Crucible & other furnaces & kilns

Hot glass artists make their clear glass by melting a mix of raw materials called ‘batch’ in what is called a crucible furnace.

What's a crucible?

A crucible is a pot made of refractory materials which can withstand not only the high temperatures, but also chemical attack by the materials being melted.

Why is it called a ‘furnace’? Industry jargon dictated that kilns cooked solids whilst furnaces produced liquids. Hot glass artists make molten glass, hence furnace. See, it’s just a name.

Crucible melt furnaces are widely used in industry, for melting aluminium, lead, glass and other materials.

In the smaller size units used for glass, the heat, from either electric element or gas flame, is usually introduced low down so that it flows through the bottom and sides of the crucible into the contents.

The batch used by hot glass artists contains some quite aggressive fluxing agents to lower the melting point of silica, (from around 1700°C to around 1200°C, 3100^oF to 2200^oF) so the crucible used in a glass making furnace has special properties; which makes them expensive.

These crucible furnaces are usually gas fired using LPG or natural gas, are kept running indefinitely; often being recharged at night so that a fresh batch of glass is ready for the next day.

Crucibles or flower pots in kilnforming

There is one area in kilnforming where similar techniques can be applied to make use of offcuts of scrap glass, by melting them in simple clay vessels such as flower pots.

The refined glass is free of the fluxing agents, so the clay pots won’t be attacked.

There are two general arrangements using small pots as crucibles:

• Using the conventional flower pot with a hole in the bottom, arranged so that the molten glass runs out through the hole and through a hole in the floor of the kiln. 
• Using small laboratory crucibles and gathering the molten glass from the top.

Both use a type of top loading kiln with elements in the walls but not in the lid.

Fig 1

Fig 1 shows a simple square top loader arranged for the first purpose and to accept a nominal100mm (4”) flower pot as a crucible.

This kiln is used to make stringer and thread from glass offcuts for use as decoration on kilnformed objects, so is mounted high on a stand to allow the molten glass to be manipulated by the operator as it flows from the hole in the kiln floor.    

The molten glass can also drop into a bucket of water where it will shatter to make frit.

Fig 2

For simplicity of construction, the element is in the walls, so the pot has been propped up off the floor to concentrate the heat toward its bottom.

The pot is positioned centrally above the  hole in the floor. The kiln lid is close fitting to minimise heat loss due to air flowing straight through from bottom to top.

The floor and lid in the unit shown are made of fibreboard but can also be made of IFB cemented together.  The fibreboard happened to be available. For the amateur builder, IFB will be cheaper. It will also be less affected by glass spilling onto the floor.

Fig 3

The pot is charged by placing a sizable shard over the bottom hole and then filling with smaller scrap pieces. Smaller pieces will melt faster than the large shard, so that there should be a reasonable quantity of molten glass above it when the plugging shard finally melts. 

The charge shown is a mixture of compatible offcuts of various colours which will melt and flow together, producing variegated threads or frit.

Fig 4

Temperature in this case is controlled by a single ramp and soak controller, as shown in Fig 4. For more on them, see Simple kiln controls.

The set temperature shown is 970°C (1780°F), but glass flow will be best at temperatures around 1100°C (2000°F). The temperature is easily controlled up or down by adjusting the set point. Just remember to enter a soak time of many hours before starting; otherwise the kiln could turn off in the middle of the fun.  

Fig 5

Mounting the kiln on a tall stand makes it convenient for the operator to manipulate the emerging stream. The thread can be grasped with long nosed pliers in one hand, twisted into shapes and cut free using side cutting pliers in the other hand.

The hot thread can then be laid on a wooden surface where it will cool without sticking.

The stream can be wound around short pieces of wooden dowel to make coils which can be later cut into sections to make circles or other shapes. The options are enormous. When fired onto a base glass as decoration they will collapse into a more two dimensional form.

Wood is the best material to use as a former or support for cooling glass thread, as it will not draw the heat out of the glass too rapidly, but neither will it be unduly affected by the small heat content of the thread. Minor charring is the most that will occur.  

Yes, that’s me in the photo; and I’m not wearing safety gloves.

Many people have used this kiln. Initially gloves have been used but quickly discarded by all as being cumbersome and an impediment to manual dexterity. Nobody has been burnt. Sometimes too much safety can be an overkill and a menace.

If the glass was touched, the heat content of the portion of thread momentarily contacted would be quite low. There is more risk to people picking up strands from the bench before they have cooled.

Fig 6

Fig 7

The contents of the crucible is small, so to compensate for this a method was devised to allow it to be recharged whilst at temperature.

The lid is raised as shown, right back against the stops, the recharging chute installed and a container of charge poured in. This will flow straight into the pot with little risk of any going outside and onto the floor.

Crucible life

Because of the different COE of the crucible and the material being melted, the flower pot can be used only once with safety. They may crack at any time from shortly after the pot reaches room temperature to a week or so later. Rarely will the pot survive indefinitely, and if it does it should not be reused; it could be on the verge of cracking and break when being reheated. The cost is so low that throwing them away is not a great loss.

Flower pots are no longer made in Australia, and pots without holes aren’t imported. Capable potters can hand throw them, but they can be costly.

Suitable alternatives, of the type used by laboratories for processing mineral samples, can be sourced from scientific laboratory suppliers. They are made from numerous compositions, so one with COE compatible with glass should be available. Laboratory supply houses may be the best place to try.

Top loading crucible furnace

In another application described below, the vessel is used truly as a crucible, so pots without holes are required.

The kiln is a more conventional square Australian style top loader, with elements in the walls, and is used to maintain a supply of molten glass for lamp working; in a manner similar to that followed by hot glass artists with their full size crucible furnaces.

  Small gathers can be made, simply by lifting the lid; using a thin piece of wire instead of a heavy punty.

To protect the operator, a refractory batt is fitted inside the brick walls has been fitted inside the brick walls and rests on the top of the crucible. A hole matching the inside diameter of the crucible allows access to the glass but blocks most of the radiant heat when the lid is lifted.

Innovative thinking indeed.

For those considering a similar project, a few points:

• Keep the element coils toward the bottom of the walls.
• For long life, the element should be designed for 1200°C (2190°F) operation, but it may be too long to fit in the short length of slot, so a shorter element suitable for lower temperature operation may need to be used, even though its life will be reduced.
• The heat baffle can be made from the thinnest kiln shelf available, with a centre hole cut by water jet.
• Even though the temperature is getting toward it’s limit, a type K thermocouple will work OK and be the cheapest. However, Type N has a higher rating and will last longer.
  
• Use a digital temperature controller with push button Up and Down control. Most instrumentation suppliers can provide them. The one shown in Fig 4 above is a BTC 9090 and can accept Types K and N input. Use search to find more info on this device in the site.