Thickness of Powder Application

 

Why does my powder disappear when I fire?

 

Powder may appear to disappear after firing as Donna Brown found out with the pieces of her work shown here. Glass powder is finely ground glass sheet. The full colour of glass sheet is seen only when the glass is 3mm thick. So, to get the same intensity of colour you need to have the powder nearly 3mm thick.

This image shows the powder application before firing.
Picture credit: Donna Brown

There is not enough powder applied to the honeycomb. Everyone needs to run some tests to see how much powder is needed for strong colour. By running some tests of different thicknesses of powder you will be able to see how shading effects can be produced with powder. You should run the tests on both light coloured and dark coloured bases. Opalescent glass requires more powder than transparent. Opaque powders are better than transparent colours to show on dark colour.

This image shows the result of the firing, showing a thicker application of powder was required to give the full effect.
Picture credit: Donna Brown

In this particular application, I would put the powdered colour down and then the honeycomb grid on top for better definition of the honeycomb.

Fading powders  

Colour dilution   

Homemade Glass Powder

Summary of a question: Frit made in an electric coffee grinder and cleaned with magnet still produced black specks in the finished piece. What’s happening?

 

 Most coffee grinders use stainless steel blades. Most culinary stainless is not magnetic. So, you are left with flecks of steel in your powder that the magnet cannot remove.

 My experience with homemade powder has not been positive. This has led me to buy powders. But I still use home-made frit. You can wash and sieve out the powder and steel particles at the same time into a bucket or basin. This will leave you with clean frit from fine to as coarse as you want.

 Do not put the residue down the drains. It will block your drains after a time. Instead, you can let the glass settle to the bottom of the bucket and pour off the clear water. Let the remaining water evaporate, and wrap up the sediment for waste collection. Or if you have a garden, you can empty the water and sediment onto the ground. It makes for good drainage over time.

 

The contamination in home-made glass powders make it best to buy powders and make your own frit.

  

Glass Milling Equipment and Techniques

 

Glass Milling Equipment and Techniques

Posted Krista Grayson on Aug 26, 2021

Milling is an effective top-down process for producing glass powders with particle sizes ranging from nanoscale to micron-size. This article provides a guide to the various types of milling processes used by Mo-Sci to produce our range of specialty glass powders.

Alongside microspheres and ingots, one of the primary forms in which Mo-Sci provides glass is as a powder. Powdered glass has numerous applications throughout research and industry. For example, glass nanofillers offer unique advantages in developing composite materials for medical and dental prosthetics.1,2 Such composites are not only incredibly strong, but exhibit superior biocompatibility due to their surface roughness, hydrophobicity, and chemical inertness.

While glass powders and nanofillers can be produced via “bottom-up” methods such as the sol-gel process, such techniques are not always practical, especially when producing glass particles with complex compositions.3 In these situations, a “top-down” solution is preferred: larger particles of glass can simply be broken down into smaller particles in a process known as milling.

Note that in the world of machining, “milling” has two meanings: it can refer either to grinding (e.g., using a ball mill) or cutting (e.g., using a CNC mill). Here, we’re strictly using the term in the former sense.

Glass Milling Processes

Jaw Crusher

A jaw crusher produces the coarsest grind out of any of the processes in this list. Much like a nutcracker, a jaw crusher uses direct application of compressive force between two mechanical “jaws” to fracture solid materials. The jaws are typically arranged vertically so that the gap between them narrows as material falls through the system. The particle size decreases as the material moves down through the crusher. Crushed material escapes through the narrow gap at the bottom between the jaws after it has been processed. Jaw crushers are heavy-duty machines often used for mining and quarry applications, and are suitable for continuous use with materials across a wide range of hardness. Jaw crushers are typically used to process larger fragments of material and are not capable of fine grinding. They may be used as a preliminary step before more controlled grinding in a ball mill or jet mill.

Hammer Mill

With more precision than a jaw crusher, a hammer mill (or hammer crusher) can crush aggregate material into smaller particles using hammers. Commonly used for processing papers, organic waste, and foodstuffs such as grain and fruit; hammer mills can also be used to produce glass powder. Available in configurations with one or several rows of hammers, hammer mills are high-durability devices capable of coarse grinding.

Ball Mill

Ball milling is one of the most commonly used techniques for milling glass, thanks to its adaptability and relatively low cost. A ball mill for glass processing consists of a rotating or vibrating ceramic-lined drum, inside which are a number of balls that act as the grinding medium. These can be made from a variety of hard and durable materials such as alumina or zirconia. Glass is added to the drum, and the continuous impact and attrition between the balls and the glass as the drum rotates breaks the glass down into small particles.

