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Stony expression

By Mathew Haslam
There is a wide range of possibilities when designing with stone, but you need to understand how to select the material and finishes so that they are both as appropriate and as beautiful as possible.

Material compatibility with the climate is one of the most important factors to consider when specifying a material for a project. Cold weather can cause rock to crack and break open when water becomes trapped and expands in freezing conditions – porous materials, such as soft limestones, are most susceptible to this and can also be sensitive to staining. 

As well as resistance to the elements, public realm schemes and those that receive a high volume of pedestrian traffic will need a highly durable material that can withstand repetitive use day in, day out. Other materials and options can be considered for installations in private residential schemes where heavy traffic isn’t such a consideration. Easiness to work with, the intricacy of the design and the desired finish are also factors to consider. 

Granite is a great example of a hardwearing, versatile material, suitable for almost any kind of external application or scheme to provide a sharp looking, stylish design. It contains a blend of minerals that contribute to its unique variety of colours – from pure white to deepest black – and comes in a number of textures. If you want a sustainable product, granite is a good choice – not only does it have a long lifespan, it can also be efficiently and cost effectively recycled or repurposed. 

One factor that you should consider during the initial design phase is that dark granites are quarried in smaller pieces than light granites, which can affect the overall look of the final design. It’s also worth noting that costs vary depending on material specifications – the darker the granite, the more expensive it gets.

Although granites are hardwearing and durable, care needs to be taken during the transportation of granite. Creating a subtle chamfered edge by cutting off the sharp corners can prevent pieces getting chipped on site. Hardscape recently used this technique to produce a 50-metre granite seat art installation, which included bronze illustrations and typography depicting the history of the Cunard Building in Liverpool.  The stone’s properties made it the perfect choice for intricate and detailed design. A series of inscriptions was made in the granite bench recounting some of the defining moments in the building’s epic story, dating back to a period when Liverpool was considered as the gateway to the world.

It’s important to consider additional features such as artwork, text and metal inlays – for which some materials are more suitable than others. The granite seat at the Cunard Building was inset with a specially designed piece of artwork including unique bronze illustrations – other metals work equally as well as inlays.

Porphyry and sandstone are also hardwearing, natural materials and popular choices for paving thanks to their resistance to heavy loads. During the initial compression process when they are formed, the materials absorb almost no water at all – making them able to withstand freezing temperatures. This characteristic makes the materials ideally suited for wet areas, such as those containing fountains; in addition, they are naturally slip-resistant. 

Creating colour
Introducing a variety of colours into a scheme is a great way to add interest. Quarried stone comes out of the ground in beautiful naturally occurring tones and hues – some stones even contain small rock and sediment layers – which add detail and individuality to a design. 

Porphyry – Greek for purple – is a rock consisting of striking, large-grained crystals, known for its rich purplish-red or greenish-brown tones, formed during volcanic activity. The colour characteristics of the material depend on its level of iron content and its purplish-red or greenish-brown swirled patterns have made it a popular choice.

Sandstone contains naturally distinctive coloured bands, including red, brown, white and black - meaning that no slab ever looks the same and it can be used to create truly unique designs. It’s important to remember that the colour of the sandstone selected at the beginning of a project may alter throughout the installation period; over time its appearance and character changes. Rather than bleaching, sandstone darkens richly with age. 

A technique using recycled glass with RAL coloured epoxy can also be used effectively to add interest and character to hard landscaping materials. For example, Liverpool’s Eberle Street has used a dramatic, intricate paving and lighting scheme pattern to honour the mythical Emerald City from the Wizard of Oz, with homage to the Yellow Brick Road and Judy Garland. 

Crystalpave – recycled glass slabs bonded to a concrete base – was used 
to create the artwork throughout the paving. The resin in the surface layer can be made in almost any specified pigment colour or left as a natural, clear resin.

Finishing touches
Most physical properties must be taken into account at the material selection stage as these are unchangeable. However, other qualities can be determined during the finishing process such as adding texture and a finish to the surface. These include:

Diamond sawn – a cut that showcases the full spectrum of colours and character of a stone, with a smooth surface that reduces the need for regular cleaning. 

Shot blast – a secondary process to diamond sawing, this adds a stippled texture, exposing the grain to create an aged look. 

Honed – A smooth, even, and non-reflective finish – similar to an eggshell – achieved by polishing the stone with a series of increasingly fine abrasives. This process also deepens the colour while maintaining its unique characteristics.

Bush-hammered – A high-impact pitting device creates an evenly textured, non-slip surface, suitable for high-traffic external areas. This finish also lightens the colour and dulls any patterning found in a stone’s natural state. 

Hand-riven – A traditional process which splits the stone along its natural bed and is usually produced with a ‘fettled’ edge – a soft organic line, instead of a dead-straight edge – to complement the traditional character.

For a finish that looks like two different colours, etching – a process that uses highly pressurised air mixed with an abrasive – slowly grinds down materials to create shadow. This technique can also be used to sandblast letters as small as 2-3mm wide. This is more suitable for hardwearing materials as they give a sharper look. 

In summary, achieving a high quality and durable natural stone landscape relies on selecting the correct stone, ensuring that it is beautiful as well as fit for purpose. These considerations have been broken down into the following do’s and don’ts: 

Do consider:
  • Who will be using the area and whether it will be exposed to harsh weather conditions. 
  • Most materials are available in a huge variety of shapes, sizes and design specifications but these come with longer lead times. 
  • Different mineralogy produces various stone strengths and usage applications, while technical standards allow classification of stone applications and size specifications. 
  • A good supplier can provide advance details on this. 
  • Whether the material looks suitable. For example, stone can be given an aged look for regeneration projects to blend in with existing surroundings.
  • The environmental impact of obtaining raw materials, processing and recycling issues.  
  • Ensure you work with companies that source materials ethically – quarries are located across the world and unethical business practice can be a problem.

Don’t:
  • Assume materials will always look the same. Some stones change colour while others are susceptible to chemical degradation. Due to the nature of most paving, vegetation may grow in the joints, or on the paving itself in shaded areas which remain damp. Regular maintenance and good cleaning practices will enhance the long-term appearance. The original colour can often be brought back by power washing.
  • Be too rigid with expectations. 
  • Stones from the same quarry come out of the ground in different sizes and colours so consider this if you want a consistent pattern. 
  • Limit your imagination – there are a variety of processes and finishes that can transform materials to meet design specifications, including colour and texture. These are just some examples of the vast capabilities and considerations for specifying stone and hard landscaping materials.

Mathew Haslam is managing director of Hardscape.

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