3rd Historic Mortars Conference 11-14 September 2013, Glasgow, Scotland

C19th cement based artificial stone at Swiss Garden,

Bedfordshire: Mortar Techniques and Materials used to

Imitate Stone

Simon H Swann

Simon Swann Associates, simon@swannassociates.co.uk

Abstract

Swiss Garden, Old Warden, Bedfordshire, has samples of 1830’s Austin Artificial stone and 1870’s

Pulhamite Artificial rockwork which have recently been surveyed. The paper examines the use of mortars

and mortar techniques which have been used to imitate stone and rockwork. Techniques include the

selection of binders, careful selection of aggregates, the addition of pigments, and the careful application

of mortars.

Keywords: Austin Artificial Stone, Pulhamite Artificial Rockwork.

1. Introduction

A recent survey of C19th garden ornament and landscape rockwork has allowed the investigation into

techniques and materials used in the imitation of stone and artificial rockwork. The Swiss Garden (Old

Warden, Bedfordshire) site is of particular interest because it offers two historic phases of early cement

mortar based imitation materials and techniques. The Garden was laid out in the 1830’s and had a second

phase of development in the 1870’s. The early phase of garden development employed Felix Austin’s

artificial stone ornaments, while the later phase had works carried out by Pulham and Son which involved

artificial rock landscapes and the grotto and fernery interior. The survey and investigation was carried out

by Simon Swann Associates and reports compiled (Swann and Bardwell, 2012) were submitted to the

Architect as part of the Heritage Lottery Fund stage 2 investigation.

2. Austin’s Artificial Stone

The Austin ornaments at Swiss Garden are dated between 1830 and 1836 and are of particular

interest because it appears that Austin used early Portland cement (Patent; J. Aspdin 1824) as the binder.

The ornaments are cast, usually in several sections and then put together to form an ornament, such as

the “Dolphin Tazza” (Figure 1) or “Eagles”. All Austin ornaments found at Swiss Garden are in a colour

that would match that of a light coloured limestone such as Portland stone. Austin clearly intended to

make this imitation as is indicated in introduction to his 1838 catalogue: “Austin’s Artificial stone is of a

light tint, requires no painting or colouring, will not sustain injury from the severest winter, and, being

impervious to wet, is particularly applicable to all kinds of Water-works.” (Austin 1838). The intention

to imitate Portland stone is made clear by Loudon in 1834 “Mr Austin states that he has brought the kind

of composition which he employs as artificial stone, to such a degree of perfection, that his imitation of

Portland stone is very nearly, if not quite, as durable as the natural material itself”. (Loudon 1834).

The comments and references above are important and help the understanding that at about the time of

the patent for Portland cement (J. Aspdin 1824) one of the main thrusts for binder development was to

develop binders which had the strength and qualities of Parker’s Roman cement (Patent 1796) but with a

colouring that could be described as matching natural stone such as Portland stone. Roman cement had a

distinct brown colouring which quickly lost favour architecturally. The other main incentive was to make

cements from readily available materials (such as lime and clay), rather than one single rock source with a

restricted mineral reserve (such as septaria). As such the early artificial cements resembled natural

cements in many ways as this was the model on which they were developed. It is perhaps important to

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remember that the terms “natural” and “artificial” cement are defined by the production process rather

than the materials analysis of the set cement.

Figure 1; Austin Dolphin Tazza, Swiss Garden (shrubs for scale).

The survey allowed the examination of both the surviving intact ornaments and a range of ornament

fragments, many of which were of Austin origin and some of which, were clearly marked with an

impressed mark giving date and name of manufacturer. Most of these fragments could also be related to

ornaments listed in the Austin catalogue of 1838. An article by White ( White, 1853) on Cementitious

Architecture, appearing in the “The Dictionary of Architecture” states that: “Austin’s Artificial Stone, a

composition invented by Mr Austin, of New Road, London, for the manufacture of architectural

ornaments, figures etc, used in external decoration of houses, and for garden embellishments, about the

year 1814, when Roman cement formed the chief ingredient: Atkinson’s cement was employed about 5

years later, and about 25 years since Portland cement was adopted, which material is still used by his

successor, Mr Seeley”. The article appears in Volume 1, which was published 1853, and provides two

interesting pieces of information. Firstly; that although initially Austin used Roman cement ( i.e. Parker’s