Ball mills are supremely versatile, they exist in a wide range of sizes, can perform wet or dry milling, and can accommodate a wide range of different grinding media. Ball mills are suitable for either batch or continuous processing, and grind size can be controlled by changing the diameter size of the balls. Industrial ball mills may be used to coarsely crush relatively large pieces of material, whereas lab-grade ball mills are capable of finely milling glass to micron level and below. So-called “high energy” ball milling can even be used to reliably grind the material into nanoscale particles.4

Centrifugal Mill

Most centrifugal mills lie firmly in the realm of labware. Inside a centrifugal mill, shearing action between a rotor and a fixed sleeve breaks material down in a relatively well-defined manner. High rotational speeds (up to several tens of thousands of RPM) enable fast and finely controlled grinding of small batches of material, often using sieves to enable the extraction of particles with sizes in the tens of microns region.

Jet Mill

Another precision piece of equipment, a jet mill uses a high-velocity flow of gas or compressed air to cause glass particles to collide with each other inside a chamber. A process known as cyclonic separation enables particles to leave the gas stream once they have reached a sufficiently small size. Consequently, jet mills can continue milling particles of glass until they reach the desired size, resulting in a glass powder with very little variation in particle size.5 Like ball mills, jet mills are capable of grinding glass powder down to sub-micron particle sizes.

Milled Glass Powders from Mo-Sci

At Mo-Sci, we are experienced in the use of all of these milling methods to produce a wide variety of specialty glass powders, including those made from complex formulations which are difficult to find elsewhere. As well as using different types of mills, we make use of various milling media (such as alumina, zirconia and yttria-stabilized zirconia (YSZ)), and perform wet milling in water or alcohol to achieve superior results.

To find out more about our capabilities in the specialty glass powders market, get in touch with the Mo-Sci team today.

References and Further Reading

1. Velez, M. et al. Processing of yttrium aluminosilicate (YAS) glasses for dental composites. Cerâmica 57, 1–9 (2011).
2. Erol-Taygun, M., Zheng, K. & Boccaccini, A. R. Nanoscale Bioactive Glasses in Medical Applications. Int J Appl Glass Sci 4, 136–148 (2013).
3. Vital, A. et al. Ultrafine comminution of dental glass in a stirred media mill. Chemical Engineering Science 63, 484–494 (2008).
4. Yang, L. 2 – Nanotechnology-enhanced metals and alloys for orthopedic implants. in Nanotechnology-Enhanced Orthopedic Materials (ed. Yang, L.) 27–47 (Woodhead Publishing, 2015). doi:10.1016/B978-0-85709-844-3.00002-1.
5. Wachtman, J. B. Materials and Equipment – Whitewares Manufacturing. (John Wiley & Sons, 2009).

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.

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.

Making Thin Sheets

The question of how to make thin sheets arises from time to time.  Unless you are a glass manufacturer, it is unlikely you can make large, thin glass sheets.  But you can approximate making thin sheets by two methods that I know.

Sintering

One of these is sintering.  This is firing the glass to a low temperature and soaking for a long time.  The common form of this is powder wafers. 

By using a screen to deposit an even layer of glass powder you can make very thin, but delicate sheets of glass.  The procedure I would use is a screen of about 45 – 60 threads per inch.  This is coarse enough to allow the powder through, but not so fine as to “reject” large amounts of the coarser particles. 

You can screen the powder directly onto a kiln washed shelf, or onto Thinfire or Papyros.  You will not be able to move the unfired powder on a sheet of paper or fibre paper without changing the thickness and shape of the screened powder.  It must be laid down onto the separator directly on the shelf.  You can of course, move the shelf to the kiln if you can get in without tipping it.

Method

Support the screen about 3mm above the surface to allow the powder to fall through.

Make a ridge of powder at one end of the screen.  Using a smooth straight edge wide enough to cover the whole of the screen, lightly spread the powder from the starting end to the other. Then repeat drawing the powder to the starting end.  Make about five repeats of this – that is 10 passes, to get enough powder laid down to form about 0.5 to 1mm sheet.  You will need to experiment with the number of passes to get what you want.

Do not try to press the powder through the screen.  That will only wear the screen out quickly and may tear it.  Each pass should be a light spreading of the powder.  It is heavy enough to fall through the screen without additional force.

You could, of course, just sift the powder over the area you want to cover and judge by eye how even the layer is.  It is possible that your observation is good enough, but it is more likely that you will have thick and thin areas.  Often even at sintering temperatures, the thin is pulled toward the thicker, leaving small or large holes.   By screening the powder, you know you will have an even layer

Firing

The kind of schedule to use to sinter the glass particles together without changing their structure is the following:

220°C to 482°C , soak for 60 mins
55°C to 593°C, 10 minutes
28°C to 665°C for 5 mins
as fast as possible to 482°C for 30 mins
28°C to 427°C, no soak
55°C to 370°C, no soak
110°C to 50°C, no soak

This will work for most fusing glasses.

