PeterH

You may find this of interest, especially the sections What is Heatwork?  and  Put Witness Cones in Every Firing.
Temperature vs Heatwork – Why We Use Witness Cones
https://suemcleodceramics.com/do-you-put-witness-cones-in-every-firing/

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing.
https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf


Did you previously fire by observing the cone drop or just use it as record of the firing?

Looking at the cones, that's direct feedback.
With the bung having to be in, then getting stuck, relating the pyrometer readings and clock times (for temperature rate of change) against the cones might be the best you can do.
That's assuming you have a pyrometer - from there, take notes on the readings (time and temperature), then compare against the cones afterward.
After a few trials, you will be able to repeat your heat work by watching the pyrometer and the clock!
...that's assuming that your pyrometer is consistent and reasonably accurate.
Heat work
Take a long look at a cone chart, noting the relationship between time and temperature.
In short, it takes a higher temperature to slump the cone when the rate of change is faster; the cone will slump at a lower temperature when the rate of change is slower.
 
"... Are we talking about welders glasses for IR & UV protection?"
Yes, also protection against any flying bits! The odds that something would go ping and fly into one's eye are minimal, yes, but not worth chancing.
For sure, don't look into the yellow hot kiln without protection!
I'm seeing several welding supply shops in northern UK, where a decent pair of Fronius UV wraparound glasses are about fifteen pounds.
Added: or, as Bill points out, these (IR and UV)
Warrior Safety Spectacles Smoked (weldsafe.co.uk)

Even if you just get the little glass plate that goes in the welding visor, that’s better than going without. 
If you’ve got to have the pieces for Christmas,  it is possible to do it, but it’ll be under less than ideal conditions for your kiln. The good news is, once you replace your elements, and likely the relays too, you’ll have essentially a brand new kiln. Insulating bricks don’t expire. 
 

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing.
https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf


Did you previously fire by observing the cone drop or just use it as record of the firing?

Hi np.04,
Is your firing governed by a controller?
If you are able to set cones where they can be watched*, that might be a real help - you could start your cool down when the heat work is completed.
*be sure to wear kiln glasses against the harmful rays and any flying bits!

As someone who’s been firing an analog kiln outdoors in the Canadian winter for the last few years, I can confirm that -9 is not enough to make a difference in your firing speed. I don’t start getting much slow down until about -25 or so, and even then, it’s only about a 15 minute difference.
Since your elements are likely worn, you can still continue, but definitely include the cone packs this time, and make sure you’re checking the firing every 15 minutes for the last 2 hours or so. You want to be able to turn the kiln off yourself once the cones fall.  If you fire overnight to avoid peak electricity rates, maybe start it about 2-3 hours later than you would normally, so you’re awake for the end. 

Lots  of pix at https://tinyurl.com/2rkyb27z ... two  particularly lo-tech ones are

It would be interesting to know if  "fingerdips" work
Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
Apparently suckers can be useful in some circumstances

@PeterH  his is similar to one he has  
Magswitch
I almost wonder if it might be too strong 
 

A cone temperature chart confirms that this is a real effect. A cone number relates to an amount of heat-work and the maximum temperature needed to achieve this depends on how fast the kiln is heating towards the end of the firing.
https://www.overglazes.com/PDF/Orton-Cone-Chart-C.pdf


Did you previously fire by observing the cone drop or just use it as record of the firing?

I haven't heard of this before, do you have a trade-name for them?

This explains a/the mechanism, and may be a starting point for some DIY-er . 
How to Build an ON/OFF Switch for a Permanent Magnet
https://interestingengineering.com/video/build-on-off-switch-for-permanent-magnet
The ones I saw on the net looked expensive, and possibly overpowered for the application.

I’ve been using neodymium magnets as glazing tools too, thanks to @Min suggesting this to me several years ago. Can confirm that this is a game changer! I have my inside magnet wrapped in fabric held on by a rubber band. The fabric helps cushion the magnet so it doesn’t disturb the liner glaze, and the rubber band “stem” is like a handle. My outside magnet has a big eye bolt as a handle. 
I’ve learned to keep them stored on two different shelves, otherwise they will seek each other out and stick together, and it takes a lot of strength to pry them apart! 