Roman cement Patent 1796), he then progressed to Atkinson’s cement. Atkinson’s cement (also called

Mulgrave cement or Yorkshire cement) was another type of natural cement, akin to Parker’s Roman

cement but which was derived from nodules within the Yorkshire Lias formation near to Sands End, and

which formed a lighter coloured cement of a more grey tone. A date for these various changes in binder

use can be derived by assuming that “25 years since” means since the publication of the edition, i.e. 1853.

This implies Austin changed to Portland Cement use in 1828, some four years after Aspdin’s 1824 patent.

Accepting this chronology we have Austin’s use of materials being: Roman cement predominantly from

1814 to 1819, Atkinson’s cement from 1819 to 1828 and from 1828 to 1853 he and his successor used

Portland cement. Francis (1977) writes that cement supply up to 1838 was from the original Wakefield

works, and this would imply that all the ornaments found within the Swiss Garden are potentially

manufactured with Portland cement from the original Wakefield Works of Joseph Aspdin, where the

cement was manufactured from 1825 to 1838. However it can be seen, from observations at Swiss

Garden, that Felix Austin was still using Roman cement, particularly in conjunction with tile creasing,

internally, on many of his ornaments in the 1830’s.

2.1 Austin Materials and construction techniques: The combination of cement and ground Portland Stone used by Austin forms a composition that has

been referred to as “Puddingstone”, and this term will be used in this paper to indicate the typical Austin

composition. (N.B. This has no relationship at all with Hertfordshire stone known as Puddingstone). The

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outer surfaces of casts, where not badly weathered, give the appearance of having the Portland stone

incorporated as finely graded stone, where as inner materials and bulk castings more typically incorporate

large Portland stone aggregates, and this matrix may be referred to as “coarse puddingstone”.

Figure 2: Surface detail of Austin ornament (5mm Portland stone aggregate).

Although the general impression of colour from Austin ornaments is pale uniform colour and a

reasonable match for limestone, a closer inspection indicates a degree of warmth in the colour of the

composition, with shades of browns and warm pink colour in which the stone aggregates appear to add a

degree of surface complexity and irregularity. In places it can be seen that the binder matrix has been

gradually eroded leaving the stone aggregates more visible and pronounced (Figure 2).

There are frequently red roofing tile sections found within the casts and sometimes darker possible

brick fragments or sections. Internally on thin walled hollow construction casts, such as pedestal sections,

it is normal to find tile creasing with red roof tiles set in Parker’s Roman cement.

The two layers of tile creasing set in Roman cement can be seen

to the internal surface.

Externally the surface is made up in light coloured “pudding

stone” type material.

Figure 3: A cross section view of Austin Artificial stone pedestal wall - a “thin wall hollow construction

cast” (c. 80mm)

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Most of Austin’s ornaments are cast in sections and then assembled into one piece. Austin casts

appear to take three main forms of construction;

a. Hollow casts with no internal lining but which may include sections of red roof tile in the body

of the composition; particularly where the thickness of the casting is more substantial than

normal, for instance, where mouldings or sculptural projections occur on the external surface.

For example in the casting of the Dog of Alcibiades the legs may be solid cast while the main

body was hollow cast with sections of red roofing tiles on the inner surface in places embedded

in the pale pudding stone composition.

b. Solid casts, in which the internal space is filled with coarse “Pudding Stone” or other materials

(some of which may have been added during repairs or assembly).

c. “Thin wall hollow construction casts” (our terminology) in which the face of the objects has

been pushed into the mould and the internal side of the cast is backed up with roman cement tile

creasing. This construction is typically used for pedestals, and associated constructions that are

large and hollow (Figure 3).

In conclusion it can be seen that Austin took considerable care in the production of mortar for his

works; to achieve a colour and surface texture that could imitate limestone he employed an early artificial

cement, probably Portland cement (Aspdin 1824) as the binder and ground Portland stone as the

aggregate. The use of Parker’s Roman cement (1796) is evident on some interior surfaces where it is used

as a binder to bond tiles in the form of tile creasing.

