Wednesday, 11 September 2019

How Close to the Edge

“How close to the edge of my shelf can I place a large piece?”

It depends in one sense how thick the piece is.  A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge.  Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature. 


The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece. 

This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. -

The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.

If you have greater differences in temperature than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.

I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.

Wednesday, 4 September 2019

Controlled cooling

It is sometimes stated that you can simply turn the kiln off below 370C and let the kiln’s natural rate of cooling take over the cool down.

This works for most flat 6mm pieces in most kilns, but as you work thicker or with greater contrasts in thickness, lots of tack fused elements or in a small rapidly cooling kiln, you do need to control the cooling toward room temperature.

The first thing you need to know is the natural cooling rate of your kiln.  

The rate of cool is not just about the annealing soak. The soak at annealing temperature is to equalise the temperature throughout the blass to have a differential of not more than 5C. 

The rate of cool is about avoiding thermal shock, too. The glass needs to maintain the temperature variation to less than 5 degrees Celsius difference throughout the glass as it cools.  This requires a slow controlled cool.  

You may program a cool of 100C to 370C thinking that the kiln will maintain that rate or less.  If the natural cooling rate of your kiln at 370C is 200C/hour, you risk thermal shock due to the rapid increase in the cooling rate.

You really do need to know the natural cooling rate of the kiln from the point you turn the programmer off to room temperature to be safe from thermal shock.

The alternative to turning off at 370C is to program the schedule all the way to room temperature.  The kiln will use no energy unless the kiln cools too quickly on its own.  At which point the program will kick in to slow the cooling of the kiln.

Wednesday, 28 August 2019

Finding Your Kiln’s Natural Cooling Rate

You need to observe how your kiln behaves while cooling without any power to be sure when you can safely &turn it off and let it cool without power.

Assuming you have programmed your kiln for a shut off at 370C, you need to observe every quarter hour or so to record both time and temperature.  From those observations you can calculate the cooling rate at the various temperatures.

Say at 6:00 your kiln was at 370C;
At 6:15 it was at 310C;
At 6:30 it was at 265C;
At 6:45 it was at 230C;
At 7:00 it was at 200C;
At 7:30 (you missed the quarter hour) it was at 160C;
At 8:00 it was at 140C;
At 9:00 it was at 125C;
At 10:30 it was at 110C.

To calculate the rate, you divide the temperature difference by the proportion of an hour between observations, as demonstrated in the following table.

Kiln Name/Description
Shelf composition
Amount of glass
Rate of
of an hr

Although this is an example, it shows how the cooling rate slows down as the kiln cools. 

If you were cooling a flat piece 12mm thick, you might get away with turning the kiln off at 370C, as a flat piece can cool as quickly as 300C/hr.

If you were cooling a piece 19mm thick, the natural cooling rate of the above kiln is too fast. 19 mm thick pieces need a cooling rate of 150C/hr, so according to the figures above you need to programme this kiln down to 230C to get the appropriate final cooling rate.

If it is a tack fused piece with a 6mm base and areas of two layers of tack fusing, you should fire as though it is 24mm thick.  In this case, the final cooling rate needs to be 90C/hr.  For the kiln in the example above, that rate is not achieved until below 160C, so that is the minimum temperature for switch off.

This method can be used for any temperature range.  For example, you may want to know the rate of cooling from the top temperature to the annealing temperature.  This method will work there too. You may want to record the temperatures more frequently than every quarter of an hour though.

You really need to know your kiln’s natural cooling rate before you can be confident of switching the kiln off at 370C.  This blog shows a method of determining the natural rate of cooling. 

Wednesday, 21 August 2019

Bubble Mystery

A question was asked about a collapsed bubble. There were two pieces in the kiln and one (strips) was fine and the other (flat plate) had the collapsed bubble.  Both on the same dried shelf.  The question also asked if the collapsed bubble piece could be flattened by fusing again.

Collapsed Bubble
The bubble collapsed because it had not burst by the time the cool toward annealing had begun.  As the air pressure under the bubble dropped, and the weight of the thinned glass bubble sank down as there was not enough air pressure to hold it up.

The glass is now thinner at the centre of the bubble than the main part of the piece, and thicker at the edge of the bubble. I don't think it is possible to successfully flatten it to become an even thickness across the whole piece. To get the same thickness across the whole piece would require high temperatures and long soaks there. 

