Homebrew Castable 21" Kent (UK)

[david s]

Thanks for documenting the build with all those pics. Did you find the casting picked up a lot of sand on the inner surface? Also did you fill any voids on the inner surface after de moulding? And did you add any fine polypropylene fibres to the homebrew as burn out fibres to give the casting some steam spalling protection?
Looking pretty good.

[bamforp]

After completing the dome mouth forms it was time to build the sand castle! I used some left over bricks to bulk out the volume and then started to build the dome using sharp sand with a handful of fireclay. The gauging tool I made can be seen in the photos and this worked really well for me. I could easily see where I needed to add more sand and I could also use it as a scraper to remove any excess. I made the tool so that it was a 90 degree arc so I could reference everything off the plane of the firebricks.

I did run out of sharp sand towards the end and tried using some soft sand that I had to hand but quickly realised this was a mistake. Whilst it did mould nicely, it didn't have the inherent strength of the sharp sand and I could see it was going to peel off very quickly when I came to cast the homebrew.

The last photo shows we had a visit from next doors cat overnight but it is impressive how strong the sand/clay mix is and it was an easy fix in the morning.

[bamforp]

The next day I cast the homebrew dome. I used a mix of 2 parts sharp sand, 1 part soft sand, 1 part lime, 1 part cement, 1 part fireclay. I added the soft sand as I was concerned the sharp sand I had was very sharp with a lot of fairly large particles. To that mix I added roughly 1g/litre of alkaline resistant glass fibres plus half a handful per 10 litres of polypropylene fibres. The first photo shows my setup and I dry mixed roughly 10 litres at a time by hand on the spot board using a good pair of chemical resistant gloves and a mask. The fibres were quite 'clumpy' so mixing by hand gave me a chance to break up the larger clumps.

I then added water to a roughly 3 litre batch in a bucket which was about the limit I could stir by hand. I used a couple of old tins to gauge the mix volumes to make sure I had a consistent mix across the whole cast which I thought would be quite important.

After applying the lower ring of homebrew, I found the technique that worked best for me was to overlap each new handful of material onto the existing material by at least 50% and then squeeze it up to advance the leading edge of the cast if that make sense. This seemed to work much better than applying a new handful onto 'virgin' dome and then trying and bring the new and old material back together.

[bamforp]

After reading a lot posts on the forum I came to conclusion that a WFO without cracks is a rare animal, especially in single piece cast domes. One approach to mitigate the impact of cracks appears to be to cast the dome in multiple sections so that the various parts of the dome can move independently of each other according to the differing rates of thermal expansion they are experiencing. Building the moulds to cast the dome in multiple sections was not a task I thought I was up to so instead I opted for a hybrid approach in the spirit of experimentation. I cast the dome up to around 70% of the height and then introduced a barrier between the upper and lower parts of the dome using wet newspaper. I then completed the rest of the cast as normal. Hopefully the photos illustrate the concept better than I can describe in words.

My reasoning is that the most severe temperature gradients will be towards the top of the dome so by introducing a weak bond between the upper and lower parts of the dome it will create an opportunity for the thermal stresses to resolve without creating uncontrolled cracking elsewhere. It may turn out to be a complete dead end but it only added half an hour to the casting process so I thought it was worth a try.

The final photo shows the completed dome wrapped in cling film to retain moisture for an extended cure.

[david s]

I like your idea. When I first started casting my domes I did them in one piece and came to the same conclusion as you. I now cast them in three pieces which allows for some expansion, but also has the benefit of the three pieces being of a more manageable weight. The top piece of my castings is pretty much exactly where you have placed your slip joint and my home oven with its one piece dome, which is around 15 years old has a fine hairline crack running pretty much exactly around where the top casting in the newer three piece ovens happens to be. This confirms our theory about the ideal placement of the join. With a fire inside the oven and heat by convection rising and with flame impingement on that top section, it will get way hotter and much faster than any other part. In a brick oven all the mortar joins become pathways for cracks to develop and for this reason the mortar should always be weaker than the brick units. You will probably find that you will get a fine crack at exactly that position. Better to engineer its placement than allowing it to find its own path. Exactly the same technique is used on concrete paths where cracks are encouraged to occur by grooves made in the path.

