Kono II

I decided to take a day off yesterday and tackle the kono / hibachi / yakitori BBQ / grill I'm intending to make.

Hay happened to have a perfect sized black box that would create the inside former, so I nicked it. She won't notice, unless it takes longer than a week for the percrete (perlite and cement) to cure.

I started off by wrapping the inside of the large recycling box and the outside of the smaller black box in clingfilm, which will hopefully easy the separation of the grill from the boxes, as well as keeping the boxes relatively clean.

Not the easiest of tasks, as the damned clingfilm sticks to everything.

Hay had some grey Portland cement, but I didn't really want a mottled thing comprised of white, perlite aggregate and grey cement - plus it's more than a year old and cement does go off with age. 

I should really have some white, refractory cement, due to its heat resistant properties, but I saw of YouTube that most people get away with ordinary, white cement. That involved a click and collect trip to Travis Perkins in town.

I mixed the dry ingredients - 10 scoops perlite, 4 of white cement - thinking I'd have plenty. The perlite alone looked as it it would fill the entire recycling box. However, once water was added (4 parts) it immediately shrank in volume. I used an old builder's bucket to mix the perlcrete in, but a clean wheelbarrow would have been better - I ended up having to use my hands and cement knackers skin in the same manner as hand sanitizer.

It turned out to be only enough to put a layer on the bottom, on which I rested the black box, and to come half way up said internal black box. I made another batch of half the previous volume - wishing I'd bought a pair of overalls.

The perlite is a bit like polystyrene beads, but much firmer. It's light as a feather and floats on water. Apparently it's some kind of expanded volcanic glass.

Finished filling the sides with a slightly wetter mixture and then carried it to my engine room (where all the gubbins for the underfloor heating, etc. is installed and is consequently quite warm) where it will harden and cure for at least a week before I attempt to remove it from the boxes. The longer the curing, the higher the strength - ideally it should be a month for max strength, but most people seem to get away with a week. Perlite has water retentive qualities and thus it adds to the drying time - any water present in the perlcrete risks cracks when it's heated by a charcoal fire.

As this is my first attempt, it may simply fall apart through the walls being too thin, or the mix not being sufficiently fireproof, or I may have to demolish the boxes in order to release the grill. The latter is my greatest fear, although I have already flexed the inner box last night while it was drying and managed to lift it a centimetre and push it back down, so I don't think that will be an issue. The outer box may prove more difficult, but I don't mind having to destroy that it I have to, although there are 4 holes in the bottom of the box which will aid extricating the grill.

The 3 bags of perlite cost me £25, the 25kg bag of cement was another £15, the boxes were free. However, I'm still waiting for the aluminium angle I ordered as reinforcement for the edges (£40), the aluminium brazing rods (£10) and a blowtorch (£15 - including 4 gas cannisters). The blowtorch will double in the kitchen for caramelising food, so that won't be included in the final price. I will also need some form of sliding draught control for the holes I'll be drilling in the ends - say another tenner, max.

I still have enough perlite and cement to make another, should this attempt prove abortive. Overall cost should, assuming all goes well, be in the region of £100, plus my time. Much cheaper than the ones on eBay. I dare say I could have made it a tad cheaper.

As they say, watch this space.