Another possibility is to use a pressing solution

My suggestion is to add elements or repurpose it. I don’t think any repairs would present a good-looking piece.

The on-line diagnosis of the possibilities for the cause of the bubble was extensive and sometimes inventive.  It was finally determined the bubble was from under the glass, that is, between the glass and the shelf. A slight depression in the shelf is the usual explanation.  The user tested the shelf for smoothness and found no depressions.

It was clear the bubble came from under the glass.  All the suggestions about how bubbles can form under glass were given, but none seemed to apply.

How can you get a bubble on a dry shelf that is perfectly flat and that has not been subjected to too rapid or too high a temperature?

The answer is that a little spot of grit or tiny ball of fibre paper can keep the glass raised up enough for air to be trapped.

It is not enough to test the shelf is flat.  You need to use clean kiln wash with a clean brush to avoid any grit being brought to the shelf. It is also a good reason to vacuum the shelf before each use in case any dust or grit has fallen onto the shelf. Covering the shelf or putting it into a cupboard will also reduce the possibility of small bits of grit falling onto the shelf.

Of course, if you smooth the kiln wash with a nylon or similar fine cloth, you will remove any specks of grit.  A vacuum of the shelf after smoothing is still a good idea.

It is as important to keep tools and materials clean as it is to clean the glass you are going to kilnform.

Wednesday, 14 August 2019

Simultaneous Firing of Different Moulds

Often you have moulds of different sizes or depths that you would like to fire at the same time to use the space or save time.  If the moulds are of distinctly different sizes or shapes, you will not save time, as the likely outcome is that some will be over-done and un-shapely or, conversely, that some will not have completed their slump.

The main things that act against firing moulds with distinctly different firing requirements are:

·        Moulds with different spans require different temperatures or different soak lengths.

·       Moulds of different depths, even if they have the same span, require different soak lengths.  

·        Moulds of different shapes, even if they are the same depth, require different soaks or different temperatures. 

As an example, if you have two moulds that require less time or lower temperature than three smaller ones. If you get the smaller, relatively deeper ones fully slumped, the larger, shallower ones will be more marked by the mould than necessary.

The best thing you can do if you want to make full use of the kiln space each time you fire, is to save up the glass until you have enough to put in a full kiln load.  This may require more moulds of the same size than you currently have.

Usually trying to fit in a lot of slumping into one firing relates to a concern on how much electricity will be used in multiple firings. However, the kiln does not use huge amounts of electricity.  A 50cm square kiln will normally use less than 10Kwh for a slump with a long soak.  This will cost much less than a glass of beer or wine.

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.

Regular shapes

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

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.

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, 7 August 2019

Firing uneven layers

Firing uneven layers requires more care than a piece equally thick all over.

My rule of thumb is to add the difference between the thick and thin to the thick and fire for that. So, a piece with a 6mm base and a total height of 12mm gives a difference of 6mm added to 12mm gives a firing thickness of 18mm. If you look up the bullseye site annealing for thick slabs, follow the schedule for 19mm. The initial heating rate can usually be half the final cooling rate shown in the table.

The Bullseye recommendations are more conservative.  They recommend that the firing rate should be for something twice the thickest part of the piece.  In this case, the firing would be for a piece as though it were 24mm. Again, the initial rate of advance would be equal to half the final cool segment in the Bullseye table Annealing Thick Slabs.  

If you are slumping a thick piece, you can use the initial rate of advance all the way to the slumping temperature and then anneal according to the thick slabs table.

Wednesday, 31 July 2019

Terminology for degrees of fusing

Can anyone describe what a contour fuse is?

No one can satisfactorily describe, to a high level of acceptance, what a contour fuse is. For me it is just before a full fuse. That will not be acceptable for many, just as describing something as a rounded tack fuse is not a contour fuse for me.  A sharp-edged tack fuse is sintered glass. This will be important to observe as you move to other glass processes such as pate de verre.

There is not yet an accepted terminology and will not be as long as people choose to invent new descriptions for what are essentially the same things.

The closest you can get to a sensible range of descriptors is in the Bullseye document "heat and glass" where the temperature ranges are the important constants.
The fourth column of this document gives names for the process. It would be a good idea to adopt these terms, as Bullseye is the company doing the research in the area of kilnforming.

