Matt Walker tiny cookstove build in Portugal

No, there is lots of room in a Batchbox.
Little sticks, bigger sticks, and logs all burn like crazy in a warm batch.

The Walker stove uses a much smaller core than a Batchbox.
There is a limit on how large a piece of wood is used.
But nothing wrong with using small sticks, you will need to poke it more often and add more sticks but dry wood is dry wood.

Joylynn Hardesty wrote:Ages ago, folk would bundle and tie together a number of small diameter sticks to burn as logs. Would this form be appropriate for a batch box?

I'm hoping someone with more experience will chime in, but I might experiment with this.

Two potential drawbacks I can think of are

a) having to find something to tie them with, bearing in mind I like to find ways to do things on the cheap, and

b) the fire-box is rather small and the bundles (am I allowed to call them faggots?) are likely to refuse to stay in shape after they are tied and end up not being easy to slide in through the very straight edged entrance.

Possibly we could make a little fire-box sized box we could load up with whatever seems appropriate and slide it in straight from the box? Or make up bundles in advance and then drop them into the box to re-shape them before sliding them in?

For now I've been concentrating on cutting up small sticks that need clearing for fire-safety, with the idea of raiding burned trees from the forest as needed. But there are also other sources of free wood.

This turned up a few days ago, for instance.



Sometimes there are wood offcuts and bits of old roof that don't get grabbed by the other workers before the client wants them cleared out.

Which means that Austin gets to take them away on the truck.

Which means that unless the boss wants them for his wood burner, we might be lucky enough to get them!

Rock for scale.

The smaller bits can go straight into the house for burning this winter.

Bigger ones will have to be cut first.



Vermelha likes this bit.

I think she's planning some sort of experiment.

One that involves fire, if I know her. Just you behave yourself Vermelha!



She just couldn't resist, could she...



Sigh.

Yes dear, very pretty.

And you did it all by yourself without Austin supervising.

Not sure that I approve. And I'm quite certain he won't either!

Austin was working this morning, but he took the opportunity after lunch to mortar a layer onto the core.

Whilst it's apparently perfectly ok to dry-stack the firebricks in the core, both of us feel that somehow it's better to mortar them in 'properly'.  Austin because he's a perfectionist, and me because I want this stove to be a bit of a showpiece and am expecting people to turn up and poke and prod at it, and I think they will be unconvinced if we tell them it's supposed to just hold itself together by art and not need to be put together 'properly' the way they expect bricks to be put together.

So fireclay-and-yellow-sand mortar it is!



There he is concentrating and making sure it's all perfection personified...



And finally the magic doorway.

Er, port. Or is it portal?

He's busy making sure all the measurements as exact as possible for this bit!



After finishing that layer, he grabbed a drink, sat down, and his phone pinged as more worky-work apparently needed doing urgently. So now he's half way out the door again. Ooops...

Burra Maluca wrote: So now he's half way out the door again. Ooops...

That's the nice thing about the mortar just being sand and clay - any left can be re-mixed and is ready to go for the next layer!

Having got the first layer of the core in, my thoughts now turn to insulation.  I have perlite which is little beads, so you can all share if you will your favourite recipe for making that into insulation material to put into the gaps around the core which are 1" - 1½" wide.  

muito obrigado as they say around here

Hi Austin;
You can use the perlite dry, just fill the void with it.
Or mix powdered clay & water with the perlite and create a thick slurry, heavy on the perlite.

Ha - I was off searching for previous answers but you've already replied!

I found this, in another thread, written by a very wise man whose word I trust on the matter...

thomas rubino wrote: Dry clay is sprinkled over perlite, add water.  The mix is more perlite than clay.  You want to keep the insulating property's of the perlite with the sturdiness of the clay to hold it together.

A softball size piece should hold together on its own but still bust apart if enough pressure is applied.

thomas rubino wrote:Hi Austin;
You can use the perlite dry, just fill the void with it.
Or mix powdered clay & water with the perlite and create a thick slurry, heavy on the perlite.

This reminds of my question I once had but never tried.

If one had access to a very large kiln, that would run up to say 2250 degree's in a super controlled manner (say in 12 hours)  Has anyone taken 15% fire clay and then a combination or single part at 85% of Perlite-Pumice-grog-sand-or any mixture of these items.  And formed (cast) any shape required. This has to be done very dry and then let dry before going into kiln.

While I don't know the end results, the end product, should be as close to porcelain (kiln heated fire clay)  as you could get.  I fully understand that a near production grade vibrating table would be required.