[bamforp]

After 24 hours, I removed the polystyrene moulds and the sand and bricks from the inside of the dome. The polystyrene moulds worked extremely well for me - they have a bit of give so coupled with the use of the wood strips to raise the moulds, they came out in one piece with just a bit of gentle wiggling.

After reading about the trials and tribulations some people have had with layering the sand mould with newspaper or clingfilm prior to casting the homebrew I decided to cast directly onto the sand mould with no release agent and I have no regrets. I was easily able to remove any residual sand from the inside of the dome with a gloved hand and a stiff brush which meant I was quickly able to inspect the surface of the dome to check for voids.

I am not sure how well it shows up in the photos but I was very pleasantly surprised to see there were no significant voids apart from on the front face of the dome mouth which were easy to fill with a homebrew mix using soft rather than sharp sand. I did fill some of the more uneven surfaces in the dome with the same mix but I don't think this was really necessary and was more for cosmetic reasons than anything. The final photo shows how the weak barrier I described in my last post shows up on the inside of the dome.

The other thing I should mention in conclusion of this stage of the project is that I would definitely recommend casting the dome and gallery separately particularly for newbies like myself. I feel I learnt a lot about how to mix and handle the homebrew whilst casting the relatively simple dome structure that will stand me in good stead when I come to cast the more complex gallery plus access to the dome is so much more straightforward without the gallery in place.

[bamforp]

I like your idea. When I first started casting my domes I did them in one piece and came to the same conclusion as you. I now cast them in three pieces which allows for some expansion, but also has the benefit of the three pieces being of a more manageable weight. The top piece of my castings is pretty much exactly where you have placed your slip joint and my home oven with its one piece dome, which is around 15 years old has a fine hairline crack running pretty much exactly around where the top casting in the newer three piece ovens happens to be. This confirms our theory about the ideal placement of the join. With a fire inside the oven and heat by convection rising and with flame impingement on that top section, it will get way hotter and much faster than any other part. In a brick oven all the mortar joins become pathways for cracks to develop and for this reason the mortar should always be weaker than the brick units. You will probably find that you will get a fine crack at exactly that position. Better to engineer its placement than allowing it to find its own path. Exactly the same technique is used on concrete paths where cracks are encouraged to occur by grooves made in the path.

I am mightily encouraged you think it's worth a try. I couldn't really see any downside to giving it a go so lets see how it works out.

[bamforp]

Thanks for documenting the build with all those pics. Did you find the casting picked up a lot of sand on the inner surface? Also did you fill any voids on the inner surface after de moulding? And did you add any fine polypropylene fibres to the homebrew as burn out fibres to give the casting some steam spalling protection?
Looking pretty good.

Hi Dave - I really didn't see any downside to casting directly onto the sand dome - the residual sand was very easy to rub off and it has the advantage that I could immediately inspect the surface for voids without having to peel off any newspaper or clingfilm. I will certainly go back and give it another more rigorous clean once it has fully cured to ensure no grains of sand fall into my pizzas but in my opinion casting direct onto the sand is the way to go.

I didn't really have any significant voids on the inside of the dome so I just smoothed out a few uneven surfaces with a smooth mix just to make it look pretty.

I did add some polypropylene fibres to the mix along with the AR fibres. I do have some concerns about how well the fibres were distributed in the mix as they were very clumpy out of the packet. I did my best to break up the clumps by hand and they did seem to disperse better in the wet mix than the dry mix but you can see some fibre clumps on the surface in some of the photos I have posted.

[david s]

Yes that happens. They are hard to disperse and trying to make them do so in the dry mix is not very successful. I pull them apart a fair bit when adding them and mix by hand in a barrow (that way you can inspect the mix closely and adjust the consistency easily) for about double the time you would normally. If you look at the profile of the cast you should see lots of little hairs sticking out.

