How to Build a Lime

Mortar Foundation

and Cob Oven

 

Register for a Workshop

Tools and the like:

Buckets with lids, Rubber gloves, Drill with paint mixing attachment, Screens, Mattock, Wheelbarrow, Rock hammer, Wire brush or Churn brush, Tape measure, Level, Shovel, Spray bottle, Tamper, Trowels, Tarps, Carpentry Tools

Materials:

Hydrated lime, Course sand, Water source, Brick dust or grog, Rubble, broken urbanite or 3" rock filler, Stones, bricks, blocks or big chunks of urbanite, String Chalk line chalk, 21 Fire bricks, Newspaper, Clay, Soil, Straw, Wood for door, Door hardware

 

Recipe for Lime Putty:

- 2 Parts hydrated lime (builders lime - not garden lime)
     {Hydrated, builders, masons & quicklime-all the same}
- 1 Part water

- 1 Part lime putty
- 4 Parts course sand
- 1/4 Part brick dust or grog
- Water as needed (see mixing & laying tips)

Recipe for Lime Plaster & Pointing Mortar: 1 Part lime putty / 3 Parts course sand / Water as needed

 

Recipe for Fire Cob:

- 4 Parts course sand
- 1 Part clay
- Water as needed



 

 

General Helpful Info.

Clear away the top 12 to 18” of topsoil for the garden to get to the harder sub-soil below. Most soils contain some clay and sand. The ratio of sand to clay will vary depending on the stickiness of the clay. Soil tests can be found in any of the resources listed below.

Recipe for Cob:

- 4 Parts course sand
- 1 Part clay
- Water as needed
- Straw to taste

Mixing Cob Tips:

Cob can be mixed easily on a tarp. Mix together sand and clay first by turning the tarp over on itself and dancing around the center. Add water as needed first, and then sprinkle in as much straw as the mix will hold without falling apart.

 

These instructions are for a 27" base oven and are by no means comprehensive. But rather, are intended as a frame of reference for workshop participants as a review. The resources are BUILDING WITH COB by Adam Weismann & Katy Bryce, BUILD YOUR OWN EARTH OVEN by Kiko Denzer, and THE COB BUILDERS HANDBOOK by Becky Bee.

 

1. Dig a four and a half foot circle 18" deep or down to the frost line. Roughly level and tamp.

2. Fill to within 1" of ground level with drain rock, rubble, or broken up urbanite. Tamp every 6".

3. Face (shape) stones with a rock hammer and clean surfaces with a wire brush.

4. Using the largest stones 1st, lay the first course in a 4 1/2 foot ring, at least a foot wide. Level.

5. Spray the first course with water and cover top of stones with mortar.

6. Lay each course 1 stone over 2 with no vertical seams. Wet each course and top with mortar.

7. Build stone plinth (above ground part of foundation) up to the main baker's waist height.

8. Fill with drain rock, rubble or urbanite to within 6" of top. Tamp every 6".

9. Fill gaps and top 6" with sand for insulating base. Top 1/2” should be a finer sand to bed bricks in.

10. Strike joints and clean stone faces with a wire brush after 24 hrs.

 

11. Starting from center, set firebricks level according to the diagram, with no spaces between them.

12. Mark 27”, 33”, 45”, & 49” diameter rings on bricks & sand with a string and chalk line chalk.

13. Build form, the void that will be your oven, with damp sand to 191/2” high (vertical to 41/2”.)

14. Compact sand with hands or a board and cover with damp newspaper.

15. Mix the 2 batches of cob fairly stiff. Apply 3” thermal layer first (fire cob) pressing cob into itself.

16. Compact cob with a board, being careful not to crush form. Score surface and apply next layer.

17. Mark out the door 121/4” high X 191/2” wide and extract sand when cob feels firm to the touch.

18. Cut out the door like carving a pumpkin so it doesn’t fall in. It can be made out of wood or cob.

19. Allow oven to dry for about a week. Apply a lime or earthen plaster and pointing mortar.

20. BUILD YOUR OWN EARTH OVEN by Kiko Denzer has fire building tips and a great bread recipe!