Gilding Sizes

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

Oil-based Size: The most popular size amongst professional gilders is oil-based. These sizes are usually clear, made from linseed oil and 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 size is left before being gilded upon. 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 the following day. It’s never as easy as that of course, and temperature, humidity, the material you are gilding upon, and many other factors can (and will!) reduce or prolong the open-time of size. Only experience and experimentation provide the best time for gilding an object.

Oil sizes are applied using a good quality brush and left until the required amount of ‘tackiness’ is achieved. If it’s left too long however, size will become over-dry, and a fresh application will need to be made. This is tricky, because the closer size is to drying - yet maintaining enough tack to hold the leaf - the better the gilding. Getting leaf on in this ‘sweet spot’ allows production of a much shinier and hardier finish.

If leaf is applied too early, a matt finish is produced. Although this is used as an effect in its own right – it helps convey proportional values to a piece - the downside is that because the size isn’t fully set the leaf can easily be scratched upon the surface, even many days after gilding.

As much as is possible, a dust-free environment for size application must be used, as any contaminants in the air will stick to size and mar the final finish.

Being turpentine based, brushes are washed out in turpentine.

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 dries quickly and is usually ready to gild in just 15-30 minutes.

The main advantage is that it has an ‘open-time’ indefinitely, allowing you to apply the size and gild at your leisure thirty minutes, twenty hours or even twenty days later! It’s a quick and easy product to use and brushes are simply cleaned in soapy water after use. It’s easy to use for the beginner, but the finish won’t be as brilliant as can be achieved with oil-sizes. An indefinite open time also means it can be scratched or damaged after leaf has been applied. This size can also be used with different types of metal leaf – gold, silver, copper etc.

Although I have used oil size, all projects in the ‘Oil Gilding’ section may also be completed with water-based sizes instead if you so wish.