Glass Volume 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.

This post gives some alternatives.

Revised 30.12.24

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 here. In 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.

Circles – Some Methods of Scoring and Breaking Out

 Scoring the circle

Setting out the centre point and radius of the circle

Set up a centre point. You can do it by estimating. But it is more accurate to measure a centre point that is at least 1cm from the edges. Use that crossing point to measure out the radius of the circle.

 

Setting out the circle measurements

Place the pivot point of the circle cutter on the cross hairs. Lock it into place.

 

Locking the pivot into place on the cross hairs

Adjust the cutting wheel along the length of the circle cutter bar to the marked radius. The measure marks on the bar are not always accurate and anyway wear away quickly.

 

Adjusting the scoring wheel to the correct length

 

Tightening the set screw at the top of the turret

Tighten the set screw.

 

Add a touch of oil to show the score line. Make a test circle by pushing the bar around with no pressure. This shows up any obstructions around the end of the bar. 

Preparing to test the sweep of the cutter arm

Score in an anti-clockwise direction. This avoids accidental loosening of the set screw if it is under the button or handle. 

 

Start with the scoring head under the arm with which you are securing the centre pivot. This allows you to do the whole score in one motion. The pressure you use should be no more than in your normal scoring.

 

Oil trace of the score can be seen in the upper left quadrant

 

Running the score 

Running the score is a glazier’s term to indicate the way in which a scored line gradually separates under gently applied breaking pressure. This can be seen when gently applying pressure with cut running pliers. The score line is made progressively visible by the gradual separation of the glass. This results from the light passing through in a different way than when it is not separated from top to bottom.

 

The glazier’s method to get a clean break of a circle is to score on one side with a trace of oil. Then turn the glass over and press with your thumbs on the score line. Running the score from the back requires a little skill and a certain amount of courage or determination.

The object is to bend the glass just enough to crack it along the score without breaking it anywhere else. The best surface is a short pile carpet square, a rubber mat or a single layer of towel or an unfolded newspaper. These provide a surface with a little “give” to allow the glass to deform.

But if you have too soft a surface, it is easy to break through the circle. A too soft surface is given by a household carpet, several layers of towel, or any other surface with a lot of “give” to it.

You may need both thumbs to start the run depending on the pressure you can exert. Try one thumb first. If that is not enough use both thumbs.

 

At the start of the second pressure point

If you place your thumb opposite a corner, you have greater leverage to start the run of the score. This leverage makes the running of the score easier as less pressure is required. You will hear a loud click at the opening of the first part of the score. 

Score has been run completely around the circle

Continue around the circle by pressing at the end of the opened score, until the whole score has been run. You may hear quiet clicks as the score opens. Other times there will be no sound, but you will see the score run away from the pressure point.

Once you have run the score from the back, turn the glass over to have the scored surface up. This makes it ready for the relief scores. It is much more successful if all the scores – circle and relief - are made from the same side. Unless you are scoring float or other glass that is smooth on both sides, this flipping back will be the easiest anyway.

 

Make the relief scores on the front.. Sometimes only one relief score is required to release the circle from the surrounding glass.

 

Another method is to cut the corners off so that you have an octagon around the circle. This gives you the opportunity to run the score from the top with breaking pliers. Ease the score open progressively around the circle.

Opening the score with pliers

This method is easiest for opalescent glass where you cannot see through to the score. By opening the score from the top, you do not have to estimate where the score line is. You can see the oil trace indicating where the score is and where to place your breaking pliers. 

You should start at a place where there is only a centimetre or two between the edge of the glass and the score. This may mean that you have to move around the broader areas of the rim so that the score runs both back to the first opened score and forward. It is in effect, opening a new score four times. But with gentle persuasion the scores will run toward one another. Do not use heavy pressure in griping the pliers, or in bending the glass. Gradually increase the bending pressure until you hear the click of the glass separating.

These two methods of running the score give the cleanest break-out of circles.

  

Relief scores

There are multiple ways to create the relief scores.

Perpendicular

Score by drawing the cutter from the circle out to the narrow edge, leaving a small gap between the opened score of the circle and the start of the score.

 

You can also score a longer line to the corner. Again, leave a space between the circle and start of the relief score.

 

Tangents

 A third way is to score tangents from the edge to the opened score of the circle. 

Tangent scored from both sides of the circle

Tangent broken from both sides of the circle

Alternatively, score a first tangent and run the score

Further tangents scored and broken away

All the tangents run and broken away 

This reduces the risk of breaking through the centre of the circle. But it does leave little nubs of glass at the point of each tangential score. And for that reason alone, is the least satisfactory of the methods of breaking out circles.    

