Copyright law is different in different countries. In the UK, if you have written it, you have the copyright, as I understand it, and I think it lasts for 50 years after the death of the author. Recipes are written, hence they are copyright. So give credit where it is due by quoting author, title, etc. In the same way if it is from a website, give the link!

It is one reason why there are so many recipes that vary only in one ingredient, or by very small amounts--it isn't copying then, technically, I suppose.

I found this recipe last year, when I had to pick all my tomatoes early because of blight. I really like it, and it uses vast quantities of green tomatoes easily. There are other pickle and preserve recipes in the book. They are often
for huge quantities, like the one for apple butter that starts with 20
gallons of apple juice--even with US gallons, that is a lot!

                   French Tomato Pickle (Old)

Mennonite Community Cookbook, by Mary Emma Showalter, The John C Winston Company, Philadelphia, 1950.

I will give the original quantities, then mine in brackets. t=teaspoon, T=Tablespoon

1 peck green tomatoes, sliced--this is 8 US quarts, nearly 8 litres   (8cups)

6 onions, sliced (1)

1 cup salt (1/4cup)

2 US quarts vinegar (2cups, 16 floz)

1 pound sugar (1/4 lb.)

2 teaspoons powdered mustard (1/2t)

2 tablespoons curry powder (1/2T)

2 tablespoons turmeric (1/2T)

2 teaspoons cinnamon (1/2t)

2 teaspoons cloves-I assume ground cloves! (1/2t)

2 teaspoons allspice, "             "       (1/2t)

Slice the green tomatoes and onions (I wanted a relish, so I put them in the food processor to get the texture I wanted)

Mix salt through them and let stand overnight.

In the morning, drain and allow to stand 15 minutes in a weak vinegar solution. (Your guess as to correct dilution for vinegar is as good as mine. This does NOT use the measured vinegar)

Drain.

Combine vinegar, sugar, and spices and bring to a boil.

Add vegetables and simmer slowly for 3 minutes.

Pack in jars and seal. (I did sterilize the jars first!)

This is a rich pickle.

The recipe is attributed to Mrs. Menno M. Brubacker, Waterloo, Ont., Can.

Well done, , for finding that!

But do note that it says 70 years after the death of the last author of a work. So if you published your two year old's sweet sayings, and the two year old lived to be 110, it looks as if the coypright would last 178 years.

I was told that copyright lasts 75 years in the US, but haven't checked the details

You cannot copyright an idea, so you couldn't copyright "mayonnaise". You can copyright the expression of an idea, so you could say for example "take exactly 1/7th of a teaspoon of mustard, very slowly add best quality olive oil, and beat furiously..." and that would be copyright, as I understand it. Recipes would be literary, I think, in the same way instruction manuals would be.  It would be plagiarism to quote exactly without attributing it. If an author isn't listed or known it is still copyright for 70 years, unless it is made public within that time when it would be copyright for 70 years from date of publication.

The main thing to do to avoid problems is to say where you got the recipe!

Thanks to DD for some of the above information.