Materials Characterization

Microstructural analysis of the render of the

Bolinder palace in Stockholm

Bo Nitz

Optiroc AB, P.O. Box 707, SE-169 27 SOLNA, Sweden

Received 20 March 2004; accepted 13 September 2004

Keywords: Render; Ransome stone; Light optical microscopy

1. Introduction

The residence of Jean Bolinder (1813–1899) at

Blasieholmen in Stockholm was built in the years

1874–1877. The construction proprietor, Bolinder,

was a mechanical engineer and was the owner of

machine shops and foundries. He was also interested

in the latest techniques and methods of building

construction. He had widespread contacts all over

Europe with prominent people in industry. For the

design of his residence, Bolinder engaged perhaps

the most outstanding architect in Sweden at that

time, Helgo Zettervall. Zettervall was also interested

in advanced technology including the use of iron and

brick walls for framework. The masonry for the

brick walls was made with cement mortar, which

was unusual for that period. The practice became

widespread decades later. Almost the entire facade is

made of cement-rich renders made on site or

1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.matchar.2004.09.005

E-mail address: bo.nitz@optiroc.se.

fabricated from pre-cast artificial stones. A few busts

are the only gypsum ornaments. Advanced shapes of

ornamentation, decorative beadings, column heads

and skirt-boards were made of strong render or pre-

cast in very strong mortar/concrete, which in those

days was called artificial stone. Columns and

pilasters were made on-site from strong lime/cement

render.

The smoothing sub-coat was made of weaker

lime/cement mortar. Either natural stone or artificial

stone or strong mortar with high cement content

was put on top. The entire methodology of

construction is completely opposite to today’s

recommendations of render system composition.

However, this rendering has lasted for more than

100 years, and it would probably have lasted longer,

if general maintenance had been practiced. Unfortu-

nately, gutters and down-pipes, in particular, were

inadequately repaired.

The objective of this study was to analyse the

render composition and determine its quality, as a

basis for proper recommendations for repair work and

for the choice of renovation mortars.

53 (2004) 187–190

B. Nitz / Materials Characterization 53 (2004) 187–190188

2. Sampling

Three samples were chosen for structural analysis:

(1) Sample No. 1 was cut out from a beading of a

balcony.

(2) Sample No. 2 was a corner of a pilaster, which

had broken off.

(3) Sample No. 3 was a column head.

To the naked eye, Sample Nos. 1 and 3 looked

like pre-cast artificial stone. Sample No. 2 was

assumed to be a site-made render. The selection of

these samples was based on visual examination, and

on overall quality. The relative strength of candidate

samples was also estimated by scratching the surface

of each piece. To make the right choice of

renovation renders, it was necessary to cover the

range of cement-compositions and strengths of the

facade components.

3. Methods and preparation

A very useful tool for helping with a structural

overview of a cement-base building material is light

optical microscopy (LOM) performed on thin-section

samples. One thin-section was prepared from each

sample. As the degree of hydration of the cement

clinker grains in the artificial stone was low, two

polished and etched sections were prepared to

analyse the clinker quality. In addition to LOM,

small, 13 mm cube samples were prepared for

determination of density and strength of Sample

No. 1.

Fig. 1. This figure illustrates the carbonation front in the Ransome

stone. The brownish part at the top of the image is carbonated. This

shows that even after 125 years the paste in the stone has not

completely carbonated.

4. Microscopy

Qualitative and quantitative analyses were per-

formed on each sample to determine as accurately as

possible the age of the mortar, the constituents of the

samples and their composition. Because the samples

were about 125 years old, it was assumed that their

degree of hydration would probably conceal certain

constituents with low percentage content. However, it

was found that the hydration had proceeded to a

certain degree and then more or less stopped. The

carbonation front had not penetrated deeper than 5–7

mm in Samples 1 and 3. This finding indicated an

almost impermeable structure into which CO2 would

penetrate extremely slowly.