In each of these methods of running a relief score, you need to tap under the relief score to run it to the edge of the circle. Normally, the quarters or other fractions will drop out one by one. Occasionally the rim will drop away after the scores of the first two quarters are run.

 

Breaking out without running circle first

There are at least two ways to cut a circle without running the circle score first.

The first is to cut the corners off the glass to create an octagon, but do not run the score.

First corner scored and removed

 

The four corners removed

Starting to run the circle with breaking pliers

Then use breaking pliers to run the scored circle. Once the score is run, make a single or multiple relief scores, and carefully run it. the circle can then be removed from the octagonal rim. This provides a clean cut.

 

corners and rim removed 

It does not risk breaking through the scored circle to the interior when tapping the relief score.

Many people run scores at a tangent to the circle without having opened the score of the circle. These are then run, in turn, to and along the edge of the scored circle.

They can then be broken off in turn, if they don’t simply fall off when they all are run.

 

tangent removal 

Finally, some people tap under the scored circle to run the score, as in the first of the tangent removal methods. This leaves shells – or ledges – on the glass circle. These must be removed by grozing or grinding. Grinding leaves a rough surface which takes copper foil tape less well than clean cuts. It also increases the possibility of devitrification when fusing.

Tapping leads to shells as in the centre left of this picture

The least satisfactory method of running the score of a circle is to tap under the score before breaking the circle out.

 

In conclusion, running the scored circle first and without tapping will provide you with the cleanest cut circle. This will be so however your make your relief scores. But, making relief scores before running the circle score risks breaking through the circle.

Finally, it should be noted that cutting out larger circles is easier than cutting small ones. Better, cleaner results are obtained because the curves are less tight.

 

Measuring for Circle Cutting

Often there is uncertainty about which way the cutting head should be placed on the bar of the circle cutter to get the right diameter.  And the distance markings on the arm often get worn away.

It is for these two reasons that I have given up trying to get the right diameter circle from the measurement markings on the arm of the circle cutter.  Instead I measure and mark out the centre point and the radius of the circle directly onto the glass.  Only a few tools and supplies are needed.

Glass, measuring stick, marker pen, oil and circle cutter are all that are needed to measure the circle

First you need to decide on the centre point, leaving at least 2cm at the edge of the piece the circle is being cut from to allow a clean circle to be broken out.

The four black does are for measuring from the edge to the axis

Once you have done that, mark an axis at right angles at the centre point.

This shows the axis established and the radius marked out on the left.

Measure the radius from centre line , mark that on the line. 

Place suction cup at the centre of the axis. In the case of the cutter I use there are four markings to assist in the centering of cutter.

Move the cutting head along the arm until the wheel sets right on the radius mark.  I find that getting low helps a great deal in seeing the placement of the wheel.

Tighten the locking nut.

I put a drop of cutting oil on the wheel, so that in a preliminary run, I can both see where the scoring line will be and be sure everything is far enough away that the arm does not hit something on the way around.

Score the circle, making sure your fingers are only on the knob. If your fingers slip down, they can loosen the locking nut.  Some people score in an anti-clockwise direction to ensure they do not loosen the locking nut.  An anti-clockwise motion means that if your fingers do touch the nut, it will be tightened rather than loosened.

This photo shows the circle scored and the spacing between the edge of the glass and the score line.

Once you have set the cutting head on the arm of the cutter, you can cut as many circles as you wish of the same size without needing to do further measurements.
 

Further information on breaking out the circle is given in this blog post.

Centering Holes for Drilling

When using larger core drills, it is not possible to see the centre point for drilling. So a different arrangement for marking the place to be drilled is required.  This example is to locate the hole for a clock spindle accurately between the points marked by the stringers.


Find the centre point and extend lines at right angles to each other across the centre point.



Then measure the radius on each arm and make a perpendicular mark on each of the radial arms. Depending on how long those lines were, you have something approaching a box.

Approximately center the drill over the center point. In this case it is a portable drill, but the principle is the same for a pillar drill.

Lower the drill bit over the hole. The radius marks allow the drill operator to see the edge of the hole and use any three of the marks to centre the drill bit within the hole.


This procedure ensure the accurate positioning of the hole.  This is especially important when fitting to existing fixing points rather than making new ones.

Other posts on drilling are here.

Measuring Openings

Measuring rectangular openings

Timber and metal openings can vary in their dimensions. So measure each opening at the top and bottom, and at the left and right as well as the middles of each side and top and bottom.

To be perfectly sure the window has right angles – is square – take the diagonals from opposite corners, e.g., top left to bottom right and top right to bottom left. These measurements should be within 5mm of each other for the window to be considered square.

You can check for ”squareness” with a try square, although that is not completely accurate.