There is an entire wood fire place company that totes they have such a product, but nary say a word on how it is made, which is understandable.

Just thinking out loud.

cheers

Rosa doesn't normally get involved with anything not very 'girly' but she decided that Thomas' recipe for perlite core insulation sounded so much like making cookie dough that she should get involved.

So with the help of her trusty stirring stick, she's getting to work with some perlite and fireclay.



Roxa added some water and suggested that the trowel might be a better shape for this sort of stirring job.



Once they'd got the mix just right, Austin helped them fill the gap around the core. There will be more to do when the core is finished, but it's easier to do this in stages as we build it.



And now Roxa and Austin are turning their minds to thoughts about doors and door frames...



They've been busy reading Thomas Rubino's book about building your own batchbox door but knowing those two they will come up with something a bit different, if only because Austin enjoys that sort of thing so much and can't resist experimenting and tinkering and generally playing with all those tools and things he's collected over the years...

More scrumped firewood for use as rocket fuel!

Austin had a bundle of offcuts of roofing battens, and I claimed this bunch of thin strips that were part of the packaging on pallets of roofing tiles.



Over the years I've developed quite a good technique for resting one end of such thin bits of wood on a large brick and then stomping them to length by applying my booted foot in the middle of the unsupported bit.

It's the little things...

Been contemplating doors...

First, I want the oven door to be flush to the outside of the front of the stove, so there's no awkward frame around the opening, which either will restrict the opening or make an extra step you have to lift pots etc. over.  Also having the door on the front surface makes it super easy to fit.

With that decided I turned my thoughts to the firebox door.  Fitting that flush on the front so as it's similar to the oven door is aesthetically desirable.  So then I got to thinking, and my current thought is to extend the front of the firebox part of the firebrick core until it's flush or very-near flush§ with the front face of the stove, like in this mock-up I stacked up to see what it looked like:


I can get insulation either side of it, where it goes through the firebox-door-opening.  

The advantages I see are:
* works nicely with the proposed flush-fitting door.
* extends the firebox length by about 4"-5" which means wood could be a little longer.  
* the extension will support layer 11 of the shell bricks and thus in turn the brick top layer without having to incorporate a steel bridge piece there.  

However, I'm interested in whether anyone sees any disadvantage to doing that

I'm leaning to switching to clay-sand mortar for layer 11 and the top layer, a) that part gets hottest so will avoid any possible issue with the refractory mortar, and b) I might in the future need to get the top off to work on it.

§ I'm hoping to put glass in the door, and that needs to be attached inside the door so will need a space to allow that to happen.

If I understand, you intend to extend the fire box by 4.5” I think it would be worth contacting Matt to find out his opinion as that is a massive change in dimensions and could very easily change the design enough to prevent a clean burn.
Most rocket stoves require quite precise dimensions, it might still work but send out extra pollutants into the air, or smoke up the glass or throw out smoke from the chimney.
It is a bit like reloading a hot fire, it might not look too bad inside the fire box but it will be over fuelling like mad and polluting the atmosphere.

Fox James wrote:If I understand, you intend to extend the fire box by 4.5” I think it would be worth contacting Matt to find out his opinion as that is a massive change in dimensions and could very easily change the design enough to prevent a clean burn.

Yes you understand right.  However, the bit I'm proposing to add is kinda there anyway unless you fit the door right inside the aperture in the outer shell.  I did also email Matt, so will find out what he says.  Previously he said that surface mounting the door on the outside was fine.  Also Burra was looking at Thomas Rubino's batch box book and found a bit that said that extending the firebox is OK up to a point (I think 25% was mentioned), provided the other internal dimensions are correct, which I have been quite careful with, the sizes and the port are all pretty much spot on according to the plan.  

Austin Shackles wrote:

Fox James wrote:If I understand, you intend to extend the fire box by 4.5” I think it would be worth contacting Matt to find out his opinion as that is a massive change in dimensions and could very easily change the design enough to prevent a clean burn.

  Also Burra was looking at Thomas Rubino's batch box book and found a bit that said that extending the firebox is OK up to a point (I think 25% was mentioned), provided the other internal dimensions are correct, which I have been quite careful with, the sizes and the port are all pretty much spot on according to the plan.  

Not making any suggestions, but I for one, did take advantage of this additional 25% extension, (mostly due to my already split fire wood length)  and things worked fine. But EVERYTHING else was strictly held to.
  Remember if you make 2 changes at the same time, and something don't work, you have 3 things to check to correct,  Original- Change 1 alone-Change 2 alone,  so the unknowns can pile up in a hurry.
Best of success to all!