Palmer (Palmer, T., 2012) confirms the identity of ground Portland (oolitic) limestone aggregates

which had an approximate grading of 5mm down for the fine exterior mortars, though the majority of

Portland stone particles would pass through a 1 mm sieve. Some minor amounts of sandstone or clear

calcitic limestone were also found in the aggregate but these have been treated as incidental contaminants.

Eden (Eden, M., 2012a) analysed the binder matrix petrographically, and found it contains coarse

textured particles with distinct crystals of belite and an orangeish brown matrix phase. Some of the

cement grains contain traces of crystals resembling alite. The binder also contains very finely crushed

angular crystalline fragments that may be of crystalline slag; these may have contributed to the set and

strength of the material by pozzolanic action. One sample contained occasional lumps of undispersed

cement grains reaching maximum size of 12mm.

3. Pulhamite Artificial Rockwork materials used at Swiss Garden

The artificial rock landscape and the internal work forming the grotto have been attributed to James

Pulham and Son, a firm of rock landscape gardeners who worked predominantly between 1840 and 1940.

Many of their most important works were built during the second half of the C19th, and much of their

landscape work included what is now referred to as Pulhamite Artificial Rockwork (referenced from here

on as PAR). The firm worked on the Shuttleworth Estate and in the Swiss Garden during the 1870’s and

most of the artificial rockwork and grotto work can be either indirectly or directly associated with this

firm.

Different techniques are used in the construction of the exterior rock landscape and the interior grotto

works, as a result the two investigations have been presented separately.

3.1 Exterior Rockwork: PAR is a form of imitation rockwork constructed by the Pulham family firm, usually constructed to

imitate naturalistic rockwork, and employed in landscapes to provide “rock garden scenery”. PAR has a

constructed masonry background to form the basic rock shape and landscape form. This is often built in

brick with stones slabs to provide cantilever rock projections but sometimes other materials such as lime

concrete or stone rubble construction are used. The masonry is then coated in a cementitious render which

is normally of varying colour hues, with inclusions of specialist aggregates to give a distinct surface

finish. The main features are usually rock cliffs with projecting and overhanging rocks and recesses, plant

pockets often formed in projecting rockwork, water features, caves and associated items. One of the

principal characteristics is the formation of rocks into naturalistic geological strata, sometimes

differentiated by colour or surface texture. These rock strata are clearly represented and are normally

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“inclined” or “dip” in one direction, between rocks there may be clefts or fissures and all these features

combine to give a naturalistic and geological quality to the artificial rockwork. The rendered surface of

the rockwork is carefully worked to appear realistic, and may include impressed markings such as fern

leaves, or significant shells as an inclusion in the render both designed to imitate fossils. Other features

also occur on the surface such as modelled grotesque faces (which are usually partially hidden), or visible

exposed brick burrs (partially molten bricks).

There is no precise materials definition for Pulhamite as a range of materials and binders were used

through-out the period of production. The best definition would be landscape rockwork which is

constructed by the Pulham family firm.

The Pulhamite system is well described by Pulham in A description of A Naturalistic Pulhamite

Fernery, Conservatory, or Winter Garden ( Pulham, J., undated) as: “the building up of natural stone in

complete imitation of a portion of rocky cliff with stratified or unstratified stone or rock, and joined,

where necessary, with Pulham’s cement, made of the same colour and texture, as lime or sandstone, and

tufa. Where no real stone or rock exists, at or near, and too expensive to get then the Pulhamite formation

is adopted. The core is formed by building up burrs, rough bricks, rubble etc, to the rude rocky shapes;

then covered with cements of the colour, form and texture of the rock, which may be considered to be the

most natural or nearest to the locality, or as may be desired to imitate, as of red, grey or brown sandstone,

or of a limestone; the strata being varied in tones of colour as in nature; forming a background

harmonising with the green of foliage. Sometimes real rock or stone is used with artificial, for economy

and effect; in thin strata, where large blocks of real stone are too expensive, this adds to the naturalness of

the appearance, and not too much cost”.

