Gilding Sizes

To adhere the metal leaf to the surface an adhesive size is used. This is known as a 'Gilding Size'. There are many different types available and each has their own distinct properties. These include both water and oil-based versions. Overall, oil-based sizes are far superior to the water-based versions and are the type that I am using in the following examples.

Oil-based sizes are provided with a guideline ‘open-time’. This is indicated on the tin and can range from one hour to twenty hours. This number loosely refers to how long the size is left before being gilded upon. That is to say a ‘1-hour size’ can be gilded around one hour after the size is applied, whereas a ’20-hour size’ is gilded around twenty hours later. In the 20-hour size scenario, you will typically size the object one day and gild it the following day. It must be noted however that temperature, humidity, the surface you are gilding upon and many other factors can (and will!) reduce or prolong the open-time of size. Only experience and experimentation will provide the best time to gild an object.

Oil sizes are applied with a good quality brush and left to dry until the required amount of ‘tackiness’ is achieved. If it’s left too long however, the size will become too dry and a fresh application will need to be made. This is tricky, because the closer the size is to drying - yet with enough tack to hold the leaf - the better the gilding! Getting the gold on in this ‘sweet spot’ allows the production of a much shinier and hardier finish. If the gold is applied too early, a matt finish is made which can be used as an effect all in its own right and can help convey proportional values to an artistic piece. The downside to this is that because the size is not fully set, the gold can easily be scratched off the surface - even many days after the piece has been gilded. It is also really important that size application is done in as best a dust-free environment as possible, as any contaminants in the air will stick to the size and mar the final finish. Being turpentine based, brushes should be washed out in turpentine.

On a final note, about those water-based sizes. Even though it is called oil gilding - due to it traditionally being oil sizes used - there are now modern water-based sizes available. These are milky in their appearance, but dry clear. The size is applied in the same way as oil, but dry really quickly and will usually be ready to gild on in just 10-15 minutes. The main difference however is that it has an ‘open-time’ indefinitely. This means you can apply the size and gild at your leisure twenty minutes, twenty hours or even twenty days later! It is a quick and easy product to use and is simple to clean brushes out in soapy water after use and is great for the beginner. Please note however, the finish will not be as brilliant as oil sizes, and an indefinite open time means it can be easily scratched or damaged after the leaf has been applied. It can also be used with all the different types of metal leaf – gold, silver, copper etc. Although I have used oil size, all projects in the ‘Oil Gilding’ section can also be completed with water-based sizes instead if you so wish.