It's time for an update post.  Layer 10 is on and the second level of the core.   Here are the dragons inspecting it, well some of them are, Serra is still ensconced in her new-found cave in the oven.

Since taking that I also filled in most of the insulation around the core.

Layer 11 and the top layer I will set in sand-clay mortar in case I ever need to get it apart.  Next up is to source some steel for the pre-port tube.  

The boys went shopping over the weekend, and when the got back the dragons, as usual, snuck down to see if there was anything interesting lurking in the back of the bus.

They were delighted to find this - a collection of interesting looking metal tubes!



Vermelha wants to use some of them for huffing into the rocket mass heater. She says that if you huff at the back of the fire, the fire will burn hotter and maybe Iggy could invent a way to use them to direct her huff to the right place.

Iggy thinks that if Austin bought them, he probably has something in mind for them already. But then, Austin started showing Vermelha where the huff should come out and working with Iggy on a design, so maybe he already knows about such things...

Meanwhile, Roxa went off in search of interesting tools that could be used to work with the new tubes, and persuaded Austin to fetch this one out of storage and bring it into the house where it belongs.



Austin is showing her how it works by drilling a hole in the side of a bit  box section.

There. One hole.

With the same internal diameter as the hole in the round bit of tubing, hopefully...



Iggy explained to Austin that we want a tube to deliver air near the top of the back of the burn chamber, so Austin measured it carefully.



Then he cut a piece the right length.

About there should do!



Then it will sit on the bit of box section something like this...



We also need a little collar to hold it in place, so they made one of those next.

They do eventually burn out at the top, so a collar means they can be pulled out and replaced without too much hassle.



This one is still hot from all the grinding, hence the glove.



It's cut from the same bit of tube, so it needs a split so we can open it up enough to be able to slide the tube into.

The edges need tidying up a bit so there are no sharp edges.

Roxa is quite fascinated by this job...



It needs opening up else the tube won't fit inside it.



There, that looks about perfect!

Time for a break I think, we can weld it up later.



When we went downstairs again, we found Roxa  wondering whether or not she's allowed to indulge in a bit of time with Austin's favourite vice.

Those two fiery ones in the back look like they're up to something, too...



YOU WERE SUPPOSED TO WAIT FOR AUSTIN TO WELD THAT!

Sigh. Dragons, eh...

Vermelha does look exceptionally pleased with herself. Though I suspect Rubeus helped.



There - one secondary air intake!

I did wonder if I could get away with referring to it as the afterburner but my aeronautic engineer friends gave me a huge lecture about how afterburners are where additional fuel is added after the initial fuel is totally consumed, and that no air/oxygen is added at the afterburner stage in a jet engine, just a ring shaped tube with holes that pump fuel directly into the hot exhaust.

So I backed down a bit and for now I'm just calling it a secondary air intake.

There will, of course, be a primary air intake in the usual sort of place in the door. This one just makes it go up to eleven. And beyond!

Vermelha is experimentally huffing up it and Iggy is checking that the huff comes out the other end. In actual use the huff gets drawn in from the hot end rather than huffed from the cool end. But the dragons learn a lot from playing, just like the rest of us...



Whilst we had a wonderful time playing with all the tools and making firework displays in the house, with me standing by on spark duty with a fully loaded watering can in case of problems, not everyone wants to do that.

Fortunately it's possible to buy them ready made from Thomas Rubino - DRAGON TECHNOLOGY QUICK CHANGE SECONDARY AIR TUBES!!!

He also has a thread discussing them here on permies.com - Quick Change secondary air stub!

We decided to try to sell the old woodstove, which is a very nice old cast iron Edinoliva, as it was taking up space and we are very much past the point of no return in the building of the rocket mass heater.

I had this theory that if I completely smothered both it and the stove pipe in the sticks I've been cutting for fuel this winter, then someone would show up to buy it unexpectedly and we'd be embarrassed by having to dig it out. Which in the long term is better than having it sitting there when we could use the money to buy new stove pipe with. So it still looked like this when someone contacted us to ask if they could come and see it in an hour.



I was in full hermit mode, especially as I know from experience that visitors will take up half my daily spoon ration and I couldn't really spare any more, so I hid upstairs when they arrived. The snippets of conversation I heard were rather fascinating, and encouraging, though!

