Is fireclay necessary?

[Carl]

Dmun,

My dome's a clay one, and I only used regular sand and cement mortar for the chimney brick. I used the waterglass to mix a mouldable insulation using vermiculite, fireclay, waterglass and portland that set within an hour or so, but was nice and sticky like clay beforehand. I used this liberally to fill a lot of gaps, coating the inside of the chimney bricks and smoothing the chimney passage. The oven's had a good couple of hot fires now, and the insulation seems to be doing fine. There should be some pictures on the blog somewhere - or on my post here (Another UK oven started).

I've kept a lot of the waterglass back, and will be painting it over some of the crumblier bits of my second hand firebricks on the hearth to stop in crumbling any more, and avoid masonry in my pizzas...

Carl

[dmun]

I ask because it's been brought to my attention that Water glass is an affordable binder for refractory mortars as well as the vermiculite:

Water glass is a useful binder of solids, such as vermiculite and perlite. When blended with the aforementioned lightweight aggregates, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.

When I went to look into the stuff, it came in a dozen different formulations, solid and liquid, and haven't had a chance to research it further. I thought maybe you had done so already.

[Carl]

I first came across water glass for exactly that reason - a cheap way for me to make refractory stuff. I specifically wanted a mix that I could coat the inside of my chimney with (I was concerned that portland alone would break down at the high temperatures), and wanted something mouldable, insulating, resistant to very high temperature, but that would set quickly without shrinking and cracking (like fireclay alone would). You can use a range of setting agents - the portland cement in my mix serves as the setting agent. I think it's great stuff - the waterglass sets at 220C and doesn't melt until > 1200C. I'll be using the same insulating mix to make my oven door, maybe this weekend.

My research led me to this page which has details of how to use water glass and perlite to make refractory stuff. My mix was based on the information on this page, plus a little experimentation until it felt right. I used vermiculite, not perlite, and a liquid form of waterglass, not solid. My water glass came from clayman supplies

An aside: the different grades of liquid sodium silicate are measured in degrees twaddell (degrees Tw on the website above) - a quality old english way of expressing a specific gravity of a liquid. I used the 100 degree twaddell stuff, in case it helps.

You can also use it to flameproof wood (I'll be doing this with my peel, brush etc), and it's non toxic, so you can preserve eggs in it! Strange stuff.

[diy]

You can buy Vitcas Heatproof Mortar is ready to use mixed refractory cement ( mortar) , supplied in a smooth trowelling consistency which will form a strong air- dried bond with all grades of dense and insulating firebricks. It is rated for use up to 1400 ºC, well in access of any temperature to be found in domestic applications.

www.vitcas.com or www.refractorymortars.co.uk

[aureole]

... but not so great for external applications or where large gaps have to be filled.