A new terracotta pot or a freshly cast stone ornament announces its newness, all sharp edges and bright clean surfaces. A skin of moss is the quickest way to take the edge off and lend the weathered, long-settled look that otherwise takes years of weather to earn.
The slurry method
The technique is the same moss milkshake used for graffiti and walls. Blend a couple of handfuls of clean moss with enough buttermilk or natural yoghurt, thinned with water, to make a paint, then brush it over the surfaces you want aged. The dairy helps it cling and feeds the moss a little while it takes hold. Paint it into crevices, around rims and over textured areas where moss would naturally settle, rather than evenly all over, for a believable result.
What it works on
Porous, rough materials take moss best: terracotta, unglazed ceramic, natural stone, concrete and cast reconstituted stone all give the moss something to grip. Smooth, glazed or sealed surfaces are far harder, since there is nothing for the rhizoids to hold and the slurry simply slides off. Pick a piece destined for shade and damp, because the same conditions that grow moss anywhere apply here; an ornament in full sun will stay stubbornly clean.
Keeping the patina
Once painted, keep the piece in shade and mist it regularly for the first few weeks while the moss establishes, exactly as for any new moss. After that, a position that stays cool and damp will maintain the patina with little help, while a dry spell will send it dormant and dull until the moisture returns. Over a season or two the moss thickens and the piece looks as though it has stood in the garden for decades, which is the whole idea. The underlying method is set out in the slurry and spraying guide.