From the above description three different Pulham approaches to building rockwork can be defined

(see Table 1):

1. Natural rockwork which maybe bonded (“joined together”) together to form larger boulders,

strata or elements. A good example of this is the work at St Stephens Green, Dublin.

2. Totally artificial rockwork (The “Pulhamite formation”), where a masonry structure is covered in

cements of colour form and texture of natural rockwork. A good example being Madeira Walk,

Ramsgate.

3. A technique using a combination of natural and artificial rockwork, often in thin strata.

Table 1: The three principal techniques used in the manufacture of Pulhamite Rockwork.

Pulham Rockwork at St

Stephen’s Green Dublin;

carefully bonded together

natural rockwork.

Maddresfield Court Pulhamite; all

artificial rockwork (PAR).

Swiss Garden, Pulhamite at the

approach to grotto fernery, using

artificial for base stratum and

natural above, in thin beds.

The last example described above, using natural rock or stone with the artificial, is the one that most

accurately describes some of the Pulham rockwork at Swiss Garden and can be seen to be adopted in the

rock garden (Figure 4) and the Pulhamite approaches to the Grotto Fernery (Figure 5). Much of the

cascade was extensively restored, but would appear to be more artificial than a blend, while the punt

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harbour’s rock face is mostly artificial. While at the underpass the use of Pulham’s typical partially

rusticated and less formal renders, in imitation of rough faced masonry, can be seen.

Palmer (Palmer, T., 2012) has indicated that the natural rock in the location is an ironstone from the

Woburn Sands Formation. It is the darker, better cemented upper beds that provide rubble building stone

in the local area (e.g. Old Warden Church), and which is the principal material used in the historic

landscaping within Swiss Garden. Significantly these ironstones typically have a dark brown and reddish

colour, but may have a fine or more gritty surface appearance.

This ironstone is used in the Pulhamite rockwork at Swiss Garden either as thin beds of stone jointed

with coloured PAR cement or larger rocks; both usually forming a strata of rockwork below which is a

strata formed in PAR, typically with red or red brown colouring but also with significant colour variation.

This colouring and variation is often not noticed because of the green algal coating and because 1990’s

repair phases have used a green universal wash to assimilate new surfaces.

Figure 4: The Rock Garden showing PAR clearly as a lower strata (e.g. lower left side), with natural

ironstone and vitrified brick slabs or similar material in upper strata bonded together with PAR mortar

(Daffodils for scale).

Figure 5: Pulhamite rockwork at South Entrance to grotto, showing large artificial base strata with thin

strata of natural ironstone above, bonded with coloured PAR cement mortar - note small cleaned area

(rock work c. 1m high)

Site observation and technical scientific examination of these imitation stone mortars has revealed a

number of interesting techniques. Firstly pigments are used to colour the cement based mortar; most the

mortars are reddish, reddish brown or pink tones but with some lighter shades of pale pink and grey and

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paler colours also being present. The surfaces colour in the historic PAR appears to be quite variable and

not uniformly the same colour, it is assumed this was intended to imitate the variations found in natural

rock, as Pulham states “varied in tones of colour as in nature” (Figure 6).

Figure 6: Detail of Pulhamite surface cleaned to show surface colour variation and surface visible

aggregates (cleaned area approx. 100mm sq).

Pigment analysis by Hassall (Hassall, C. 2012) found that all pigments were based on iron oxides, red

ochres and burnt umber being the most common. These were used to make the red and darker reds and

brown tints. The shade or intensity of red was largely determined by the amount of pigment added. Paler

shades of Pulhamite with a grey colouring were formed with yellow ochre and some black oxide. And

some samples appeared to be un-tinted mortar of a light whitish colour. Both site observation and analysis

revealed that the colouring and variation in colour was formed by “blending” or “chopping” mortars of

different shades, or applying one coloured mortar partially over another colour, without complete mixing

of the colours. “Blending” may be an inappropriate term because it implies well mixed colour, on the

contrary there is often a clear distinction between different coloured mortars. The clear cut line between

these mortars, when viewed in cross section, implies they were applied “fresh on fresh” (Figure 7).