[bamforp]

Yes - they show up particularly at the sharp edges of the profile. I've zoomed in on one of my photos and they are quite visible.

[bamforp]

Some more updates to my WFO in Kent build. The dome has been curing for at least a week wrapped in clingfilm to limit the moisture loss - I also placed a bowl of water inside the dome to try and boost the humidity on the inside too. The form for the gallery is made from the same polystyrene foam I used previously, so I did not worry about including any releases in the form design as I knew I could just hack it out once everything had cured. I did add some wetted newspaper at the mouth of the dome to try and limit the bonding of the newly cast gallery onto the dome and then started the cast. The gallery cast was pretty straightforward but I did get a little sagging at the front and sides. I improvised a board and wedge to support the vertical face at the front of the gallery - the photo makes it look likes it is still not plumb vertical but in fact it was pretty straight. The final photo shows one of the two holes I cleared before the cast had set to act as vents from the ceramic blanket layer into the gallery. It also shows the plug used when casting the mount for the flue but more of that in my next post.

[bamforp]

Some more photos showing the gallery after it had cured and the foam form had been removed. The second photo shows the cast in the flue area. My plan is to seat the stainless flue in the recess and then pack around the annulus with a 5:1 vermicrete mix to allow for some differential expansion between the steel flue and the rest of the gallery (another tip I picked up from David's posts). Once again I am pleased to say there are no significant voids that I can see on the surface of the gallery and if you look closely at the third and fourth photos you can see where the vent holes I included exhaust into the gallery. Overall I am pretty pleased with how this stage went but if/when I do this again, I would aim for a thinner cast (maybe 35mm vs the 50mm I ended up with) to try and avoid some of the issues with sagging - you can see the sagging on the right hand side of the gallery in particular.

[bamforp]

Onto the cast decorative arch - another idea I have pinched from David! Once again, the form for the arch is made from polystyrene foam. In fact, I have created all of the forms and guides /tools for the project from a single 1.2 x 2.4m piece of 50mm insulation which cost less than £12 and I still have roughly half of it left over for another project. The arch was cast with a 10 mm gravel concrete mix and I included some old tent pegs for a bit of reinforcement. I also added some wire loops as per David's design to help eventually tie the arch into the render layer.

The next step will be to decorate the face of the arch with mosaic tiles. I am currently pondering what would be the best adhesive and grout to use for the mosaic so any suggestions would be very welcome.

[david s]

Your build is progressing really well.
Regarding the arch I damp cure mine for a week and before doing any mosaic I coat the surface with a sealer then uses quality outdoor tile adhesive. I was concerned about the heat interfering with the bond between the concrete arch and the tiles, but have never had a problem, although in my build I have an 8mm expansion gap between the gallery and the arch which apart from allowing the inner parts of the oven to expand without placing any pressure on the arch also reduces heat conduction. Also when I cast the arch I use a mix that contains 50/50 perlite and 7mm aggregate for the inner half, but no perlite for the outer half. I use two 6mm steel reinforcing bars in the outer half. Your tent pegs should work well. The more insulative mix also reduces heat transfer to the outer half and to the mosaic tiles.It is far easier to do the mosaic work on the arch when it’s horizontal then when it’s all finished mortar it in place.Use glossy tiles and black grout which make removing any smoke stains easy with a little damp newspaper.

[Mr. Slowhand]

Excellent looking oven. I stumbled here, as I am casting my flue, but the rest of the oven was brick made. Looking at this, the casting would have been a waaaay easier project. That is OK, we will keep that for the next one....
Question on your chimney placement:

[QUOTE=bamforp;n447891 The second photo shows the cast in the flue area. My plan is to seat the stainless flue in the recess and then pack around the annulus with a 5:1 vermicrete mix to allow for some differential expansion between the steel flue and the rest of the gallery (another tip I picked up from David's posts).[/QUOTE]

I presume the chimney will not go through that hole, but sit on it. The annulus is the size of the isolation between walls of the chimney? So the chimney will be held by the 5:1 vcrete, how high are you going to put the vcrete?
Apologies if I misunderstood, again, great looking oven!!

Marko