Casting over consecutive days.

[Hattori-Hanzo]

Hi all

I'm nearly at the stage of casting my dome with home brew. I'm just finishing off the sand mould and had a question about casting the dome.

Is it possible to cast the dome a little at a time over the course of a few days or is it better to do it all in one hit.

As I only get a couple of hours spare in the evenings I would not have time to cast the entire dome in one hit but could do several inches up at a time over a few days until complete.

I wondered if it would help to prevent slumping doing it this way but I'm also concerned the layers won't bond together well?

Or am I better waiting until I have time to do it all at once.

Another question about the sand dome, If I have to leave it for a couple of weeks before casting will I need to keep the sand wet and how wet should I keep it?
I won't put any news paper on until I'm ready to cast.

Thanks.

[david s]

It is better to do it in one go, but also possible to do in bits. You will find it pretty fast once you get the hang of it, so how about a compromise and do the base around 6” high. As you say the slumping is a problem and this will give you a solid base onto which you can build higher. The first 6” or so are the most difficult because they’re standing vertical. The higher you go the easier it becomes.
You will have to keep the sand damp, so keep it well covered. Get yourself a spray bottle to keep the surface damp. When covering with wet newspaper tear it into strips about 30 mm wide to prevent creases because wider strips won’t conform well to a compound curve. Some folk have used cling wrap, but I find wet newspaper works better. A day with no wind is much better as the wind dries the newspaper out and blows bits away. Keep your spray bottle handy.

[Hattori-Hanzo]

Thanks for the advice David.

I've just got the flue entrance to finish on the sand mould which I hope to do over the next few days.
I've been covering it with a tarpaulin and keeping it wet with a spray bottle like you've suggested, I didn't know If i would over saturate it, the weather is all over the place in the UK at the moment.

I actually have little vertical parts to cast as I'm doing a kind of hybrid oven and used a soldier course of fire bricks to form the start of the dome. The home brew will start directly on top of them, thats the plan anyway

Thanks for the tip with the newspaper, ideally I want to try and get the dome as smooth as possible inside, tearing into strips makes a lot of sense. I did consider using cling film over the top too but I'm guessing its difficult to get it nice and smooth?

Another quick question, what consistency am I looking for with the home brew? When I mortared the firebricks I had a fairly wet mix, I'm guessing I want to go as dry as possible for casting to avoid slumping?

Thanks again.

[david s]

That’s a difficult one to answer. Too wet will slump too much, too dry will give you air voids. Wriggling a handful against the mould surface helps. “Ball up” consistency is that which when thrown in the air a couple of feet retains its ball form. If you have the bottom part of the dome already done (sorry, I’d forgotten that) then you should have the casting work done in under an hour.

[Hattori-Hanzo]

david s Any tips to applying the newspaper?

I thought this would be the easy part. I'm having a hell of a time trying to get the wet newspaper to stay in place on the sand dome.
I've tried thin strips and larger pieces with no luck. The trouble I'm having is the paper wont stick to the sand at all and either slips out of position or creases up into a mess.

[david s]

You need to overlap the strips. Try mixing a little flour with the water then. This will make it stick like shit to a blanket.

[UtahBeehiver]

Umm, flour to water, that is a good one, I guess it is kind of like old fashioned wall paper paste.

[Hattori-Hanzo]

cheers guys I'll give that a try. Hoping to get the casting done today.

[UtahBeehiver]

It would be nice if you took pics to document your process for future builders.

[Hattori-Hanzo]

Well I've managed to get the casting done.

Applying the newspaper was tedious, I think I was trying to get it too perfect and flat, a swift breeze wasn't helping matters either. In the end slightly wider strips (75mm) doubled up, plenty of water from a spray bottle and perseverance got it done.

The cast was a lot more enjoyable though an hour was optimistic Took 4 in the end but I was mixing tiny batches by hand in a bucket, lost count of how many it took.

I have a couple more questions if you can help.

How long can I leave the cast before removing the sand and door former? I probably wont be able to work on it again until next weekend but If these need to come off sooner I can do it one evening.

For the flue opening I've wrapped my flue with foam sheet (around 4-5mm thick) then covered it in cling film and cast directly around it.
I've been rotating the flue every 30 minutes to prevent it from sticking but I'm worried it will set solid over night. Ideally I don't want to be up every half hour through the night, am I safe to remove the flue at the latest point this evening or should I leave it in place and hope it comes free in the morning?

UtahBeehiver I've been taking a few pictures along the way which I planned to show in a build thread. I need to sort through them and upload when I get the time.

[david s]

You can prevent the thing sticking by wrapping some plastic around it before wprking up to it. I think it’s better to wrap some cardboard around it first though so it becomes a slightly loose fit once the cardboard is removed. This then allows the pipe to expand without placing stress on the casting. Keep the whole thing covered well to retain the moisture in the casting for a week. This allows the cement to hydrate and strengthen the casting.

[Hattori-Hanzo]

david s Thanks for the reply David.

I read a post by you in another thread where you recommend wrapping the pipe in card board before casting up to it. I didn't have any thick card board to hand so used the foam sheet instead which I then covered in plastic kitchen wrap. (hopefully it won't stick)
So I'm best leaving the pipe in place until the cast has hardened more.
I plan to secure the pipe using your method of cutting slits and bending the metal out then secure it in the Vcrete layer.

Also just to clarify, it's okay for me to leave the sand in for a week before removing.

I've got a heavy tarpaulin which I'll cover the dome with, looks like rain is on the cards tomorrow.

Cheers.

[david s]

Yes, wait at least a week before removing sand, it won't hurt to leave it even longer (see table)

[Hattori-Hanzo]

Excellent, thanks for all the help.

I cant see the table too well, its small and cant enlarge it.

[david s]

Try this.