Figure 7 ( Hassall, C.): Section of PAR mortar showing variation in colour intensity (average aggregate

size 3mm).

Eden ( Eden, M., 2012b) cement based matrix of the PAR coating is rich in belite and is probably

early artificial cement ( low calcination temperature) or a form of natural cement, but not Parker’s Roman

cement (1796). The two samples analysed by SEM indicated Ca0 to SiO2 of 45.80 : 32.00 and 40.30 :

41.00. Thin sections revealed that a cement slurry was seen on many of the samples between the

brickwork and the main pigmented PAR coating, and this is understood to be an important craft

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application to aid the adhesion of the cement based PAR coatings. As is normal with PAR the brick

backing is built with hydraulic lime bedding mortar.

The final materials addition of the mortar to aid the imitation of the natural ironstone is the addition of

aggregates with suitably sized and coloured grits, which are intentionally exposed on the surface of the

PAR and resemble the more gritty samples of the Woburn Sands Formation ironstone. Palmer (Palmer,

T., 2012) found that the aggregate was 90% quartz sand but the remaining 10% of the aggregate particles

consists of a mix of rounded Chalk fragments and crushed flint (probably also from local gravel), forge

scale, crushed ironstone, and pieces of hydrated cement clinker. The PAR exhibits variation in the extent

to which the large visible aggregates are present, their presence on the surface being visually significant

in some places and not visible (possibly not present) in other places. It is assumed during the application

of the PAR cement mortar that the aggregates were intentionally exposed by sponging, brushing or water

washing processes.

In conclusion, mortar techniques used to imitate natural rock surfaces include; the addition of a range

of pigments to produced differently coloured mortars, the careful manipulation of different coloured

mortars to produced surface variation in tone and colour, the use of a slurry applied to backgrounds to aid

the adhesion of the PAR mortars, the addition of special aggregates to imitate the surfaces found within

the natural stone, the careful treatment of these surfaces to expose and make visible these aggregates, and

the use of well proportioned mortars with a suitable binder, both in terms of colour and durability to

survive the aggressive environment.

3.2 The Grotto Fernery The cruciform Grotto Fernery is constructed internally as a slightly offset brick tunnel with a central

crossing formed by a conservatory wings and a central glass dome. The construction relates mainly to the

early phase of development within the garden but in the 1870’s Pulham created the current interior grotto

in imitation of a natural tufa cave or cavern. The imitation material is essentially covering the brick tunnel

or cantilevered stone slabs may be built into the brick tunnel lining or other related brickwork. The

principal features of the grotto are stalagmite and stalactites, as well as projecting plant pockets, all the

brick walls being covered in the tufa material ( apart from some sites behind plant pockets where some of

the original phase ashlar struck lime render is still visible at lower levels). In a few hidden areas the

original brick lining is visible.

The whole interior is designed to appear as a rocky underground cavern or cave (Figure 8) with

stalactites and stalagmites and plant pockets to allow extensive fern growth, with light coming from

above and the sides through the dome and the conservatory.

Figure 8: Interior of the grotto/fernery (height from floor to dome approx. 4m).

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A large proportion of the surfaces, rocky projections and stalagmites are natural tufa rock, which has been

cut to form the required shapes. Cut and tool marks can be seen on some tufa surfaces if carefully

examined. Careful observation of the surface of the grotto reveals that areas between the natural tufa is

made up with “artificial tufa” applied as a hydraulic lime mortar, designed to match the natural tufa

surfaces as far as possible in both colour and texture. The surface texture of the mortar implies it has been

cast on (i.e. thrown on) in a suitable consistency to leave an appropriate pitted and lumpy surface texture.

There have been anecdotal reference to the incorporation of compost or similar material into the mortar to

help create the open textured and rough nature of the surface, but this hasn’t been supported by any

analysis ( Palmer, T., 2012; Eden, M., 2012 c and d).