The very tight renders with general low perme-

ability and the Ransome stones in particular arises

from additives used in their preparation. An English

engineer named E. Ransome is regarded as the

inventor the Ransome artificial stone, the first func-

tional stone [1]. The stone consists of water glass,

cement, quartz powder, clay and pozzolanas among

others. The water glass makes the stone very tight

with very little capillary porosity. It is almost certain

that water glass was added to Sample Nos. 1 and 3 in

the fresh mix. It is also probable that the renders had

water glass in the fresh mix to obtain the tight and

hard surface.

Sample No. 2 is the corner piece of a pilaster that is

definitely made and shaped on the wall. Under the

microscope, Sample No. 3 looks at first to be pre-cast,

but structural logic implies that this is not the case. It

must be assumed that the column heads are placed,

shaped and worked on the wall. Sample No. 1 has

cement that looks different from Sample Nos. 2 and 3,

based on the clinker studies. In addition, Sample No.

1 has no aggregate and this suggests a different origin.

The aggregate and cement in Sample Nos. 2 and 3

looks as though it came from the same source but the

cement differs from Sample No. 1. It is therefore

Fig. 4. This figure shows the render of the column head. It is

certainly more worked on than the pilaster render because there are

no cavities, only air voids (epoxy-filled circular yellow features in

the figure). A more appropriate definition of this material is concrete,

as the material is so strong and dense. This was made on the wall,

even though the microstructure suggests that it was pre-cast.

Fig. 2. This figure shows a wooden stick in the Ransome stone,

placed there during construction. These are centering pins used to

align the stones when setting them in place. The blue stripes in the

stick are annular growth rings in the wood.

B. Nitz / Materials Characterization 53 (2004) 187–190 189

certain that Sample No. 1 is the only material not

made on-site.

5. Composition defined as lime/cement/aggregate

The compositions of the materials were determined

from microscopy and from knowledge gained in

earlier studies. The accuracy is not high, and

estimated at F10–15%. The approximate lime/

Fig. 3. This figure shows the topcoat of the pilaster render. The

paste is tight and dense but the render has cavities. The consistency

of the fresh render was probably like dough, so great effort is

needed to work out the air inclusions.

cement/aggregate compositions of the three samples,

per 100 kg, are as follows:

Sample No. 1 5/95/0,

Sample No. 2 5/95/350,

Sample No. 3 5/95/350.

Apart from cement, lime and aggregate, additional

materials were detected in the structure but the amounts

Fig. 5. This figure shows a polished section of a cement clinker

grain in the artificial stone. Compared to today’s clinker, this has

low quality with many eutectic formations probably due to

insufficient cooling.

Fig. 6. This figure shows the clinker from the cement of the column

head. The clinker phases are somewhat better formed with fewer

eutectic formations. The cement in the Ransome stone is coarser, up

to 0.7 mm, compared to 0.4 for the site-made renders.

B. Nitz / Materials Characterization 53 (2004) 187–190190

were not possible to measure or even estimate. The

detected materials were probably clay and chalk.

6. Images from microstructure analysis

Examples of the microstructural investigation are

shown on the color plate (Figs. 1–6).

7. Compressive strength

As Sample No. 1 seemed so strong when scratched,

three 13 mm cubes were cut. The compressive strength

was measured to be as high as 73 MPa even though the

density was as low as 2250 kg/m3.

8. Discussion

How is it that render compositions, made in

contradiction to present-day recommendations, stand

up so well for more than a hundred years? First, one

important factor is the wall surface. As the wall

surface has so much unevenness from ornaments and

decorations, hygro-thermal forces have not resulted in

widespread cracking. Secondly, the craftsmen of the

1800s took greater care to perform the rendering work

and completed it with more accuracy than today. This

means that pointing work was so well done that the

joints have stood up to the elements very well and, on

the major part of the facade, no water has penetrated

into the weaker render layers underneath. Less than

5% of the facade was damaged after 125 years of

service.

The topcoat render is to be regarded as very tight.

The pilaster coating, which is site-made, has not been

carbonated completely after 125 years. This means

practically no water penetration occurred. Icicle

formation and ageing over the years has misshaped

gutters and down pipes. Leakage has showered parts

of the wall and this has contributed to localized

deterioration.

Reference

[1] Manufacturers and building. New York7 Western & Company;

1870.