effect of different venting or glaze finishes in electric kilns

[MadMetal]

So this question is strictly about the effects of different vent systems with electric kilns in tweaking depth and color of glazes . It assumes studio safety venting already handled. I also appriciate there are many recipes and techniques with glazes to obtain certain effects

Given that some of the variables in glaze texture is heat, cool down and oxygen or lack of, i was wondering about the effects of the different vent systems . updraft, down draft, positive and negative drafts..wow, so many variables.

So for those running both vent types (up\downdraft), do you find running the various draft systems together or independently through the firing stages helps in creating more depth and texture to the various recopies?

With overhead vents, does setting the vent fan to be a positive\negative draft yield different results? It seems there are so many combinations one could use, what have you found to be the best setup for pushing the spectrum in electric kilns?

Also to help slow cool down, outside of the controller box programs, is there such a thing as a kiln blanket to help slow the loss of heat. If so, is it a safe & useful thing to use?

Appreciate all input and knowledge as a result of your trials with "99 way not to make a light bulb"

[Callie Beller Diesel]

The problem with a kiln blanket is that it would typically be made of some sort of fibre refractory, and you’re supposed to disturb those as little as possible, the dust they create being bad for your lungs and all. In a universe of infinite possibilities, I’m sure someone uses one, but most don’t.

If you’re concerned about the electricity use in regards to digitally cooling cycles, usually there’s targeted areas where you want to keep the kiln at certain points for longer, so it’s not a continuous use of power. Typically the aim is to encourage or discourage crystal growth in the glaze matrix, or to somehow mature either clay or glaze. There’s a LOT of different possibilities, so you’d have to have an idea of what end result you want to narrow it down to get better directions from here.

 

As far as vents go, I can’t speak from personal experience because I don’t have one. My kiln is not from this century, and it’s in an outdoor unheated shed.  Hopefully someone else can chime in on that. I don’t think they change atmosphere, but they can alter the rate of cooling and even out firings. 

There are methods that you can use to create a reduction atmosphere in an electric kiln, but they tend to increase the rate you have to change your elements. Some of them are toxic (throwing in a couple of mothballs), or they might void your insurance (Bunsen burner in a hole under the kiln to create redux), or are illegal in my area (that Ferberizer thing). I have seen some interesting experiments with creating localized reduction in a saggar, but I don’t know how that would scale to a larger production situation.

I found that when I switched from fuel firing to electric, you’re using your clay and glaze materials to get the interesting results, rather than using simpler chemistry, reduced oxygen and extended heat. Instead of thinking about high temperature redox reactions and how the atmosphere reacts with simple materials, with electric you’re thinking about light, and how optics work with glass and how the material behind the glass affects that. You’re thinking about how glass is formed in a furnace, and you’re thinking about how minerals are formed geologically with heat and pressure and time. You’re thinking about localized surface reactions like the eutectics created at the interface between 2 layered glazes. 

Electric is essentially asking a different set of questions than gas. The fixed parameters you’re working in are different between the two approaches, and you can get some similar results working with either. It’s two possible paths with the same possible end result.

 

 

[neilestrick]

An overhead hood will not have any effect on the work. A downdraft vent will pull air into the kiln, providing a cleaner atmosphere. Many glazes respond well to this and will be brighter and have better color development. Positive vs negative pressure won't matter, because it's all pulling from the kiln. The difference is where the blower motor is located, either close to the kiln (positive) or at the exit to the building (negative).

Cooling rates will have a noticeable affect on some glazes. Slowing down the cooling will allow for more crystal growth, which may or may not be a good thing. As always, test, test, test. The best way to slow down cooling is to use a digital controller and program a specific cooling rate. It allows for far more control than trying to cool by other means such as added insulation. A 25F/hr difference in cooling rates can be noticeable.