The natural tufa pieces appear to be bonded directly to the brickwork with Parker’s Roman Cement

(1796), which was well known for its adhesive qualities and rapid set. The brick lining to the tunnel also

appears to have been made “irregular” by the application, in places, of a coat of Roman cement render

that projects and undulates. This render coat is covered in the artificial tufa mortar. Much of the surface

material in the tunnels is “artificial” mortar with a greater proportion of “natural” tufa material towards

the central area, particularly under the dome and associated conservatory cross wings where there is more

light.

Although many of the principle stalactites are cut and made up from tufa, many of the more minor

stalactites in the tunnel are formed in Parker’s Roman cement core material with artificial tufa mortar

coating. The nature of shrinkage cracking on some of these items would imply that the Roman cement

cores to these stalactites were probably cast, in a mould, from liquid cement to form the basic shape, then

probably adhered to the ceiling with more Roman cement, prior to the thrown application of the imitation

tufa mortar. The brown nature of the Roman cement cores probably explains the anecdotal references to

oak armatures being used on these stalactites, the Roman cement having similar colouring to dark oak.

Some of the stalactites form large pillars, and these are often formed with jointed pieces of natural tufa, it

is assumed the jointing or bonding is carried out in brown coloured Roman cement which is then covered

with small areas of artificial tufa mortar.

The plant pockets can project extensively from the walls of the grotto and are formed by cantilevered

stone slabs, using York type sandstones or similar. The upright edges of the pockets, which contain the

earth filling, may be constructed in tufa and sometimes brick. All surfaces that are not natural tufa are

then coated with artificial tufa mortar to give visual consistency. Some of the larger projecting pockets are

supported with stalactite pillars and in some cases minor supports for pockets are provided by visually

hidden bricks applied to the wall with Roman cement, these sometime form minor columns or pilasters

but rely for the strength on the adhesion of the Roman cement to the tunnel wall.

4. Conclusion

A range of methods have been described which illustrates an extensive range of techniques and materials

used in the imitation of artificial stone and rock surfaces. These include the use of carefully selected

binders that are not only of suitable colour but also durable in aggressive environments, the addition of

pigments and carefully selected aggregates, and a range of craft techniques that show the imitation

mortars were well designed and carefully applied.

References

Austin, F., 1838, Collection of Ornaments at Austin’s Artificial Stone Works, New Road, Regent’s Park,

London. Commercial publication.

Eden, M., 2012, GMRS/Sandberg unpublished reports as follows:

a. Report 46017/k, Report on the Petrographic Examination of two samples of Austin Artificial

Stone Ornaments (Austin Planter 1830 and Austin Dog 1832). 21 pages.

b. Report 45599k-A, Pulhamite and Render analysis. 67 pages.

c. Report 46092k, Grotto Mortar Analysis. 29 pages.

d. Report 46186k, Grotto render samples. 20 pages.

Francis, A.J., 1977, The Cement Industry 1796-1914: A History. David and Charles. Pages 76-90.

Hassall, C., April 2012, Swiss Garden Pulhamite Pigment analysis report. Unpublished report. 6 pages.

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Loudon, J.C., 1834 the Architectural Magazine and Journal of Improvement in Architecture, Building,

and Furnishing, and in the Various Arts and Trades Connected Therewith. Vol 1,Cornmarket Reprints,

1972.Originally published by Paternoster – Row 1834. page 159.

Palmer, T., 24th

May 2012, Report on Geological Matters arising from the on-site study and collection of

samples from Swiss Garden, Near Biggleswade, Bedfordshire. 13 pages. Unpublished report.

Pulham, J., A description of A Naturalistic Pulhamite Fernery, Conservatory, or Winter Garden. James

Pulham Broxbourne, Herts. Not dated. 4 pages.

Swann, S., and Bardwell, T., 2012 Report: Survey of Pulhamite Artificial Rockwork and Artefacts at

Swiss Garden, Old Warden, Bedfordshire, Simon Swann Associates for the Shuttleworth Trust,

supported by Heritage Lottery Fund. Unpublished Survey report. 350 pages.

White, 1853, Article on Cementitous Architecture in; The Dictionary of Architecture, Edited Papworth

W, 1892, London, Architectural Publications Society. Published between 1853- 1892 in 11 volumes)