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Greg R

rigidizer question

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I just soaked my ceramic blanket in rigidizer. How hard should expect it get. Instructions say full cure takes 24 hours. Just wanna make sure its totally done before i apply refractory.

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Note that it makes it more rigid, rather than hard: When you press a thumb into blanket, it doesn't take much force to press it down by, say, 1/8" and it springs back. With rigidizer, it'll take noticeably more force and it won't spring back. 

The rigidizer seems to get fairly rigid as soon as it is fully dry, then get more rigid once it has been fired to high temperature. 

If your plan is to rigidize, then coat with a castable refractory, I'd not worry too much about it.

The general consensus seems to be that the refractory bonds better to wetted blanket and the usual practice is to spritz the blanket with water before applying the refractory. If you wait until the refractory has crisped up at the surface, spritz it and apply the refractory, you should be fine.

If you rigidize and apply the refractory while it's still wet, you should also be fine, but your drying time is likely to be much longer: moisture can escape from the exposed blanket face much more easily than it can escape through the layer of refractory. 

Drying time is VERY location-dependent. Some places, you'll need to slow initial drying down by covering the forge with plastic to give the (castable) refractory time to set properly. Other places, you'll struggle to get it dry at all. I'm in the wetter bit of England, just North of Manchester, and fall into the latter category.

Dry is IMPORTANT: if the low-permeability layer is not dry, it'll flash off steam inside and blow it apart. It may not be impressive or even immediately apparent, but it'll leave cracks where the steam forced its way out. I'm pretty sure this has caused the early demise of many forges built by impatient would-be smiths.

Using a fireclay-based refractory cement/mortar as a coating, the steam flash-off causes paper-thin bubbles to rise, harden and break up. If I really need to make a forge with refractory cement, it gets dried over 8+ hours in the oven (the actual time taken depending on how long I can be sure the wife will be out).

 

Edited by timgunn
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