Most reduction firing effects can be achieved in oxidation. A good speckled brown body will give you the look of reduction fired stoneware, and most glazes can be worked out with some knowledge of glaze chemistry. Copper reds can be done very well in electric kilns by using silicon carbide for reduction. Celadons can be done using stains. The only common cone 10reduction glaze I haven't seen a convincing cone 6 version of is a tenmoku.

[PeterH]

A picture  to illustrate Neil's point about controlled cooling.

From Alternative Firing Schedules https://www.georgies.com/pdfs/catalog/Page PDFs/03-36.pdf

In a similar vein Glazing and Firing Techniques Atmospheric-like Effects in an Electric Kiln https://tinyurl.com/9jrshrb
... which references The Many Faces of Iron http://carol.knighten.org/docs/ManyFacesofIron.pdf

[Bill Kielb]

On 8/13/2021 at 10:42 AM, MadMetal said:

With overhead vents, does setting the vent fan to be a positive\negative draft yield different results? It seems there are so many combinations one could use, what have you found to be the best setup for pushing the spectrum in electric kilns?

Also to help slow cool down, outside of the controller box programs, is there such a thing as a kiln blanket to help slow the loss of heat. If so, is it a safe & useful thing to use?

The internal kiln draft will In General ensure oxidation and In General allow for brighter colors. The hood provides an effective way to capture fumes and can also be designed to capture much of the waste heat of the kiln. Vents that remove air from the kiln internally influence infiltration by making the interior of the kiln slightly negative with respect to the air outside the kiln. Both fans can be shutoff at some point in the firing to save energy.

Slow cooling generally encourages crystal growth or a matte look.  Kilns for the most part do not control the rate of cooling, instead they use a drop and hold methodology which seems adequate for ceramics.

Color variation in firing is quite often (not always)  most influenced by top temperature. Potters use rate techniques and holds at lower temperatures to influence the developed color. (Especially for glaze colors sensitive to temperature) It’s not uncommon for a potter to fire to cone five with a 15 or 20 minute hold to achieve cone six heatwork without reaching the top temperature of 2232f which for some glazes  has a dramatic effect on their color.

Drop and hold schedules also can provide a fix for glazes that tend to pinhole  and not heal due to fired surface tension issues.

The best system for removing fumes, the hood IMO. The only effective system, to remove heat, the hood again. The counterflow or downdraft is probably the best to ensure oxidation and removes a decent amount of fumes. Both fans can continue to run  post firing to speed up cool down and waste more electric. Blankets over top often cause expansion overheating issues so unless the kiln was designed with extra insulation often folks who try this cause some additional stress and damage to their kilns. Rated blankets will slow things down though and generally kilns insulated with an additional layer of insulation board have noticeably longer cool down cycles because of the insulation. Bailey and Coneart are two that come to mind.

Finally electric kilns generally never undergo a positive or pressurized draft. They generally do not have enough power to tolerate much fresh air. Down draft vents withdraw a fraction of a cfm to do what they do. Most of their air is taken from the room and a tiny percentage from the kiln.

[MadMetal]

Thank you everyone, exactly the information I was looking for!

[neilestrick]

On 8/14/2021 at 9:36 PM, Bill Kielb said:

Kilns for the most part do not control the rate of cooling, instead they use a drop and hold methodology which seems adequate for ceramics.

Not sure what you mean here. Most slow cooling schedules involve a specific rate of degrees per hour for the cooling. They may drop at first, but then they cool at a specific rate. The pre-programmed cooling cycle in the Genesis drops to 1900F, then cool at 150F/hr to 1500F. No holds.

[Bill Kielb]

44 minutes ago, neilestrick said:

The pre-programmed cooling cycle in the Genesis drops to 1900F, then cool at 150F/hr to 1500F. No holds.

I just mean
If a kiln does not cool naturally at least that speed then the current controllers can’t power cool them at a higher rate. One can’t force the kiln to go faster than it’s natural cooling speed with the common controller. In your example I couldn’t make it go 300 degrees per hour if it did not do so on its own. They drop and hold or drop and soak. Not really a controlled rate but it seems fine for most ceramics.