Lime mortar
A traditional lime mortar is a combination of lime putty and aggregate (usually sand). A typical modern lime mortar mix would be 1 part lime putty to 3 parts washed, well graded, sharp sand. Other materials have been used as aggregate instead of sand.
The theory is that the voids of empty space between the sand particles account for a 1/3 of the volume of the sand. The lime putty when mixed at a 1 to 3 ratio, fill these voids to create a compact mortar. Analysis of mortar samples from historic buildings typically indicates a higher ratio of around 1 part lime putty to 1.5 part aggregate/sand was commonly used. This equates to approximately 1 part dry quicklime to 3 parts sand. A traditional coarse plaster mix also had horse hair added for reinforcing and control of shrinkage, important when plastering to wooden laths and for base (or dubbing) coats onto uneven surfaces such as stone walls where the mortar is often applied in thicker coats to compensate for the irregular surface levels.
Shrinkage and cracking
If shrinkage and cracking of the lime mortar does occur this can be as a result of either:
- The sand being poorly graded or with a particle size that is too small
- The mortar being applied too thickly (thicker coats increase the possibility of shrinkage, cracking and slumping)
- Too much suction from the substrate
- High air temperatures or direct sunlight which force dry the mortar
- High water content in the lime mortar mix
- Poor quality or unmatured lime putty
Mixture
A common method for mixing lime mortar with powdered quicklime is as follows:
- Gather your ingredients, sand, quicklime, and water
- Measure out your ratio of sand to quicklime, for example 3 buckets of sand, and 1 bucket of quicklime for a 3:1 ratio.
- Mix the dry ingredients thoroughly so all the sand is coated with quicklime, and there are neither chunks of sand or quicklime visible.
- Reserve some portion of the dry ingredients by removing it from your mixing vessel. The amount reserved can vary, but a safe starting point is about 1/4 of the batch. This will be added in later to fine tune the dryness of the mix.
- Measure out water. How much depends on how wet you want your mix to be, and how damp/wet your sand is. A good starting point is 1 quart of water per gallon of sand.
- Add about 2/3 of the water to your dry ingredients and mix until even consistency.
- Add the reserved dry ingredients and/or the remaining water to get a mix you like. It takes time to know what works well, and the recipe can change depending on the temperature, humidity, moisture in the sand, type of brick, and task at hand (laying brick may warrant a wetter mix, while pointing may require a drier one.
- To test the mix as you are making it, you can use a trowel, or pat the mortar with your hand to see how much moisture and "cream" come to the surface.
- Remember to thoroughly wet your brick prior to using lime mortar. Old brick can be extremely porous, a 4lb brick can hold a pint of water. The bricks should be saturated, but dry on the surface prior to laying or pointing. Excess water can cause the quicklime to run and leave streaks.
See also
References
- The Myth in the Mix, The 1:3 ratio of lime to sand buildingconservation.com
External links
- Lime mortar, Wikipedia