I am retired from teaching ceramics more than 40 years. I still enjoy working with clay on a regular basis. I still seem to have above average lungs for my age, but I have tried to be careful about what I breath and I have never smoked. We are now much more informed about dust and fumes than we were when I was in college and when I started teaching. Clay is basic and fun. It has been used by kids for thousands of years. It is ideal for lots of creative experimentation.
NEW ELECTRIC KILNS
On a new electric kiln, or with new elements in an old kiln, it is good to fire it hotter on the first firing to allow the elements to "settle" down (relax) in the grooves. Otherwise, if a hot firing is not done, the elements are more apt to pull out of the grooves and start to sag into the kiln itself instead of into the groves. In my opinion, firing it even hotter than 04 (up cone 6, about 2250 F) would be better. No pots need to be in the kiln and if it empty it can be a fast firing. I think the manufacturers should do it themselves. If the kiln company wants it fired to a certain cone, they should send along a cone to use. Otherwise, we can order them.
The first firing does release a few more fumes from the elements, brick mortar, etc. Cracking the lid open until you see it begin to be red hot (about 1,000 F) allows these fumes to exit the kiln. This can also extend the life of the elements a bit. If we have much junk in the clay (such as newspaper cores in sculptures), it is best to crack the lid until the smoke clears. This extends the element life. If it is a fairly clean load, I would not crack the lid because it wastes energy.
Firing full speed with all elements on high is the most economical way to get up to temperature. However, this is only feasible with an empty kiln. Even completely dry clay explodes from steam (just from atmospheric moisture) as it passes the boiling temperature of water (212 F at sea level and at lower temperatures at high altitude locations). In my experience, full firing power and speed can be used once the kiln has reached 1,000 F (red hot when looking into the peep hole). Firing slower will waste energy. Clay that has been fired once (bisque ware) does not explode, and can safely be fired much faster than greenware. If the piece are freshly glazed their is a slight chance that the stream can blow the glaze off the bisque ware if it is heated too fast.
The computerized controllers are really helpful in that they stop and hold the firing just prior to the boiling temperature.
The top hood or bottom sucking fan vent should be monitored to be sure the majority of the fumes exit the building. Otherwise, fire only when the place is not occupied. Some people used to put kilns in their home basements without vents. Depending on what has been fired, this can ruin a house because the fumes containing heavy toxic metals condense and leave a thin coating on all the floors, walls, etc. in the house. Future generations of babies can be brain damaged as the result of a grandmother's hobby. Most chemicals are not so bad, but some of low fired china paint colors, lead, and lusters are particularly toxic. So far as we know, these are not in the materials now being marketed to schools. If you are in an old school with old materials, you need to check the safety of the materials. I found uranium oxide in a high school glaze cupboard. Lead was very commonly used at one time.
HAZARDS OF OVER-FIRING
Sooner or later I have seen every kind of kiln sitter fail. Some get gummed up from fumes. Some get out of alignment. The wrong cone can be mistakenly put in, etc. Mechanical timers also fail if the stop working in the middle of the firing. The least reliable system is to depend on a myself of another living person to be totally responsible to turn off the kiln. Teachers have a thousand things that can come up during the day that can distract them. I have not used computerized controllers long enough to have one fail for me yet. I figure that there is always the chance that I could program it wrong, read the instructions wrong, push the wrong button, etc.
In practice, I always try to use some kind of backup plan (set an alarm, use two shut off systems, have another person charged with helping, etc.). I still wear a watch, so I stick a piece of masking tape with a K on it on my watch dial until the kiln is off. Most cell phones have alarms, but I don't always carry a cell phone. Over the years, I have seen too many over fired electric kilns. Every one is a costly learning experience, but none has burned down any buildings or injured anybody.
If a kilns seems too close to a wall, install a sheet of metal or cement board an inch from the wall with an air gap at the bottom to allow air to flow and cool it. The wall will stay much cooler if their is airflow. Never store any wood, paper, or anything flammable any place near the kiln or where it could fall onto the kiln. Use metal shelving in the kiln area for pots, kiln shelves, and other non flammable storage. Install kilns on concrete floors. Remove flammable floor coverings under and near the kiln.
Plain clay and glaze dust is not like lead poisoning. It does not ruin the brain. The silicon dust is very fine and heavy, so it just settles in the lungs until the lung tissue eventually dies if you breath enough of it. I did not allow sandpaper or dry smoothing of clay in my classroom. I explained that they need to smooth the clay before it dries. They could use sponges, moist cloth, etc. to smooth clay. If they want to smooth dry clay or bisque ware, it had to be done outside or in an exhausted place. I showed them ways to clean the floor, tables, wheels, etc. with big wet sponges that are just as easy as using counter brushes. When mixing dry stuff I showed them how to lay it into the mixing container very slowly to avoid puffs of dust. Even if one student is wearing a good dust mask, the rest of us are in the room too. We have a door, so I send them outside in freezing weather for the dumping of dry materials into a container. Glaze spraying and dry clay mixing must be done in an exhausted booth, or exhausted space, or outside. The person mixing or spraying might still need a mask in an exhausted area or outside.
If message was too short and you want more depressing details, I have a web page labeled Hazards in Ceramics - by Bartel that you can search.
We have a daughter and a son who are scientists. As kids, they learned the joy of experimentation by making all kinds of stuff from clay. In two weeks I will be working with a group of friends and former students. We will fire a large outdoor wood fired kiln. We bring our bisque and load kiln on Thursday morning. We fire continuously until Sunday afternoon. This weekend I am bisque firing some pieces to be glaze fired in the wood firing. We get ash glaze from the wood. It is only the second firing for the kiln and every firing brings many wonderful mistakes and discoveries.