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What temperature rating for ceramic blanket is needed?

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I am building a propane tank forge, and found a local supplier that has ceramic blanket for a really good price. Problem is, is that it is only rated for 2200 continuous use, 2300 max. Is that going to be high enough? I do plan on covering in Sodium silicate, then Kast-O-Lite.



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In general it should be ok, but as with many things it depends a little bit on the specifics.  If you are just planning on general forging it should last a fairly long time.  If you plan on doing a lot of forge welding where you get up into the yellow-white heat range it won't last as long.   The lining and insulation of a propane forge will have to be replaced eventually due to both mechanical damage and heat issues.  2200 degrees F is higher temperature than you need for anything but possibly forge welding, and with the added protection of the Kastolite I would not hesitate to use it.  We normally recommend the 8 lb. density rather than the 6 lb. density though, so you may want to factor that in as well.

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Thanks, I'll have too look more into it, because the cheap stuff is the 6 pound.


What is the effective difference between 6 and 8, just the insulating value?(I remember there was some technical term for that from when I built my pizza oven, but I can't say what it is now.)

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2200 degF rated is 1200 degC. I suspect that it will be the "Low Body Persistence" stuff.

Most of the Refractory Ceramic Fiber blankets I have come across have ratings of either 2300 or 2600 degF, 1260 or 1427 degC. The higher-rated ones having a Zirconia content.

The RCF blankets have stronger fibers and are physically stronger, as well as having the higher temperature ratings. The LBP stuff has fibers that are designed/formulated to break up more easily and to dissolve, albeit very slowly, in simulated body fluids. The idea being that they will eventually disappear from the human body, rather than remain forever.  

Over here in Europe, it's getting much harder to buy the Ceramic Fiber blanket, as the LBP products are assumed to be safer. I'd expect things to follow a similar path over your side of the pond.

If you can find a source for the 2600-rated 8 lb blanket, and are comfortable using it, it might be a good idea to stock up on it now, while you still can. 

As far as I can tell, the main reason for the lower rating of the LBP product is shrinkage. It doesn't simply melt at one degree above the rated temperature and fail dramatically. As the temperature goes up, the shrinkage increases. Many industrial applications use "modules": blocks made from concertina-folded blanket with refractory metal supports that are attached to an outer casing when lining a furnace. Typically, the modules are around 12" square and between 4" and 10" thick. When they shrink, a gap opens up between the modules. The greater the shrinkage, the deeper the (V-shaped) gap gets. Where there is a gap, there is less insulation between the process and the casing. Eventually, there will not be enough insulation for the task. The modules can be slightly compressed on installation, which helps a bit, but does not eliminate the problem. I "think" the temperature that gives 4% shrinkage is used as the rating temperature for some LBP modules, though I don't know if this is an industry standard.

For forges, we tend to use wrapped blanket and we tend to coat the hot face with a castable refractory layer. This would seem to largely eliminate the shrinkage failure mode.

As far as I can tell, the LBP stuff should probably work fine in most forges.

The biggest difficulty we tend to face is that everybody tends to do their own thing and the only way we get to find out what others have done is through personal contact or forums like this one. Very few of us work scientifically (change only one thing, make observations, then change one more thing, make more observations, and so on). Very few, if any, of us have equipment to allow us to take objective measurements at each stage. 


I'm not sure about Sodium Silicate as a rigidizer. I have used it pretty successfully on some forges and I've had failures on others. It certainly works as a rigidizer on my Heat-Treat forges but these are only intended to run to maybe 1000 degC, 1832 degF. The melting point of Sodium Silicate is given as 1088 degC, 1990 degF. I have had forges with largely Sodium Silicate-rigidized blanket running at welding temperatures >1300 degC, 2372 degF. These were treated by soaking the blanket with a thin suspension of Zirconium Silicate and Porcelain clay in a solution of Sodium Silicate in water. A further forge using the same ingredients but at higher concentrations seemed to result in the coating cracking up and moving on the blanket underneath. I have a feeling the higher concentration of Sodium Silicate solution was largely to blame, coupled with the thicker clay/Zircopax layer shrinking more on drying. However, the forge temperature was taken higher as well and there are too many variables to make an accurate diagnosis.

For a forging forge, I'd be happy with Kast-O-Lite over Sodium Silicate rigidized blanket. I would not let the Sodium Silicate solution density exceed 1100 grams/liter, which was what I used on the "good" forges. Make sure things are allowed to dry fully before firing.

For a welding forge, I'd spend the money on a commercial rigidizer, 2600 degF RCF blanket and Kast-O-Lite 30.


Another variable is the burner. I use burners based on a commercial gas mixer ("Amal atmospheric injector"). This has a very finely-adjustable choke and allows the mixture and flame temperature to be easily adjusted. My forges therefore tend to have more even temperature distribution than most forges built using burners with no choke adjustment. If you have a small, screaming-hot, hotspot in your forge, that's where you are most likely to run into problems with the refractory materials.


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Another note regarding an observation of Tim's. Applying THIN coats of kiln washes, eg. his zirconium, kaolin sodium, silicate flame contact layer is less likely to heat check or break up. 

Water set hard refractories like Kast-O-Lite products are designed to be applied in pretty thick lifts and so don't break up so easily. 

Fumed sillica is less expensive and more effective as a ceramic blanket rigidizer. I don't know how it works as an adhesive though and sodium silicate may work better in this instance.

Frosty The Lucky.

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