Austin showed the man in, leaving his wife and children outside.

"Oh wow - is that a rocket mass heater you're building?"

"Yes."

"It's not a J-tube though, is it?"

"No, it's a variation on a batch box, which will have a stove top for cooking and a bench over there."

"Oh that's awesome. And just look at all those dragons!"

Ooops, I'd forgotten all the dragons were still playing down there.

My son arrived, along with his exceedingly useful muscles, and they dragged themselves away from the dragons and the rocket mass heater and hauled the old cast iron stove outside. When it was safely out, the little boy came into the house as he'd heard rumours of dragons.

He took one look inside and squealed in delight - "Dragões!!!"

He and Austin spent a happy few minutes hunting everywhere for as many dragons as they could find, including Rosa who was sorting out laundry on the washing machine and Serra who was hiding in her 'cave' where the oven is going to be, and Spot the chameleon who was just keeping very still and pretending to be invisible, as is her wont, while my son helped the new owner of the stove manhandle his new purchase into his vehicle.

I do suspect, however, that in a couple of years that dear old stove may be up for sale again and a new rocket mass heater build to replace it. Which I do find exceedingly encouraging.

Today's update:  Core has it's top on, and the insulation topped up around it.  I filled in the gaps around the front of the core with some offcut firebrick and clay mortar.  That's mainly to prevent the insulation stuff from falling out later.

Bricks cut and placed ready for layer 11!  This is the final layer of regular bricks, I'm intending to get some fancy shaped firebricks for the top layer.  They're only 5cm deep so looks promising for the stove top support on the top of the core being doable with firebricks placed on their sides without any cutting.

I'm thinking that to open and shut the bypass I might have a thin firebrick cut to as close a fit as I can do and still have it move, which will drop in a slot in the top of the stove in front of the chimney.  I thought about a metal thing that slid sideways but I reckon unless it's cast iron it will warp and get stuck.  

The firebox door is gonna fit flush on the outside of the brick front, and if all goes to plan have a rope seal around the firebricks at the front of the box, which are cunningly set about 3/8" inside the front opening.  Having extended the nose of the firebox out a bit, it can now support a couple of bricks as part of  layer 11.  They're cut like that fore aesthetic reasons so the ones at the sides can continue the pattern they already have.  I'll be setting layer 11 and the top layer in clay-sand mortar, as I can imagine needing to get it apart one day for one reason or another.  Being able to remove (part of) the top and layer 11 will allow reasonable access to the core, if/when needed.

Burra Maluca wrote:... When it was safely out, the little boy came into the house as he'd heard rumours of dragons.

He took one look inside and squealed in delight - "Dragões!!!"  

Please thank your dragons from me for making a young boy's day - I bet he'll remember hunting dragons with Austin for a very long time!

Austin wrote:

The firebox door is gonna fit flush on the outside of the brick front...

Having cook stoves smooth on the outside is an awesome thing, so cooks don't catch clothing or knees on protuberances. My Mother made my Father saw off the tabs on all the drawer pulls and lower cabinets in one kitchen because she was constantly getting things  caught on them! She carefully painted the cut ends so the handles looked intentional. That kitchen was done in a "style" which my mother quickly got annoyed with as it lacked sufficient practicality. She was an artist, so it isn't as if she didn't appreciate beauty - it just had to "practical beauty" if it was going to be in her cooking space!

Today's job was design a bypass from the top of the stove direct to the chimney.  This can aid lighting it especially on a marginal day when the chimney is disinclined to draw but it can also be used if for some reason I wanted to cook on the stove but not harvest all the heat, for example on an otherwise warm day when I happen to have run out of cooking gas.  From what I imagine about half the cook top would get nice and hot.  

Anyway, here be pictures.  Will  need to contrive a handle on the sliding brick (but it won't need to be heat proof, since it will only be opened for lighting and shut as soon as the fire's going well).  It might prove that the brick isn't the right size once the top layer bricks are above it, in which case I may have to cut another.

Paul has a big downer on bypasses, and I can see why for his place where multiple different people might be lighting the RMH and leaving the bypass open by mistake.  However, I think for me having it available even if it's not always needed is worth it.

Bypass closed:

Bypass open:

For the final day before going back to work, layer 11 set with clay-sand mortar and it's even pretty much level.

There follows an interlude on the brickwork until I can get the special bricks I want for the top.  However, there's still stuff to do like doors, for example.  Still not yet found a local supplier of fire door glass, who will cut a piece to size and not charge a fortune.

