I know what you mean @barry island. Those ones I mentioned in that last post were like you describe, so I didn't put those in my compost after that - only the P. Nyssen ones. There's definitely a lot of variation in the materials used, so I suppose all you can do is the trial and error method, to see what works. You'd have to remember which ones were ok though, if you use several different types!
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
There was an interesting item on Radio 4 discussing the merits of various types of dog poo bags and the issue of compostable/biodegradable plastic bags came up. https://www.bbc.co.uk/sounds/play/m001n8nb
Long story short the advice was that the only bags worth buying for the average user are ones with a high recycled plastic content, but also that poo should alway be picked up and disposed of correctly, no flushing down the human toilet and no leaving bags to degrade in nature. There was a caveat that if you have enough space for a compost heap at home dedicated to dog poo then this is a possible way to go but it needs careful management. I think it's a shame that they didn't ask a gardener what the possible uses for the compost would be since it would be very high in nitrogen and not suitable for food use. I imagine you'd end up with a lot of nettles.
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
It's a mare, I'd heard similar to the report @SalixGold quoted, and the @wild edges dog poo story. I'd been putting all those supposed biodegradable plastics in the bin. We put all 'natural' supposedly breakdownable sanitary products in there too, like condoms, tampons etc. But some plastics survive. And I've kind of given up after sieving it out, and shoving it back in the next bin a few times. The most annoying stuff is tea bags from some of the big manufacturers, which is hard to sort out. I have a pile of rejected stuff I keep meaning to burn. As for dog poo those with gardens I just don't get why they wouldn't attempt to compost it. A willow toilet / tree bog thing could keep it under wraps. My neighbours shove it in the public waste bin rather than their own. And those that do put it in their bin, and leave it to ferment in the sun.
I've just turned one of my compost bins into another and sieved out the rubbish. I hoiked out a lot of supposedly home compostable plastic brewing for over a year in the bins. I won't be attempting any more.
I have been composting the potato starch film for some years now and it does break down in the compost heap but takes far longer than the vegetable matter. I have three large enclosed and covered heaps and try to ensure that they are "hot" heaps. They are also turned regularly from one biin into another and usually I have to put the film back into the frst bin when I do this, for the first and possibly the second time. So, not ideal but it does break down eventually and I am happy to accept this rather than plastic which does not degrade at all for many years.
For over a year I composted vegetable bags from the supermarket and compostable work top compost bin bags from Joseph Joseph.
Anyway, when I came to use the compost, they had only partially composted. So I had not only full bags to remove but also little bits of bags. Hundreds of them 😖
I repeat they had composted for over a year - perhaps the manufacturers feel that two years plus years is reasonable to be called compostable but for me, No, it's not an option anymore.
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There's definitely a lot of variation in the materials used, so I suppose all you can do is the trial and error method, to see what works. You'd have to remember which ones were ok though, if you use several different types!
I live in west central Scotland - not where that photo is...