Pompeii dome brick gauge

[phi1]

I have seen a lot of photographs on this forum with a gauge for supporting the dome bricks and ensuring a circular construction.

They seem to have a base with a door hinge and then three pieces of timber which can be altered as construction proceeds.

I have searched this site and tried Google but can't find any construction details or instructions on how it is used.

Can any forum members help?

Phil 

[chas]

Not quite what you asked for Phil, but mine is a simple piece of flat timber about 1.5" x 3/8" x 20" with another about 6" long joined and overlapping with short screws. You can see it here: ukwoodfiredovenforum.proboards.com/thread/1983/norfolk-build?page=1

resting jauntily against the side of the dome-in-progress.

All you do is make a temporary platform level with the height of any upright brickwork you have, centre a screw in that and fix the two bits together so that with a notch in one end against the screw, the step in the overlap hits the radius. Every brick now laid will follow the radius and, as importantly, the stick (trammel) will lay flat across the top of the brick as the dome goes up and over. The step will give some support to the brick being laid too.

Hope it helps: if not, I'm sure someone will come along soon with another take on it...

Chas


Happy Christmas to all!

[phi1]

Thanks Chas

Some in the pictures on this forum seem to have adjustable lengths which are altered as the dome progresses and then a wire rigging to support?

One idea I have seen (not on this forum) is a caster wheel with the wheel removed and the trammel fixed in place of the wheel. This is fixed to the centre of the base to give a constant distance all the way up. This would give a 100cm radius making the dome 200cm wide at the base and 100cm high. is this correct or too simple?

Phil

[chas]

Dec 24, 2014 13:52:32 GMT phi1 said:

Thanks Chas

Some in the pictures on this forum seem to have adjustable lengths which are altered as the dome progresses and then a wire rigging to support?

One idea I have seen (not on this forum) is a caster wheel with the wheel removed and the trammel fixed in place of the wheel. This is fixed to the centre of the base to give a constant distance all the way up. This would give a 100cm radius making the dome 200cm wide at the base and 100cm high. is this correct or too simple?

Phil

Haven't seen that, but the general principle is the same: if you scribe 100cm radius at the base - the level at which you want to start the dome, some builds have a course or two of vertical brickwork before the dome proper - then yes, as the build progresses you'll get 200cm internal width at the base and 100cm internal height above the start point... in other words, a simple and pleasing hemisphere - not to mention a huge oven.

The reason there may be a course or two of vertical brickwork off which you spring the dome is the 63% 'rule': the height of the doorway should be no more than 63% the height of the dome. In your mega-oven example, if you wanted a door 60cm high, you'd need a dome 100cm high to comply. If you wanted to use a fantastic reclaim door you've bought that's 70cm high you'd probably jack the dome up on a couple of courses to make 115cm if you also want to keep the 100cm radius...

Can't think why one build needs an adjustable trammel - it might be a useful bit of kit if you're building several ovens of differing sizes - but making one from timber takes seconds -  and haven't seen wire rigging... can you post a link to the thread(s)?

I should probably say my only experience is with red bricks stuck together with potter's clay, and astonishingly, no support is needed - but mortar builds are different.

Chas

[cobblerdave]

G'day
The tool you've discribed I know as an IT or indespensable tool. If you fix the tool say to a piece of ply for example the pivot point is above the hearth and will result in an arch that place the top point of the dome higher than you think.
If that pivot point is on one of the wheel type it could be up to 50 mm higher than the hearth with will put your dome 50 mm higher. Making it adjustable is one way around this problem. Being adjustable also can be used build a flatter dome than a 1/2 hemisphere.
If you remove a few hearth bricks and mount it lower than hearth hieght the arch will result in a lower flatter dome.
A couple of things to note. Its important that the brick holder at the end of the arm places the brick in centre of that arm to get the brick face to the correct orientation.
The next thing is the brick face is flat and the tool goes in an arch, moving the tool can result in the brick fresh laid to be moved. One way is to build a removiable spacer for the face of the brick holder. Lay the brick ,remove the spacer, move the IT then replace the spacer for the next brick.
A simple method for an IT is to cut a sheet of 10 mm ply to fit on the inside of the oven and lay it on the hearth. (protect the hearth to from drips of mortar) . Cut this circle into pieces that can be removed through the door when the ovens complete and batten and screw it back together.
A gate hindge drill a hole closest to the hindge pivot point and nut and bolt it in the centre of the ply circle loosely so it turns. Screw an arm to the other end of the hinge and you have an IT that turns and pivots. It will be out say 12 mm which is a small amount to adjust at the top of the dome.
Hope something here might help
Regards Dave

[phi1]

Thanks Dave that is really helpful.

I am not a bricklayer but am very keen to have a bash at this as an enthusiastic DIY'er. As the dome goes up and gravity takes over do you leave the IT in place with each brick until the mortar has set a bit?

Phil

[cobblerdave]

G'day Phil
I would worry about bricklaying experience at all. The vast majority of these ovens are built by first timers with zip experience.
The IT method is slow but good mainly because it allows you to clean the inside surface as you go. As gravity starts to take effect you can use sticks to hold the bricks in place. Its slow but as the dome closes in you have less bricks to lay for each row anyway. I've seen wire hooks put over the brick with a weight placed on the outside of the oven to hold the brick down as well.
A hybrid method of completing the top of the dome is to cut a circle of ply and with a car jack and some blocks of wood put it up against the inside of the dome . It gives you a surface to build a sand mold to complete the last layers. You complete the last bricks in one go, then drop the jack so you can clean up the inside of the dome and do any re-pointing of mortar gaps.
Whichever method you use once that final brick goes in and the domes complete its a strong structure. The mortar isn't the glue that fights gravity but the gape filler between the bricks. The finish dome owes its strength to gravity holding the bricks together rather than the strength of the mortar.
Hopes this is of some help
Regards Dave
PS I build to a mold of plywood vanes , then burnt the plywood out later. It's not a method I would use now.

[phi1]

Dave, Thanks again for the advice.

Quite like the idea of a sand former for the last few courses.

Fantastic day here in Sussex today so got a bit more chalk dug out where the base will go. I can't believe how much spoil is produced from a 1500mm square hole, lost some soil in the garden but will have a few trips to the local landfill site.

I will keep a log of progress and post as appropriate.

The forum is a real confidence booster

Phil