notes on DIY inner forge coating

[Michiel V]

Greetings, 

3 years back I made a big soupcan forge (outer diameter 20 cm) and since I didn't have acces to ready to use refractory materials (castable, coatings or wool) for a decent price, I had to improvise. For insulation I used a mix of Vermiculite and clay (5cm), this was sufficient for getting steel hot enough to heat treat. But it was slow and I felt I would waste to much gas working like this, so I wanted to apply a coating like ITC-100. While my funds where low, I did have time to do some research. I wanted a DIY coating with following properties: high infrared reflectivity, durability and easy application (given the soupcan model of my forge even a brush was not usable). I tried to determine the composition of professional coatings based on there properties, looked at some ceramic principles and did a few test mixtures. This was the mixture I ended up using (the numbers are grams):

Kaoline 35 Silica  10 Lithium feldspar 15 Potassiumfeldspar 5 Talc 5 Wollastonite 5 Zirkoniumsilicate 7 Zirkoniumoxide 3 Titaniumoxide 10 Bariumsulfate 5 Zinkoxide 5 Aluminiumoxide 10 Bentonite 4

This was mixed with about 100 ml of water (add in parts till you get a paint like consistency), the mixture was then slipcasted into the forge. After getting a layer of about 1-2 mm the remainder was poured out and I let the forge dry for a week before firing it up. Any remaining water will destroy the coating upon heating!
A little explanation about the recipe: 
The main refractories are Silica, Aluminiumoxide, Zirkoniumoxide and the Zirkoniumsilicate. Why this mix? It's to keep the desired ceramic properties of the coating.
Titaniumoxide and Bariumsulfate where also added for the thermal reflectivity, however they perform very poorly in the high heat of a forge, so I would replace them with 15 grams Zirkoniumsilicate.
Kaoline makes the base for the ceramic part, bentonite helps to make the mixture more fluid. 
The other components are for the durability of the coating.

Overal I was very happy with the result, the outside of the forge got noticeable slower hot, while steel got noticeable faster to an orange glow.
After 3 years the coating is still fairly intact (only used once every two weeks I must add) with carefull use.
It can be scratched with your fingernail, but it never developed cracks or came lose due to heating.
I recently used the forge for melting brass, the fumes from the zincburns did cause some brownish stains.

Hopefully I can help someone with this information or inspire further research (for me, experimenting is part of the fun)

[Irondragon Forge ClayWorks]

Welcome from the Ozark mountains. Just wondering did you use any insulation like Inswool or Kowool under the coating?

I can't control the wind, all I can do is adjust my sails. ~ Semper Paratus

[Frosty]

Welcome aboard Michael, glad to have you. If you'll put your general location in the header you'll have a better chance of meeting up with members living within visiting distance.

Interesting recipe, I'd love to see how it works over an insulating ceramic wool blanket like Kaowool. 

Frosty The Lucky.

[Michiel V]

On 5/19/2023 at 12:57 AM, Irondragon ForgeClay Works said:

Welcome from the Ozark mountains. Just wondering did you use any insulation like Inswool or Kowool under the coating?

I can't control the wind, all I can do is adjust my sails. ~ Semper Paratus

Since ceramic wools like Kaowool are very expensive here, I used a 5cm layer of vermiculite bound by clay

[gewoon ik]

sounds like a cheap mix to fix up my bad coating on top of my insulation