Glazing tools and tips brought up in this thread, starting a new thread as this should probably be in the Studio Operations section.
This is how I use magnets when glazing, no tong marks, drips or finger marks on the glaze. It’s been a game changer for me with the pots I dip. Flat bottoms on pots work the best but you can do rounded bottoms, it’s just a bit less stable while dipping. 
- liner glaze the inside like usual
- rare earth magnet(s) wrapped in a scrap of plastic with twist tie inside the pot. twist tie is used as a handle. keep the twist tie below the rim of the pot
- another rare earth magnet(s) with a flat head bolt for a handle on outside of pot
- fishing magnet used for larger pots, they come with a handle so don't need a flat head bolt for a handle
-number of magnets needed determined by pot weight (and base thickness). I try to use just enough to be adequate to pull the pot from the glaze but not more or else they can be fiddly to remove. for tiny pots I use 1/2 pieces of broken magnets.
- hold the pot by the bolt and push the pot into glaze then lift it out. (rotate the pot slightly to have any excess glaze roll around the rim) 
- when the pot is dry enough to touch at base area turn it over and remove magnet(s) from either side. 20 seconds or so for most glazes. I hold the pot on the outside bottom of the pot when flipping over 
- you can slide the pot onto the edge of a counter while removing the outside magnet if you don’t want to wait for the glaze to dry enough to hold onto the pot, or stuff your hand inside the pot to move it to a ware board.
- when I first started glazing like this I got a couple blood blisters, don't pinch your fingers, those magnets are strong!!!! I use the ones from Lee Valley tools. (to separate them use an edge of a counter and push them apart)
Anyhow, that’s it, simple and it works. I tried to think of a way to market the magnet glazing but life got in the way.

I’ve seen @OldForgeCreations have some words about using dent pullers for this purpose. He’s stated that not all dent pullers are created equal, and that if you use a sub par one, they will detach. 

I disagree with this one. I’m not going to claim that all things are possible in all kilns, but there are ways to get vibrant colours in fuel burning reduction firings, and there are LOTS of examples of folks that make work in a cone 6 electric that have a more subtle, earthy palette. While it isn’t common, you can use scrap pallet wood simply as a fuel to get to any temperature you want, although it’s usually associated with things like ash built up over days. Remember that a lot of firing techniques evolved and continue to evolve because people use the materials that are most readily available. Like Min, I cannot access a gas kiln either. Mark however, pays far more for electricity than he does propane, so that’s a deciding factor in what he uses. 
I think everyone here is answering this question with their biases, based on how they fire themselves, and which glaze chemistry they prefer. Fuel firing vs electric has an effect on the kiln’s atmosphere, which in turn affects certain chemicals in the glaze. Which chemicals? It’s pretty situation dependant. Your kiln is a tool used to apply heat and atmosphere. That’s only 2 variables among many.
I think the best way to help @kevinpleongis to first ask them some questions so we can advise them better. Kevin, what kind of work are you making, what temperature are you used to firing to, and how much experience do you have? How much work are you making, and do you anticipate that going up or down in the next 5 years-ish?  Our answers will be different based on how you plan on using your kiln. 

I have a couple of those as well for dipping. I thought I could do it without waxing an area. Ha. The dent puller stuck nicely on the bottom of my unwaxed piece, but after dipping in the glaze, the dent puller detached and my bowl landed in the bottom of the glaze bucket. Of course, I tried it again to see if it was just a fluke and yes it did fall off.

Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
I bought a couple of dent pullers for that reason.  It only works on waxed, very smooth, flat bottoms.  If that describes your pots, it will probably work.   
 

Lots  of pix at https://tinyurl.com/2rkyb27z ... two  particularly lo-tech ones are

It would be interesting to know if  "fingerdips" work
Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
Apparently suckers can be useful in some circumstances

Lots  of pix at https://tinyurl.com/2rkyb27z ... two  particularly lo-tech ones are

It would be interesting to know if  "fingerdips" work
Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
Apparently suckers can be useful in some circumstances

And I understand that slow-cooling can add to the available effects from some glazes in electric kilns, by giving them more time to develop crystals.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln

From https://digitalfire.com/picture/bestenazoh

Firing for Atmospheric-like Effects in an Electric Kiln  https://tinyurl.com/cf7ef94y
Fuel-burning kilns tend to be much larger than electric kilns. Because of their size, they usually have a slower heat rise, a soak at the top temperature, and slower cooling cycles. If you want similar results from an electric kiln, especially when firing to cone 6, one of the most important things you can do is emulate the heating and the cooling cycles of larger kilns. This means slowing the temperature gain to about 100°F (38°F) an hour during the last several hours of the firing, soaking the kiln at the top temperature, and then down-firing to slow the cooling cycle.

