Tempering Lime Putty Mortars

‘Tempering’ is the practice of allowing Lime putty mortar to stand undisturbed for a period of time in the wet state before use.

The workability of pre-mixed lime putty mortars and plasters can improve if left to stand undisturbed for a period of time before use.

Whilst this tempering may be justified in certain situations, the practice is the least important phase in the production of best quality mortars and plasters, and has some disadvantages. If mortar is to be tempered, a period of one week is recommended when six month mature putty is used – tempering for a longer period is superfluous.

More important criteria for the production of best quality mortars and plasters are:

• The use of a good quality matured lime putty of correct density
• The selection of good well-graded aggregate of appropriate particle size and range.
• The correct lime: aggregate ratio.
• Thorough mixing preferably in pan-mixer or roller mill.

Situations where ‘tempering’ might be advantageous are:

• Hot-mixed lime mortars (mortars produced by mixing quicklime with aggregate) are probably best tempered for at least 1 month to allow any unconverted quicklime to slake during the tempering process.
• Workability of mortars containing porous aggregate may be improved.
• Mortars stiffen during tempering and this may be an advantage in certain applications.

Reasons for not tempering for a prolonged period are:
• Mortars can become too stiff to use necessitating the addition of water before use. The addition of water to lime putty mortar is best avoided due to the increased risk of shrinkage.
• Animal hair in haired plasters degrades during tempering necessitating the addition of more hair immediately before use.