A lump of sugar
When i had my breakfast this morning, i wanted to have a cracker with sugar. When i opened the sugar bowl i noticed that there were a lot of lumps in it. So i found myself wondering how the lumping occurs and why the lumps still exist after the sugar has dried up again.
We all know (i expect) that if you have a bowl or pot of sugar or salt and you let it get wet, that lumps of sugar/salt are going to form. If you let the bowl dry the lumps still exist until you manually break them up. This is true for sugar and salt but not for sand. If you wet sand it will stick until it dries. Then the sand will break up into little grains once more.
Being curious by nature i started looking for an answer on the internet. The problem with these things is to find the right keywords. After browsing through many recipe, bakery and cooking sites i didn’t find what i was looking for. I suspected the lumping had to do with the crystals of sugar and salt being of a different nature that sand, i tried keywords like “sugar, crystals, lumping” etc. But still no results. Then i found an article in the scholar section of Google which was titled “Sugar: a user’s guide to sucrose”. This article is an abstract of a book about sugar as you can guess. It kind of held the answer to my question.
As i expected the degree of moisture in the sugar defines the lumping or caking as it is officially called. There are three kinds of moisture that can be distinguised. The first type is called ‘included’ and is the syrup within the crystals. A syrup is a solution of water and sugar. This syrup is trapped while the crystals are formed. The second kind of moisture is called ‘bound’. This is a thin layer of syrup on the outside of the crystals. This syrup slowly crystalizes and frees the moisture contained in it. This moisture is the third kind and it is referred to as ‘free moisture’. When the sugar is in a surrounding of high humidity. The free moisture will be taken up into the bound syrup making the crystals ’sticky’. When the sugar in the bowl dries up, the moisture will be freed again thus crystalizing the syrup. When pressure is applied during this process, the crystals are pressed together when the syrup crystalizes. Causing the crystals to stick together even when the moisture itself is gone. The solution of water and sugar (the syrup) is actually the key to the lumping.
When sand is made wet, no sand is actually dissolved in the water. In fact sand is insoluble in water. This means that the molecules of the sand stick together and do not disperse among the water molecules. The is no such a thing as sand syrup. The silicon crystals (which sand is mostly made of) do not have a sticky surface that crystalizes when the sand is dried. This is the reason that sand will only stick while it is wet. Ofcourse we all know from our time as a kid that sand will stick when a lot of pressure is applied. You could make a ball of sand when pressing it hard between your hands. This stickyness comes from the different sizes of the sand grains. The grains will ‘hook’ into each other and sort of cling to each other. This is done on grain level and not on molecular level as it is with sugar and salt.
So now we know how sugar lumps come into existence and how it still sticks when the sugar has dried up. Another one of life’s mysteries solved.
source: Sugar: a user’s guide to sucrose, Neil L. Pennington, Charles W. Baker