Hey Austin;
You are looking good on your build!
Fire glass costs around $10 a square inch here, so it is not cheap anyplace except for finding glass-top ovens at the scrap yard.

Just a very quick post so you can see the bypass being tested...

Very nice,  that Dragon Gate OH I mean bypass will do the job admirably!

We're still waiting for the place that makes the nice curvy bricks we want to use on the top layer to reopen after the summer, and even then it might take a while to find where we're actually supposed to go to buy them, and for them to arrive there.

So we thought we'd make a start on the oven door.



And as I was back on spark duty, I thought I might as well take a video.



Meanwhile, back upstairs, I found that Rosa has a new friend. A little green forest dragon by the name of Charlie. Presumably having arrived on one of the late-night trains.

And also a pretty cast iron thing she got from somewhere that she thinks will look very pretty on the oven door.

She might be right...

Hinges are being welded on.

WITHOUT the help of those naughty dragons this time, at my behest...



And a little video of bright, flashy lights. Don't worry, I was hiding behind the stairs so I could shield my eyes.

There. One door!

It still needs painting, and fixing on, and a latch making, and a pretty oval decorative thing fixing on. But we're nearly there!



And it's gonna fit there. Like that...



Iggy is checking the quality of the welding job...



Austin is experimenting with latch designs while the dragons are checking out the door.



Then Iggy distracted Austin by asking about a square section steel rod thing he'd seen in the stash. Austin explained that it will be used for making a groove for the fire rope to seal the fire-box door.



While Austin was safely out of the way and designing stuff with Iggy, Rosa snuck downstairs with her cast iron thing.

She says the oven door will be MUCH prettier when we get it properly decorated with her chosen, um, whatever it is.

And of course a nice coat of paint.

She has agreed that black will probably be more appropriate than pink. And in any case I'm not sure you can even get pink stove paint...

I must admit, it's starting to look rather nice...

Oooh, pretty!

Hello.

I'm popping by.

Burra and Austin, what is your plan for above the core ?

Cooking plate, i know. But how will it be fitted ?

To explain. The internal chimney of my batch has failed.  And i'm glad i somewhat left access to rebuild. Not easy easy access. But it can be done.

HTH.

Looks like the cooktop will be supported on the top of the core pretty much entirely, except for a bit on top of the interior wall on the left.  

I did initially think of making a groove around the top edge bricks and setting a fiberglass rope in it, but I see issues with that and anyway the heights aren't gonna work with the bricks I want to use on the top.  The potential issue is that if the cooktop puts weight on the inner end of the stove top bricks, it could dislodge them.  It's a bit more tricky as the underside of the cooktop has a ridge along the long sides, which will mean some creative brick placement to get the bricks nice and close under the surface.

I've been messing with it this morning as it happens.  The holes have discs that fit in them (or, on the big one, a whole set of rings).  The proposed top edge bricks are 6cm tall which is gonna come out quite close to cooktop level with the cooktop sitting on firebricks like in the picture, only properly set in place
Looks like if I set firebricks on the top of the core, they can support the cooktop pretty well.  I'll need also to build up under it on the left in the picture.  The edge bricks I now plan to fit so they butt up against the cooktop leaving a small gap for expansion.  Not yet certain about sealing under the cooktop, might simply set it on clay-sand mortar or I might get technical and put a rope there.  The result of this will be that the cooktop can be lifted off to gain more access than is got by removing the discs from it.  Removing the discs will give plenty of access for cleaning should it be needed.

The bricks for the top layer, around the cooktop, are the ones with a curved front that are usually used on the brick barbeques that are so popular here, like the one in the photo below, taken from this site.



It took us a while to find a place to buy the bricks. Couldn't find any locally so we're having a parcel delivered, which was cheaper than driving to fetch them.



This is the supplier - rustic curved bricks

Iggy stayed indoors to help Austin with the design, but the girls were all outside admiring their new flower and wondering if it means that special fruit will be forthcoming...

Today was an unexpected day with no worky-work so progress on the stove.  
Finished the cooktop support.  I have a background of messing with IC engines and I think it shows

Firebox door fitted, but don't be fooled: it's got a bit of work left to do yet:  needs openings for the primary¹ and secondary² air, and a channel fabricating on the inside to hold a fiberglass rope to seal it against the front of the firebox³

Also when I get hold of a suitable piece of glass, it's gonna have a window, cos who doesn't like to see the fire?
Yes, I know they're both slightly on the squint.  Drilling holes in brick is not a precise art, and the fixings are prone to have a bit of variability too.  I may fix it if it annoys me too much.
Oh yes and the fresh clay mortar is because you can't drill holes in the bricks set in clay mortar without shaking them loose.  This is the flip side of clay mortar.  Also I observe that it shrinks quite a bit when it dries.