Templates

If it is not possible to tell where the right angles of the opening are, a template is called for. The material to be used for taking templates should be stiff, easy to cut, unaffected by moisture, and relatively inexpensive. This eliminates paper and some cardboards. If you can find stiff corrugated cardboard this works well. Mounting board works well too, but is expensive. Foam board is excellent, but also expensive. Hard board or other thin pressed board is inexpensive but difficult to cut with a knife. Thin plywood is also a good material for templates, especially if the opening is relatively regular. The more complicated the opening, the more cardboard, mounting board, or foam board becomes useful for its ease of shaping to the opening.

What ever material you use, you must mark which is the interior and exterior and for further checks, which is left and right. Fit this template into the opening to make sure it fits into the opening smoothly. This template will form the external extent of the built window when it is installed into an opening with a rebate.

Where the window is to be fitted into a channel, as in stone, you need to make the template of stiff material so you can determine the panel can be installed and that there will be enough of the panel within the stone channels to ensure the stability of the window in the future and still be able to manipulate the leaded panel into the opening.

Squaring Panels

When building leaded glass panels to be placed in existing wooden frames, you need to make sure the panel is as square as the opening in the frame. The first requirement is to make sure your cartoon is squared, or has right angles at each corner.

I use a “roofing iron” as it is called in the UK. It is a steel tool about 600mm on one side and 400mm on the other. Its original use was to work out the pitch of roofs and check the same pitch was maintained all along the building. The first important thing – now that all roof trusses seem to be prefabricated – is that they still are in production. The second thing is that they all have a fixed right angle.

Using this roofing iron will ensure your corners on the cartoon are right angles. This helps in the drawing of the cartoon as you only need set the iron on the base line and draw the verticals without having to measure the width higher up the cartoon. Of course you should check that the width is still correct at the top, just in case there has been a slip.

Then you have to stick to the cartoon.

When you are setting the battens to ensure the sides are held where you want them while you continue with the leading, the roofing iron again will ensure that you have placed the battens at right angles. You choose which line is to be your base, and nail or screw it into place. Ensure it is exactly parallel to your cut line and then align one side of the iron against it. Place the other batten snugly along the length of the other leg of the iron and you know you have a right angle.

When you are completed leading, but before soldering you can check on the accuracy of the angles by using the roofing iron again at each corner to check on the “squareness” of the whole panel. If the panel is out of square, you can tap on the battens not yet nailed/screwed in place to ease it all back into “square”

Templates of openings, 6

When the opening is in stone, slight variations occur in the process of taking a template. The main difference is that the rebates are concealed. The rebates are slots into the stone. Thus, the template must slip into the slotted rebate. In these cases, the stiffer the material being used to take template, the better. Usually, thin plywood is the best material, as it has to be manipulated many times and in ways similar to the final panel.

Things are further complicated, as tracery is more common in stone than in timber framed openings. A complex opening shape may require two or more parts to enable the panel to be inserted. The taking of a template will help greatly in figuring out how the panel will be inserted into the opening.

Additionally, when the template is in position, you should mark the visible portion of the opening onto the template. Mark which is the inside and which the outside. Finally, mark on each template which side has the deeper slot as this will help in installation.

Templates of Openings, 5 – Irregular Openings

Irregular openings such as trefoils and other tracery need to have templates taken with consideration on how the final panel can be put into the opening.

In the cases where the whole of the rebate is exposed, it is normally possible to put the panel in as a single whole piece.

So, the template is taken as for any other opening. It is more complex and time consuming as there are so many more sides than in a simple rectangular or circular opening.

Templates of openings, 4

Round headed openings can be considered as a special case of a circle.

The horizontal you must find is the shoulder of the window. This is the place from which the curve springs on each side. The opening is generally vertical up to this point and then begins the curve.

You need to make sure you have marked where this shoulder is on the template. You should indicate any reference points from the frame onto the template.

The join to the lower part of the window must be made obvious. Normally there will be an overlap between the lower rectangular template and this approximate half circle. You need to mark where this overlap occurs, if you do not fasten the two sheets together. This can be done by marking across the two sheets in a few places. This will enable you to join them exactly back at the studio.

Templates of openings, 3 - Circles

Occasionally the window is circular and sometimes an oval. In both cases a template is important. The circle rarely is exact. Take the template in the normal way and then ensure you mark the verticals and horizontals for the opening. You often can use the jointing in the woodwork to help with these. Also mark any other reference points from the opening. Finally, mark which is the outside and which the inside.

This procedure will ensure that you will be able to fit the panel into the opening.

Templates of Openings, 2

Irregular rectangles

If you have found or can see that the opening is not a true rectangle and cannot determine where any right angles are, you need to take a template.

