Looking at the size of the legend characters I would say it's a farthing (though not so easy to tell from an angled picture) . A halfpenny is just a bit smaller than the later florin, the farthing is roughly the size of a 6d.
Definitely a halfpenny. The easy way to tell is to look at the size of the legend characters in relation to Victoria's bust - on the halfpenny they're proportionately much bigger than on the penny.
Three questions:
1. where do you get sodium sesquicarbonate from?
2. Presumably filtered water (Brita) counts as chlorine-free?
3. dabbing with a soft towel would do instead of acetone to remove water?