Bullseye terminology gives the following:
A slump or bend occurs in the 540C – 670C range
Fire polishing and sintering occur in the 670C – 730C range
Tack fusing (a rounding of edges) occurs in the 730C – 760C range
A rounded tack fusing that begins to sink into the base glass occurs in the lower end of the 760C – 816C range.
Contour fusing occurs in the middle of the 760C – 816C range.
Full fusing (flat) occurs at the upper portion of the 760C – 816C range.

Wednesday, 24 July 2019

Getting the Right Firing Temperature

“what temperature should I use to get a tack fuse that is just less than a contour fuse?”

This is the kind of question that appears on the internet often.  Unfortunately, no one can answer the question accurately, because it depends on some interrelated variables.

Kiln characteristics
Top or side elements, size of kiln, relative size of piece, all have an effect. Also no two kilns even of the same model have exactly the same characteristics.

Rate of advance
How quickly or slowly you fire has a big effect on the temperature and soak needed to achieve the desired result. This is the effect of heat work.

There are no absolute temperatures for a given effect, given the above two variables.

The length of time and the number of soaks will affect the temperature required to achieve your effect.

OK. So, what can I do?

The only certain way to get the effect you want is to observe.
Set a schedule, guessing the top temperature and length of soak.  Know your controller well enough that you can extend the soak or end the segment by advancing to the next.  Your manual will tell you how to do this.

Peek at intervals from 10-15C below the selected target temperature. Peek at 5min intervals until the effect is achieved.  Advance to the next (cooling) segment.  Record the temperature and length of soak at which the effect was achieved.  On subsequent firings you can experiment with reducing the temperature by 5C – 10C with a 10-minute soak.  Observe and record the temperature and effect as before.

The reason for going for a 10-minute soak rather than longer is to avoid holding at the target temperature for a long time, as that can help induce devitrification.  The reason for a soak at all is to achieve the minimum of marking on the reverse or picking up kiln wash or kiln paper on the back.

If effect is not achieved by the end of the soak, extend it by using the appropriate key or combination of keys.  Keep observing at five-minute intervals until the effect is achieved.  Advance to the next segment and record both the temperature and time.  The objective is to get the heat work done with a 10-minute soak, so you will need to increase the temperature on the next firing.  The amount of increase will depend on the length of soak required to get the desired surface on the previous firing.  The longer the soak, the more temperature you need to add.  You will need to repeat the observations and recording until you find a temperature that will achieve the effect with a 10-minute soak.

Use the lessons from the observations to lower temperature, extend soak, raise temperature, reduce ramp speed, or reduce soak as required.  It will also help you judge on other pieces the approximate temperature and time required for the new layups or new moulds.

Wednesday, 17 July 2019

Bas Relief Moulds

Bas relief moulds that have an image carved into the surface are popular at the moment. They are most often called texture moulds.  The image is “carved” into the back of the glass, creating uneven thicknesses of glass that refract the light to show the image through the smooth plane of the front.

One of the problems with these kinds of moulds is that lots of bubbles are created, often very large ones.  This results from the many places where the air cannot escape from under the glass during the forming process.


There are some strategies that can help avoid these bubbles.

Use the 6mm rule
Fuse the glass into a six-millimetre thickness first.  Two layers of glass give more weight to help the glass conform to the texture of the mould.  It also resists bubble formation more than a single layer.

Use the Low and Slow approach
It more important to have low and long bubble squeezes.  The most successful strategy will have a slow rise in temperature to put as much heat work into the glass as you can before the bubble squeeze.  The bubble squeeze is the most important part of firing these texture moulds.  It will start at about 600°C rising at only about 25°C/hr to around 680°C – that is, taking three to four hours. 

Use slow rates of advance
A third element is to rise slowly toward the forming temperature.  Possibly nothing faster than 75°C.  This enables you to keep the forming temperature much lower than a fast rise will.  The usual temperature recommended is about 780°C.

By using a slow rate of advance you can probably reduce the forming temperature by about 20°C.  You will need to peek at intervals to be sure the glass has taken up the required texture. Again, it is about putting as much heat into the glass at as low a temperature as possible.

Use Long soaks
An alternative to the slow rate of advance is to use a long soak at as low temperature as seems suitable.  You will need to peek at intervals to determine when the texture is achieved.  When the appropriate texture is imparted to the glass, you need to advance to the next segment.  This means that you need to know how to get your controller to skip the following segment.  Or, if the texture is not achieved before the end of the scheduled soak, how to extend the soak time.  If you are using 760°C as you target temperature with a rise of 150°C, you may wish to soak for about an hour or more.  Remember that this is in the devitrification range.

