3mmThese dual action permanent and semi-permanent ZIG Memory System 2 Way Glue pens are ideal for all your paper crafting projects. With their Acid Free quality they are great for working with your precious photographs!How to use:For permanent use: use immediately after applying the adhesive to paper and photographs. This is when the glue is blue.For semi-permanent use:use when the adhesive becomes tacky. This is when the glue is clear and you will find that your paper and photographs can be repositioned.ZIG Memory System 2 Way Glue can also be used when applying glitter to paper glass foil and plastic surfaces embossing using Kuretake Emboss Powders and Heat Gun for gift wrapping and in your photo albums. Also perfect for use on Vellum Paper.