time (min) 5.25
Compression strength (kg/sq.cm)316
Dry Density (kg/cu metre) 1352

Plaster-water ratio (by weight) 100/60

Setting time (min) 7.24

Compression strength (kg/sq.cm)228
Dry Density (kg/cu metre) 1206

Plaster-water ratio (by weight) 100/70

Setting time (min) 8.25

Compression strength (kg/sq.cm)175
Dry Density (kg/cu metre) 1083

Plaster-water ratio (by weight) 100/80

Setting time (min) 10.50
Compression strength (kg/sq.cm)126
Dry Density (kg/cu metre) 990

Plaster-water ratio (by weight) 100/90

Setting time (min) 12.00
Compression strength (kg/sq.cm)98
Dry Density (kg/cu metre) 908

Plaster-water ratio (by weight) 100/100

Setting time (min) 13.75

Compression strength (kg/sq.cm) 70
Dry Density (kg/cu metre) 867


This table of relationships makes it clear that the less weight of water added to the plaster, the stronger the resulting mould will be. It also is clear that with less water, the setting time is reduced. So some compromise may be needed to be able to pour the mixture before it sets.

Saturday 27 February 2010

Properties of typical gypsum plasters and cements

Number 1 Pottery Plaster
% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1105 kg/cubic metre
Expansion on setting – 0.21%
Compressive strength - 126 kg./square centimeter

No. 1 Casting plaster% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1058 kg/cubic metre
Expansion on setting – 0.2%
Compressive strength - 140 kg./square centimeter

Plaster of Paris% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1105 kg/cubic metre
Expansion on setting – 0.2%
Compressive strength - 140 kg./square centimeter

Number 1 Casting Plaster% of water to dry mix by weight - 65%
Set Time – 27 – 37 mins
Dry density – 1162 kg/cubic metre
Expansion on setting – 0.22%
Compressive strength - 168 kg./square centimeter

Pottery Plaster
% of water to dry mix by weight - 74%
Set Time – 27 – 37 mins
Dry density – 1162 kg/cubic metre
Expansion on setting – 0.19%
Compressive strength - 126 kg./square centimeter

Hydrocal Cement
% of water to dry mix by weight - 45%
Set Time – 25 – 35 mins
Dry density – 1442 kg/cubic metre
Expansion on setting – 0.39%
Compressive strength – 35 kg./square centimeter

Hydroperm Cement% of water to dry mix by weight - 10%
Set Time – 12 -19 mins
Dry density – 
<641 br="" cubic="" kg="" metre="">Expansion on setting – 0.14%
Compressive strength – 35 kg./square centimeter

Hydro-Stone cement
% of water to dry mix by weight - 32%
Set Time – 17 -20 mins
Dry density – 1913 kg/cubic metre
Expansion on setting – 0.24%
Compressive strength – 703 kg./square centimeter

Ultracal cement
% of water to dry mix by weight - 38%
Set Time – 25 - 35 mins
Dry density – 1568 kg/cubic metre
Expansion on setting – 0.08%
Compressive strength – 421 kg./square centimete
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