Alternative - Frit
A completely different approach is to use fine frit and powder to give a patè de verre appearance by sintering the frit.  This eliminates the bubble problem entirely.

You will need a lot of frit if you are trying to make a sheet of 6mm from the frit.  You could just take the sheets of glass cut to the size of the mould and smash them up to get the required amount of glass.  Or you can use your cullet, by weighing and smashing up enough glass. 

The calculations for weight are best done in the metric system (in cm) as there are easy conversions between volume and weight.  Assume your mould is 20cm square.  The area is 400cm2.  The volume is that times 0.6cm or 240cm3.  The specific gravity of glass is approximately 2.5, so you multiply the volume by that and get 600gms of glass required to get a 6mm thick sheet. 

You could full fuse this into a clear sheet, although this would take a much higher temperature and longer soak that would be good for the mould. Better is to sinter the glass.

To sinter the glass, you need slow rises in temperature and long soaks.  A rise of about 75°C to the softening point of the glass (around 600°C) followed by a very slow rise (ca. 25°C per hour) to about 660°C is needed to allow the small grains of glass to settle together.   At the upper end of the bubble squeeze you need a three- to four-hour soak to sinter the glass. The thicker the layer of glass frit, the longer soak needed to ensure all the particles are heated.  The densest glass will be formed by a 50/50 combination of powder and fine frit.

Much better is to have a much thinner sheet formed from the frit.  This will be about two to three millimetres thick.  The weight of powder and or frit can be determined by the formula above, substituting 0.2 or 0.3 for the thickness.  This frit mixture needs to be evenly spread over the mould, with as much on the high points of the mould as the low ones.

If the mould has a lot of variation in height, you can sinter the frit mixture as a flat sheet first.  Then place it over the texture mould and give it a slow rate of advance to the to the top end of the bubble squeeze and soak for an hour or more, as required.  This will ensure you get the same thickness across the whole piece even though there differences in height.

The resulting piece will be very light and translucent.  It will have a fine granular feel to the touch.  It will have the same shape on both sides of the piece, with the upper surface having a slightly more shiny appearance than the bottom.

Wednesday, 10 July 2019

Stand Alone Online E-Stores

Perhaps none of the existing online marketplaces fit enough of you needs to join them.  You can set up your own and make it your only online store, or you can do it in addition to other ecommerce sites that have some of the features you want.


The advantages of your own online store relate to control and adaptability to your design needs.

You retain control of the design, layout, branding, etc., of your site.  This helps maintain your identity or brand and aligns it with your product range.

You have control of when the store is live and when it is updated. You, of course make the rules for what can be listed and how it is displayed.

You don’t have to acquire a lot of knowledge about setting up websites and online stores. Website builders offer templates and store services. You can also use professional website builders to get complete control.

You can link to Etsy or other market places from your own store.  You can funnel the traffic from these sites to own site.

Your own site will enable you to build closer relationships with your buyers. You can communicate directly rather than through intermediaries.


Nothing comes free of course.  There are some disadvantages to establishing and running your own site.

An especially important element of a store is visits – akin to footfall in real life stores.  You must get people to visit.  You get the visits by making the links with people using a variety of communications.  You need to combine social media with the creation of newsletters, direct mail, blogging, etc.  These relationship building efforts are vital to get people to your website and store.

There are costs relating to hosting fees and one-off fees for the building of the website.  The online stores also charge fees in different ways, so a careful comparison of the best-looking services is important.

There will be additional administration in comparison to an online marketplace.

Questions for E-commerce Site Building
What are the facilities for integration of Etsy offerings into your own store? Will separate loading be required?

Is drag and drop site building supported? Is there user support or a user group to support you?

Is an integrated shopping cart available?  What are the order fulfilment assistance options?

Will the site support expanded functions as your business grows?  How adaptable it the site to changes in business?

Is the e-commerce site a market place?  How will exposure of the site to potential buyers be managed?

What is the cost for the features you want?

The Balance

You must decide whether the advantages of having your own e-commerce site outweighs the disadvantages in terms of traffic, time spent developing relationships, administration and cost

Wednesday, 3 July 2019

Websites for Selling Craft

This is not a discussion of which site to choose, but a range of things you need to think about when considering which site to use for selling your craft items.  This includes whether to have your own e-commerce site instead of, or in addition to, a market place site.

Evaluating website offerings requires you to think about a multiplicity of things.  Many of these are listed here, although there may be a few additional things you need to think about for your products.


