Which clay should I choose?

We’ve put together a comprehensive guide to this process in our “How to choose the right clay for you” section.

How should I store my clay?

We recommend storing your clay in a cool, dark place. We supply all our clay in thick plastic bags, it is best to keep your clay in these bags as it will protect it from drying out and from light. We supply clay storage bins that are ideal for storing 250 - 500kg of clay; these can be found by clicking here. In colder climates we would not recommend storing your clay outside due to the risk of freezing. Clay that has frozen and thawed will have gone through significant physical changes and can be very difficult to rework to a usable consistency, if it freezes completely it can often be irrecoverable! Prevention is the best method, even covering your clay with a blanket or tarpaulin can help alleviate the risk of freezing.

Does clay have a shelf life?

As long as clay remains wrapped in plastic and is stored as per our recommended instructions it should be ready to use at any time. Some clays have more organic material in them than others and due to this may develop a mould if left for long periods of time. This mould will not affect the performance of the clay and can be wiped from the surface using a damp sponge.

My clay has become hard, what can I do?

Unfired clay can always be recycled… If you have been using clay for any length of time you will inevitably come across a bag that has become too hard to use, similarly you will find that your trimmings and offcuts will mount up. This can always be reconstituted.

There are two main methods of reconstituting your hard clay. Firstly you could invest in a mechanical pugmill such as one of these from Peter Pugger, simply load your scraps and broken up dried clay into the mixer with some water and allow the machine to mix the clay for you. The beauty of this type of machine is that it will press and de air the clay back into long sausage shapes which you can re-bag for later use.

The other option is better for smaller amounts of clay and involves allowing the clay to dry fully before breaking into small pieces and adding in to a large bucket. The clay should then be covered with water and allowed to soak for 24 hours. It helps to give the clay and water solution a good mix every few hours. After some time the clay will have absorbed the water and become a slip/slurry mix. To dry the excess water from the clay you will need to pour this mixture onto a plaster bat, this will then pull the water from the mix until you have a consistency close to what you require, at this stage the clay can be wedged by hand ready for use again.

My casting slip is too thick, is there anything I can do?

Yes. Casting slips are what is known as thixotropic, similar to cornflour they thicken upon standing. The first step is to stir the slip vigorously; in most cases this will rectify the problem. If this doesn’t rectify the problem the slip may need adjusted slightly, this is an issue that affects the entire ceramic industry and all slip manufacturers. Over time the deflocculant loses its static charge and ability to keep the clay particles in suspension, an issue that affects stoneware slips more than earthenware due to the higher clay content. Should your slip not be remedied by stirring we suggest adding 1 drop of Sodium Dispex at a time and stirring until your slip reaches the desired consistency.

Do you have a recipe I can use for making a casting slip from powdered clay?

Each powdered clay uses a slightly different recipe to achieve the correct consistency. The best thing to do is to contact us stating which powdered clay you are using and we will try to provide a basic recipe for it on an individual basis.

Why are my items taking such a long time to cast?

When this occurs the slip takes a much longer time to build up the correct thickness. It is usually caused by one of two problems. The most obvious problem that the mould is simply too wet, this usually occurs when the mould has been used a number of times in succession and has absorbed a lot of water from previous castings. The mould will need some time to dry out and the absorbed water allowed to evaporate out. This can be done by placing the mould somewhere in your studio or class room that is warm and allows good airflow. Some studios have drying cabinets which are ideal for this application. Alternatively the mould can be placed in an electric kiln and a drying program run for a few hours. We would recommend a drying program of 20°C an hour to 50°C with a hold of 1 hour. This should sufficiently dry the mould to begin casting again.

The other issue that may occur is that the slip may be over flocculated. The slip will appear to be very thin and flows easily. To rectify this issue you can add more water and clay to balance out the level of deflocculant.

What temperature should I biscuit fire to?

Here at Scarva we recommend biscuit firing to 1000°C. We find at this temperature the porosity of the clay is perfect for glaze application and adhesion. There are instances when it may be necessary to biscuit fire higher than the lower maturing temperature given for the clay and then glaze fire to the firing range of the selected glaze. This will reduce the chance of crazing on lower temperature glazes however it can make it difficult to apply the glaze, especially when dipping. Additives are available to increase adhesion; one of our technicians will be able to advise the best course of action should this be something you need to do in your work.

How fast should I fire my kiln?

We have prepared a section entitled “Firing Your Kiln” here you will find all the relevant information and suggested firing schedules.

Can I mix two glazes together?

As a rule provided the glazes are from the same range and are of the same temperature this should not be an issue. As with every experiment the mixing of two particular glazes will require testing on the customer’s part. With all glazes we suggest doing a test firing before committing to glaze firing your pieces.

Is my glaze food safe?

This is the question above any other we are asked most frequently and is something that people get very confused about. First and foremost, the term food safe cannot technically be applied to a glaze as supplied.

The final fired piece may be considered “food safe” however this is reliant on such a massive variety of factors outside of any suppliers control that there is no definitive answer. The clay body, form, surface texture, oxide addition, firing schedule and many other variables all need to be considered. Glazes with matt surfaces, crackle glazes or other non-glossy effect glazes should all be avoided for functional ware due to the possibility of the surface harbouring bacteria.

In general provided glazes are fired to maturity on a suitable clay body there should be no issue, however the only guaranteed way to have a piece deemed “food safe” is to have it tested by a certified ceramics testing laboratory. In the UK this is done by Lucideon and can be contacted by clicking this link.

Why have my pots crazed?

In most cases this comes down to the glaze being incompatible with the clay body chosen or vice versa. It happens when the glaze is under tension due to differing rates of shrinkage between the clay body and the glaze.