¹ What size does this need to be?  any clues?  I'm probably gonna drill a pattern of holes so embers are less prone to fall out (not that I really think they will, by why chance it?) so I need to know how many holes to drill.
² This one's easy, same size as the tube inside.
³ That may not be necessary.  What do the experts say on this?  It'd be a lot easier not to do that part.  There's about a ⅜" gap between the door and the firebox.

I don't have any answers for you Austin. Just wanted to say, even after being told the doors aren't straight, they look straight to me.

Joylynn Hardesty wrote:Just wanted to say, even after being told the doors aren't straight, they look straight to me.

Alas, I agree with Austen - they look wonky to me. I think the issue is that they're wonky in opposite directions, so that multiplies the problem. I suspect if the rectangular one can get fixed somehow, the lower one wouldn't be as obvious.

That said, they still look awesome and I doubt I could have done half as good a job!

Thank you for sharing this build Austin and Burra. It is very helpful as there are so few builds showing the process of putting together one of Matt's stoves. I'm working on his Continental now and appreciate you sharing all the steps. I did wonder about your use of concrete in the mortar. I was under the impression that the use of a clay based mortar was so that the clay bricks and mortar would expand and contract at the same rate, reducing the risk of cracking. I see Thomas did the same as you on his riserless core build. Did you give that any thought, or did you feel it was a minor issue and move forward with what was readily available and what you were more familiar with? The stove is looking fantastic... keep on keepin" on!

Jason Pritchard wrote: I did wonder about your use of concrete in the mortar. I was under the impression that the use of a clay based mortar was so that the clay bricks and mortar would expand and contract at the same rate, reducing the risk of cracking. I see Thomas did the same as you on his riserless core build. Did you give that any thought, or did you feel it was a minor issue and move forward with what was readily available and what you were more familiar with?

It was purely because we'd failed to source any. We figured to go ahead with what we could find, because we wanted it built before winter!

Of course, doing that is tempting fate and when we'd started building we managed to find a supplier of fireclay, which we switched to using when we built up to the level of the firebox. The same supplier also had the pretty curved bricks we wanted for the top layer, too. In fact, they have just arrived and Himself is busy bringing them into the house as I type.

The stove is looking fantastic... keep on keepin" on!

I'm sure more photos will be happening very soon!

Thanks Burra. I'm glad to know your thoughts on the matter and appreciate the quick reply. Looking forward to the rest of the build!

Photos, as promised.

I managed to time going downstairs to take photos in such a way that the last of the bricks were being brought in. Oooops...

One heap of bricks, with pretty curved noses. Which is, naturally, being thoroughly tested by two of the dragons...



They are going to be arranged around the edge of the stove top, which will involve figuring out how to make the corners.  Iggy is puzzling over it.



Austin is helping him figure it out. I think it's going to involve a lot of rather fiddly cutting.

I used clay-sand mortar for the internal firebox core assembly which gets much hotter. Also for the top layers of the stove in case I need to get it apart for repairs or such.  

The lower body isn't normal mortar it's special refractory mortar which is used for making brick ovens and barbecues and the like.  

I noted an issue with the clay mortar when drilling the brick for the top hinge fitting, the vibration of the drill loosened the brick.  Clay mortar has much less strength/adhesion than the refractory mortar.  Clay is cheaper though.  You also want some fine sand for the core build - I tried some of the sand we'd normally use to build a wall and it didn't work too well.  What's sold here as "yellow sand" (also maybe soft sand) which is commonly used for rendering seemed to work nicely.  'course, that might be why it came loose when I drilled it but I think that's just because clay doesn't set rock-hard like cement based mortar.

Progress is happening with the top of the stove.



He treated  himself to a wet-saw for cutting the top bricks as they seem to be made of harder stuff than the others. The corners have come out rather well I think!



There will be be a rope fitted under the edges of the stove-top in due course.



He's making a flange thing to fit the stove pipe too, though he's not sure if the bit of metal he used is thick enough.



Eventually that metal sheet will have bricks cut to size and shape and it will be built into the rest of the brickwork.

The weather has turned here, and the rains have arrived. So it won't be very long before I'll be moithering for a test run...