You need to think about the amount of recognition the marketplace has.  Is it the place your potential buyers know about?  Are there a lot of visits to the site?

Is it a market place offering where the website promotes the whole site and the shops within it?  Or is it a site where your own efforts to drive traffic are required? This latter element is like having your own site.

What is the competition within the site? Are there many other sellers of your kind of product? How easy will it be to distinguish your things from others?  Are there mass production sellers on the site?
How selective is the site in approving sellers?  This also relates to reputation.

Reputation and Products for sale

Is the site restricted to craft made items? How are mass production manufacturers eliminated?

What range of products are allowed? Is it possible to sell services, and digital products as well as physical goods?

Is the site focused on general products or arts and crafts?  What pricing levels are exhibited on the site?  Is the focus on arts-based items, or does it include bargain basement and cheap deals?

What is the level of security of transaction information offered to you and your customers?  It is vital that the site offers good security for transactions to give customers confidence in buying from the site.

Your identity

Whatever site you join, there will be many other sellers or shops.

Do you get your own shop? Or are all similar products grouped? Is there support or templates to set up your shop?

If you have your own shop what degree of control do you have?  How are images formatted?  What amount of text can be included?  What range of formats are allowed?  Does the site brand dominate, or can you have yours as the prime visual?  What number of themes are available to you?

What level of flexibility in store arrangement and titles do you get?  What number of pages do you get at the various plan levels? How much flexibility and customisation is allowed? What number of items per page are allowed? And what descriptions are allowed either in length or number of terms?

What are the restrictions on the number of products you can sell? Are you allowed discount codes?  Is there inventory control with the site? What is the assistance for order fulfilment?  How much and what features? Is there a system set up for returns? How much support is available?

Are searches restricted to your shop or for all shops on the site?  How are the meta tags used by the site? Are hyperlinks within site only or allowed to outside sites too? Are social media buttons available and with what flexibility?  Can you use your own domain name?

Connections with other e-commerce sites

Are connections allowed?  How easy is the linking? Can you link to multiple sites? Are links to social media – Facebook, etc. – allowed? And how are they managed? Can you link the potential customer to mobile phone sales?


Of course, there are always payments to be made. You need to look at the various options offered, and the charges involved in them.  If you are new to online selling or have low volume sales, it may be that higher selling fees rather than regular payments with lower transaction fees is better for you at the start.

Listing costs are normally linked to number of items you are offering in your shop. There may be refreshing fees – you must pay a fee to keep the product in the shop after a set period.

There will be continuing fees.  These may be in relation to each item – commission - either as you sell or related to the plan level you choose. Are the commission fees in addition to the listing fees?  Are there additional credit card fees?

Plan level costs are ongoing fees that may be monthly or annual. They are often linked to the length of contract between you and the site provider.  They will give different levels of item fees, and levels of features.  What are the costs of the plan levels? What benefits to they give, and do you need them?  What level of functionality do you get in relation to plan level costs?  How are the plan levels related to the volume or value of sales?

What is the ability to expand and grow through graded plans?  How and when can you move from one plan to another?


The costs of doing business online may be significant. They may also be related to your familiarity with online offerings.

The creation of an entry should be easy and flexible. You should find it easy to move around the listing form, and it should contain a significant amount of flexibility.  You should be able to make bulk changes.  It should be easy to move items and entries around your shop. 

How much control do you wish to have?  With less experience, you may want to have a lot of the listing, editing of pages, especially contact information done for you, or highly guided.   The kind of support is important. Does the site have a maker support community?

An often-unrecognised level of administration is inventory management. Does the site support that?  If the site does not have inventory control you will have to do it yourself. If you don’t have the stock to satisfy the order, you probably will lose the sale. 

Is the site easy to use?
An important general question is the ease of use for you and for customer. Test the sites out for how easy it is to find and buy an item.  Look at how easy it is for you to use the tools to list your products.

Reviews of e-commerce sites

There are sites that review the offerings of various sites to help you answer some of the questions listed above.  One I have found to be helpful is Ecommerce Guide.

The answers to the relevant questions listed here will assist you toward choosing a website that suits your needs.  It may also lead you toward considering a stand-alone ecommerce site if there are not enough positives in your review of market place sites. It may lead you to consider both.  But the more sites you have the more important it is to be able to link between them and move entries between sites.

A discussion of various things that need consideration on whether to sell on line at all is here.