The objective is to make a piece that will fit into the opening without bending or being too small for the space. It will be the same size as the finished panel and so you will be able to put the finished panel into the opening without needing to trim or expand the panel.

First, trim the sheet of material you have chosen to use to a size a little larger than the measured size. Place the uncut side along one of the long sides of the opening. If the opening is a portrait format, place it on the right or left side as convenient to you.

Next, adjust the bottom by marking a line on the sheet. This is where a second person is very useful. One person can hold the sheet in place on outside of the opening and the other do the marking from the inside –in the case of the rebate being on the outside and vice versa if the rebate is on the inside. The marked line should be as close to the edge of the rebate as possible. The special case of an opening in stone will be dealt with separately.

Then take the sheet to a place where it can be safely cut. A long metal straight edge and craft or “Stanley” knife are often the best aids to cutting straight lines. Replace the sheet into the opening after cutting, and make any adjustments to the size and angles of the sheet at the bottom by marking and cutting as necessary.

When the side and bottom are adjusted, start on the other side. Proceed as for the bottom.

When the side is finished, start on the top.

Finally, present the whole sheet to the opening to make sure it slips into place with no snags, or bending of the sheet.


It may be that the opening is too large for a single sheet. In that case you will need to work with two or more sheets and try them together for the final fitting into the opening. You can put them together in the window. You can fasten them together with tape or other fasteners to make one sheet. You can also make two parallel lines both at angles and at intervals across the sheet so that when you get back to the studio you can exactly reproduce the full sheet by matching the marks and then firmly fastening them together. This makes transport of large templates much easier.

You will know that a panel made to a template made in this way will fit into the opening, no matter how irregular the opening may be.

Templates of Openings, 1

If you have an irregular opening, it may be best to take a tracing of the opening. Usually this will be in a larger opening and so a helper may be necessary to hold things.

The material used to take the template must have a few characteristics:
- It must be stiff enough to have the minimum possible bend over the width or height of the opening
- It must be easy to mark with a pencil or other implement
- It must be easy to cut or shape
- It should be light to make it easy to lift it to the opening for the many adjustments that will be required.


A number of materials can be used: stiff card, mounting board, corrugated cardboard, thin plywood, and many other sheet materials. I have found stiff corrugated cardboard easy to use.


More information in this series is at:
Irregular rectangles
Circles
Round headed windows
Irregular shapes
Stone

Measuring a Rectangular Opening.

1 - Measure at both the top and bottom for width.

2 - Measure at the left and right sides for height.

If it is a tall or wide opening measure at intervals and at least in the middle of each side.
So far so good. But how do you know that it is a rectangle rather than a parallelogram? Measure the diagonals – bottom left to top right and top left to bottom right. If these measurements are equal or +/- 5mm you can consider the opening to be a rectangle.

With bigger variations you may set out the cartoon using the measurements for the opening. Still, you need to know where the right angles are, if there are any, to be able to set out the cartoon to properly fit the opening.

So you may wish to take a template of the opening.

Window Measurements

There are a number of measurements that are critical for a good design and a sound installation of window panels.

1. Tight Size: This is the full size of the glass opening with no allowances for expansion and contraction. In a wood or metal rebate frame one would measure from steel to opposing steel or wood to wood; in a stone groove installation, from the bottom of one groove to the bottom of the opposing groove. Depending on the size of the opening, this measurement should be checked in multiple areas; at a minimum at the top, bottom and middle horizontally and at the left and right jamb.

2. Sight Size is the daylight opening or the largest opening that allows light to pass through.

3. Rebate or groove details. With a rebate frame, the depth and the width of the rebate must be measured, as well as the interior return if round bars will be used (this dictates what size bar will fit and how long the bar should be). These dimensions are also necessary to determine the dimensions of the retaining molding if one is to be used. If it is a groove, the depth of the groove and the width of the groove (measured from interior to exterior) are important.

4. Panel Size. This is the ideal size of a panel that will be installed into the opening in question. Typically, this will be a function of the tight size less 3mm in both width and height for a leaded glass panel, to allow for expansion and contraction. One must also recognize if the size varies throughout the frame and make allowance for this as well. With dalle de verre, you need a deep rebate or groove and allow at least 5mm in both directions for expansion.

5. The depth of the rebate or the width of the groove are critical measurements. To allow for a proper installation, allow a minimum of 13mm to be added to the thickness of the panel to provide room for a proper putty fillet.

Based on comments from Art Femenella

6. When measuring older openings and especially doors, measure the diagonals in addition to all the other measurements. This provides a check of all your other measurements and also tells you whether the opening is a true rectangle or parallelogram.