Hand pressed terracotta process
The key to getting the best results from a material is to understand the process. Here at Priory Architectural Terracotta we hand press our clay into moulds. The key manufacturing principles to this approach have not changed significantly for hundreds of years however the individuality of each project means specific moulds are required for each component. Below is a breakdown of each stage of our method.
Quotation and survey
It is common that we supply an initial rough estimate based on your contractor’s photographs and dimensions, but we are happy to visit site and produce a conditional report that highlights areas of concern, identifies suspect units and suggests potential causes. Having identified the units to be replaced we will then provide a specific quotation. Upon acceptance of this quotation we can attend site and survey the units for all the necessary dimensions and profiles needed for manufacture or we are happy to accept delivery of pieces from site which can prove more cost effective.
Design & Drawing
Upon completion of survey or the receipt of architect’s drawings we will produce 3 dimensional unit and/or assembly drawings for your approval.
Once the approval has been received the drawings are then adapted into workshop drawings to enable manufacture. First, we create oversize models to compensate for the natural shrinkage of the clay. These models are either hand sculpted in clay, run in plaster using zinc profiles or made from hot-wire cut polystyrene.
From the hand-crafted model multi-piece plaster moulds are made. These moulds, often complex in nature, create a negative space into which the clay can be pressed. It is important that the moulds are dried out slowly before use to ensure a consistent performance. After approximately 30 uses the surface of the mould can break down and must be replaced.
Terracotta Colour Matching
To match existing colours, we start by using either terracotta clay from Wales or ball clay from Cornwall as a base for our architectural terracotta. These clays are then augmented with pre-fired aggregates and modifiers before having the colour altered. A thorough understanding of ceramic chemistry gives us the ability to add natural or synthetic metal oxides to alter the clay to produce colours ranging from reds, oranges, yellows, buffs, greys and browns. An almost limitless palette is possible.
Our terracotta’s are processed to our own recipes in Stoke-On-Trent. The clay is milled, refined and augmented as a liquid ‘slip’ before being mixed in a blunger. The terracotta slip is then filter pressed to remove excess moisture and finally passed through an extruding pugmill to shape and de-air the final plastic clay body. For our larger contracts the colour oxides are added during this process while for smaller jobs we colour the terracotta by hand in our own workshop.
Once the plaster moulds are dried, we then hand press the terracotta onto the inner faces of the moulds up to a wall thickness of 30-40mm. A lot of practice and skill goes into this process to ensure a compact and homogenous piece. We frequently build up internal web or ribs to help maintain shape stability throughout the subsequent drying and firing process. After time, the capillary action of the plaster draws the moisture from the terracotta to a point where the piece can be removed from the mould.
The hand finishing technique compacts the outer surface of the terracotta to create the fireskin layer. This layer has a denser layer of clay particles and gives the work greater frost resistance and mechanical durability. The process is also an opportunity to sharpen up the appearance and add essential fine detail.
The drying stage is one of the most important and variable stages of the manufacturing process with its duration depending upon the size and complexity of the work. The work is set on trolleys in a manner which allows air to circulate as evenly as possible around both the exterior and interior of each piece. The air this then controlled for temperature, velocity and humidity. Careful programming and monitoring allow the pieces to dry slowly to avoid cracking and warping.
Once the pieces have progressed through the drying process, they are hand loaded into our gas-fuelled shuttle kiln. Each firing is governed by a computer programme takes the temperature up to 1180 degrees Celsius over 36 hours and then returns it to room temperature during the following 48 hrs. This process turns the clay from a friable clay into a hard, durable ceramic that conforms to BS and ASTM criteria.
After firing each piece is checked for defects and size tolerance. We frequently reconstruct sections of an assembly to check for alignment and overall appearance. Once validated each piece can be individually labelled.
For our larger contracts we pack the terracotta onto wooden pallets and protect each piece with polystyrene. These pallets are then delivered by Matthew Kibble Transport Ltd whom we have worked with for many years. When completing smaller projects, we pack the pieces into wooden crates and either send by TNT or deliver by hand. Each consignment is accompanied with a detailed delivery note.