Electric kilns are built with thinner insulation and legs (to allow for air circulation), and they cool very quickly, especially at higher temperatures. If you are seeking buttery, matte surfaces but have trouble achieving them in the electric kiln, it is most likely due to fast cooling. Matte surfaces are usually caused by microcrystal growth during cooling, and, if the cooling cycle is too steep, there isn’t enough time for crystals to develop. In extreme cases, I’ve even seen matte glazes go glossy and transparent.
... note that the 100F(38C) above is "finger trouble", see 
What is the difference between fahrenheit degrees and degrees fahrenheit? https://digitalfire.com/picture/2704

Lots  of pix at https://tinyurl.com/2rkyb27z ... two  particularly lo-tech ones are

It would be interesting to know if  "fingerdips" work
Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
Apparently suckers can be useful in some circumstances

Lots  of pix at https://tinyurl.com/2rkyb27z ... two  particularly lo-tech ones are

It would be interesting to know if  "fingerdips" work
Use a car dent puller to dip your pots in glaze! https://tinyurl.com/2p9y6bn3\\\\\\\\\\
Apparently suckers can be useful in some circumstances

And I understand that slow-cooling can add to the available effects from some glazes in electric kilns, by giving them more time to develop crystals.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln

From https://digitalfire.com/picture/bestenazoh

Firing for Atmospheric-like Effects in an Electric Kiln  https://tinyurl.com/cf7ef94y
Fuel-burning kilns tend to be much larger than electric kilns. Because of their size, they usually have a slower heat rise, a soak at the top temperature, and slower cooling cycles. If you want similar results from an electric kiln, especially when firing to cone 6, one of the most important things you can do is emulate the heating and the cooling cycles of larger kilns. This means slowing the temperature gain to about 100°F (38°F) an hour during the last several hours of the firing, soaking the kiln at the top temperature, and then down-firing to slow the cooling cycle.

Electric kilns are built with thinner insulation and legs (to allow for air circulation), and they cool very quickly, especially at higher temperatures. If you are seeking buttery, matte surfaces but have trouble achieving them in the electric kiln, it is most likely due to fast cooling. Matte surfaces are usually caused by microcrystal growth during cooling, and, if the cooling cycle is too steep, there isn’t enough time for crystals to develop. In extreme cases, I’ve even seen matte glazes go glossy and transparent.
... note that the 100F(38C) above is "finger trouble", see 
What is the difference between fahrenheit degrees and degrees fahrenheit? https://digitalfire.com/picture/2704

And I understand that slow-cooling can add to the available effects from some glazes in electric kilns, by giving them more time to develop crystals.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln

From https://digitalfire.com/picture/bestenazoh

Firing for Atmospheric-like Effects in an Electric Kiln  https://tinyurl.com/cf7ef94y
Fuel-burning kilns tend to be much larger than electric kilns. Because of their size, they usually have a slower heat rise, a soak at the top temperature, and slower cooling cycles. If you want similar results from an electric kiln, especially when firing to cone 6, one of the most important things you can do is emulate the heating and the cooling cycles of larger kilns. This means slowing the temperature gain to about 100°F (38°F) an hour during the last several hours of the firing, soaking the kiln at the top temperature, and then down-firing to slow the cooling cycle.

Electric kilns are built with thinner insulation and legs (to allow for air circulation), and they cool very quickly, especially at higher temperatures. If you are seeking buttery, matte surfaces but have trouble achieving them in the electric kiln, it is most likely due to fast cooling. Matte surfaces are usually caused by microcrystal growth during cooling, and, if the cooling cycle is too steep, there isn’t enough time for crystals to develop. In extreme cases, I’ve even seen matte glazes go glossy and transparent.
... note that the 100F(38C) above is "finger trouble", see 
What is the difference between fahrenheit degrees and degrees fahrenheit? https://digitalfire.com/picture/2704

Hi Gonen,
Welcome to the Forum!
Generally, contains manganese and iron, sometimes a stain (which likely has manganese in it)*, perhaps iron chromite (likely a rarity these days).
Per my experience (one twenty-five pound bag), be prepared for a few cycles of testing.
Stay well clear of any fumes whilst firing.
 
Tony Hansen's article on manganese and clays: Manganese in Clay Bodies (digitalfire.com)
A forum thread on black clays: Black Clay Advice - Clay and Glaze Chemistry - Ceramic Arts Daily Community
More links:
clayart - thread 'black clay safety inquiry' (potters.org)
clayart - thread 'black clay' (potters.org)
 
*credit Min, here: Scarva Professional Black Clay bodies - Clay and Glaze Chemistry - Ceramic Arts Daily Community