Refractory mixing
An easy in-bag mixing method for sodium silicate refractory mixes is described. It allows the mix to have a stiff consistency with low water content. The resulting ‘plasticine’ texture makes handling and molding easy.
In my previous post about dense DIY refractories from sodium silicate, I have intentionally kept water addition to a minimum. However, I still added a little water to the mix to facilitate easier mixing. Even so, the resulting mixes were hard to mix and incorporated air. This left them ‘crumbly’ in texture making them difficult to transfer and compact in a mold. They also required more time to dry prior to curing and firing.
The following in-bag mixing method makes the mixing and mold filling easy and eliminates the need to add extra water.
In-bag mixing
After measuring out the dry refractory powder into a plastic freezer bag spread it out form a depression in the middle. Pour the measured sodium silicate into the depression and allow it to soak in a little before starting the mixing by shaking with plenty of air trapped in the bag. When the mix is free of wet patches, start rolling and folding to finish the mixing.
As an example for a 100g mix of refractory, I use a 20*25 cm freezer bag and this allows the mix to be rolled out into a ~5mm thick ‘sheet’ without spilling. I use a smooth round glass jar as a rolling pin and put the bag on a smooth flat board for the rolling process. High-quality bags would also be suitable as the mix comes out cleanly and the bag could be used many times.
Bag refractory mixing of sodium silicate with post-hole soil. The silicate is pooled in the middle of the mound of soil that is in a freezer bag.
When the mix is rolled out I fold it back on itself and where possible put the dryer parts over wetter parts and roll again. I repeat this until the mix has no ‘dry patches’ and is a uniform dense sheet with a texture of ‘plasticine’. At this stage, it can be cleanly removed from the bag for transfer to the mold.
Post-hole soil and sodium silicate after refractory mixing in a bag. In this state it can easily be worked more to make it more even and plastic.
An example of a refractory object that can be made with soil and sodium silicate is shown below. “The priceless possibilities of this technique are endless and almost cost-free.”
Conclusion
In-bag refractory mixing can be a convenient and more effective way to mix dense sodium silicate refractories. Mixing this way makes it possible to make more intricate components.
Addendum
I used the new refractory mixing method to make a refractory burner out of post-hole soil. It is quite a complex design. This new refractory has worked very well when compared to my previous refractory stoves that had structural weaknesses. This new stove is strong, hard and very heat resistant. For details please see the post below.
This DIY refractory mixing is also applicable to other interesting refractory mixes for tinkerers like you. The posts below describe these refractory mixes:
Tim
Hi Tim.
Thanks for the brilliant post about DIY sodium silicate. I dabbled and made up a thick syrupy mix as you described for a project to make a japanese Konro grill cast inside an old tin deed box.
The mortar mix used 4 parts perlite to one part kiln dried fine paving sand, one part white cement and approx 100ml of the sodium silicate solution. I used minimal water but might have added too much.
Anyway can you give me your wisdom??? The cast is fine, been sitting there for a week… I’m just letting it dry out. Now I notice a white crystal powder on the surface… must be the sodium silicate? When I gently heat it with a blowtorch it goes brownish and leaves a sticky liquid on the surface.
What should I do now? Shouid I be curing this with the blowtorch or just leave it and light a fire in it?
I have photos but no idea what to do now!!!
Thanks for your advice!
Hi Jonathan, Thanks for your comment. The Konro grill sounds like an interesting project.
Your project has given me another inspiration to try making a ceramic blower stove with an insulated fire bowl.
I think that the white crystal powder is not silicate (as the chemical change during curing immobilizes silicate). I think is likely to be sodium carbonate crystals. I get them on my castings and they wipe off easily with a damp cloth. I think the sodium from the sodium silicate reacts with CO2 from the air as part of the formation of the silicate polymers as they join to one another and no longer need the sodium ions.
I find the best way to cure thickish casting is to use gentle heat and CO2. If I understand your project correctly, you have a metal ‘shell’ covering 6 sides of your casting and this will greatly limit the entry of CO2 and the exit of water for the curing process. The process is quicker in an atmosphere that is rich in carbon dioxide. Putting the casting in a large wood-fired heater stove is my prefered way, but the fire should be kept very small so that the process is gentle to prevent cracking. Another simple way would be to start with some candles burning underneath the inverted grill with air gap spacers holding the grill up a little from the bench. This will provide both heat and CO2. Hotter treatment such as in a stove can follow. Then use the propane torch for the final high-temperature finishing of the refractory on its ‘working surface. I cover these steps in some detail in another post and I describe the chemical reaction involved:
https://timtinker.com/sodium-silicate-refractory-firing/
However, I see that I have not mentioned the candle method, so I will add that to the post.
It is interesting to think that you start with silica gel crystals and drain cleaning caustic to make your ‘thick syrupy mix’ and with heat and CO2 from the air it turns back into silica gel as an amorphous gel or glass to hold your refractory together.
I would like to see some photos of your DIY grill. If you sent the photos to me in a personal email (using my contact form) I could share them on my website with others that may be interested.
I hope this helps,
Regards, Tim
Hi David, Thaks for your news. I will be keen to see how it works. Please let us know the result. Is green sand pure quartzite sand?
When I was making the ceramic pipe shown above, I slipped with the sodium silicate and added ~40g to 100g of soil (instead of 30g) soil and it was very plastic and workable for molding. It seems to have made a very nice refractory. How do you like my ceramic burner with a lantern below it?
Tim
Thanks for the update. I used the bag method to mix up some greensand. I used a cheap $3 rolling pin instead of a jar.