although there may be some

differences in porosity but not to any

marked effect. The Morton Partnership

has successfully used the gauge mixes

for many years in historic structures

without, as far as we are aware, any ill

effects.

properties of coarse and fine sand to

retain the mixture’s porosity. If a

bricklayer can work it, it is generally

acceptable.

Hydrated lime and hydraulic lime

and mixes with a Pozzolana or weak

lime, Portland cement mixes, can all

be used perfectly safely in historic

structures if each use is fully

understood. Great care is necessary

in using modern hydrated lime on its

own.

MORTARS TO USE

It is essential when deciding on the

mortar mix to use that the material to

which it is to be applied as either a

mortar or a render, is fully understood.

The application of the render is critical

in that the surface porosity of the base

material needs to be considered be-

fore any decision is made. Of greatest

importance is that the cementatious

material used is fully understood.

Experience has shown that 1:3

hydrated lime sand mix can be

unreliable in its setting properties. If it

is used at all, samples should be tried

before use. It is clear that bagged

hydrated limes and some lime putties

are too pure and will give an

unacceptable curingtime. Lime slaked

and mixed on site is generally better.

Hydraulic limes should be used with

caution. Pre-mixed bagged mixes with

sand for specific purposes are

preferable. The use of natural additives

(pozzolana) is perfectly acceptable if

test mixes are made.

The weak cement/lime/sand mixes

1:3:12, preferably (may be 1:2:9 for

some purposes), are of similar strength

(weak) as 1:3 lime sand mortars and

can be used, but it is very important

that the sand contains the correct

SAND FOR MORTARS

Historically, references back through

the centuries suggest that the sand mix

should be 60% sharp sand, 40% fine

sand for mortar and render mixes. This

seems to have been adopted as a gen-

eral principle but because the sands

and limes had various impurities in

them which worked very satisfactorily

to give a chemical set, each mix which

was made up had to be gauged on site

to satisfy the bricklayer.

A 90-year old bricklayer told me that

they simply made up the mix following

the 1:3 principle and then adjusted the

mix so that it “just fell off theirtrowel”.

The same man told me that with the

introduction olPortland Cement in the

1930s they reverted to a 1(cement): 2

(lime): 9 (sand) mix, but again gauged

this themselves on site. He was able to

show me a flint church that was

rendered in the early 1940s with a

1:2:9 mix which had not cracked at all.

Brian A Morton CEng MICE Conservation

(AA).

CORRECTIONS

In Michael Farey’s article ‘There is

more to lime mortar than. . .‘in Con-

text 52, a colon was transposed with a

decimal point in the third paragraph of

column two on page 27. The sentence

should have read: “Generally most his-

toric records show that blue lies hy-

draulic limes were used on their own

— often 1:2, certainly not less than 1:3

and sometimes 1:1.5” (i.e. one to one

and a half, not one to one to five). The

Editor apologises for any confusion as

a result of his myopia!