This slow firing allows enough heat to penetrate the glass grains that they will stick together without changing shape or developing holes.  I admit the anneal cool is very cautious.  You can experiment with quicker cools if you want to speed the process.

  

Pressing

This is a technique of thinning already existing sheets of glass.  Typically, you will have a 6mm or thicker piece of glass that you want to be 3mm or less.  Paul Tarlow has described this kiln pressed glass very well in his books and on the fusedglass.org site.

In essence, you use a pair of kiln shelves.  Kiln wash both shelves.  Place the glass to be thinned on one shelf.  At the outer edges of the shelf put down spacers of the thickness you want the glass to be after pressing.  This will keep the upper shelf from settling down too much and more importantly unevenly.  Place the other shelf, kiln washed side down, on top of the glass.  Be sure the spacers are in places where they can support the upper shelf.

If you are thinning from 6mm to 3mm, normally you do not need any additional weight on top of the upper shelf.  But the thinner you want the glass to be, the greater the weight needs to be.  It could be another shelf, fire bricks or steel weights.

When scheduling the annealing remember you must take account of the mass of the weight on top of the glass.  You will need a much longer temperature equalisation soak and a much slower annealing cool.  

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

Kiln Elements - Aging

As elements age, they generally increase in their resistance. This increase in resistance decreases the amount of amperage and, so, the amount of heat given off by the elements. This explains  why older kilns sometimes go so slowly and may not reach their maximum temperature.

There are several factors which affect the longevity of elements and so have implications for firing practices.

• ·        Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.
• ·        Allowing the wires to become tightly wound increases overheating of sections of the element.
• ·        Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.
• ·        Firing close to the elements allows fumes to contact the elements.
• ·        Subjecting elements to reducing atmospheres will age the elements quickly.  This would be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are carbon, wax, halogens (such as chlorine or fluorine), molten metals (such as zinc, aluminium), lead glazes, alkaline metals, borax compounds.

All these elements attack the element coating.  And each time you fire the slight difference in expansion between the core of the wire and the coating creates cracks in the coating.  The exposed core forms new coating to fill the gaps.  This over time reduces the thickness of the element wire.  As the wire thins, the resistances increases, causing more fissures in the coating to occur, accelerating the aging process.

The next in this series is about how firing practices can affect the life of elements.
Firing Practices

Other relevant posts:
Nature of elements
Maintenance

Filling Gaps in Fused Pieces

Often your cutting is not as accurate as you would like so there are small gaps between the pieces as you assemble the piece.

Tammyhudgeon.com

One solution that is often used is to grind the edge of the too large piece to get the fit desired.  The problem with this is that thorough cleaning is required to avoid devitrification lines appearing on the final piece.  Also, even with extensive grinding, the fit is not perfect.

The alternative is to fill the gaps with fine frit or powder. 

artistryinglass.com

Assemble the whole piece and assess the gaps.  If they are very large, you need to adjust the glass. If they are only millimetres wide, powder and frit can fill the gap to disguise the join.  I generally use powder for almost perfect joints, and fine frit for anything larger.

I first cover the gap with powder or frit and with a soft brush work at right angles to the line of the join.  This ensures that I have filled the gap to the height of the glass.  However, the frit and powder have air spaces, and so will fuse to a lower level than the height of the glass.  So, once gap is filled, I build a small ridge over the gap trying not to extend beyond the gap.  This mound compensates for the lack of density of the frits.

The frit and powder colour must match the glass exactly to become invisible.  It can be made from your scraps or purchased at the same time as the glass. I find it more successful to do these fills with the darker glass.  It provides a more distinct edge to the joint.  It also conceals the base glass better.

It can also be used to conceal the joint in a single colour where the piece cannot be cut as one and needs several pieces to make the whole.  This is more simple as any overspill will not be noticed when fused.

This method only works with full fusings.  At tack fuse temperatures the frit will not fully combine with the sheet glass to form a smooth invisible join surface.

Pink Confetti

Because confetti needs to be so heavily saturated with colour, some of the opalescent colours tend to devitrify. The pink is particularly prone to devitrification. There are several ways to prevent this: 

• cap (which can lead to bubbles), 
• add a devitrification spray, or 
• cover with clear powder or frit.

Covering completely with a fine layer of powder gives the most even result. Using frit can provide a speckled appearance that is useful in some circumstances.

This tendency of pink opal to devitrification applies to all formulations – Bullseye, Uroboros, S96 and float.