How hot is that?

Knowledge of temperature plays an important part in successful kilnforming operations;

• Data supplied by a glass maker details the temperature at which certain viscosity related points occur.
• This information forms the basis for firing patterns used to control kiln temperature.

The temperatures given refer to that of the glass itself.

On the other hand, the temperature displayed on a kiln instrument is that at the tip of the sensor probe which is located somewhere within the kiln, but almost never actually on the glass.

The temperature indicated will NOT be that of the glass.

The temperature at various points throughout a kiln can differ widely one from another. Hot air rises, so the temperature at the top of a kiln will generally be higher than at the bottom. Thus, in a kiln with two shelves, items on the top shelf will usually be hotter than those on the lower one.

At any given level in a kiln the temperature in the centre of a shelf will tend to be slightly higher than at the sides.

The essential part of a temperature sensor is the ‘thermocouple’; two wires of dissimilar metal joined together to form a ‘hot junction’. To protect the hot junction it will frequently be enclosed in a metal or ceramic sheath so that the junction itself is within less than 10 millimetres of the tip of the sheath.

To simplify the loading of the kiln the thermocouple will extend through a wall only a short distance, usually less than 100 millimetres but sometimes less than 50mm. This may place it well away from the hottest  part of a shelf.

• The displayed value will be that at the hot junction and not anywhere else in the kiln.
• It is unlikely to represent the actual glass temperature.
• Moving the thermocouple can give a different reading. If you have alternative positions for the thermocouple, make sure you are aware of, and understand the effects of, the change in position.
• It does not indicate the temperature with great accuracy, but it’s repeatability is excellent. 

Leave it in the same position, load the kiln the same way and fire to the same indicated number and you will get very good repeatability; unless the Kiln God intervenes

At that stage the temperature indicator may as well read kilo’s of potatoes, slices of bread, any old thing.

Don’t assume it shows the actual temperature of the glass. It may be close, but it may not.