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Not all gas lamps had mantles.
Before Carl Auer von Welsbach invented the gas mantle in the 1890s, all gas lights in homes and street lights had simple gas jets like the one on the scullery page and in the picture. They all pointed upwards.
In the home these lights were covered with glass globes (or something similar) to look ascetically pleasing and to protect the flame from being blown out. People certainly did not want to entertain friends and family on a Sunday evening in a room with what would look like a medieval torch attached to the wall!
However, this arrangement was extremely inefficient: To get as much light as possible, the gas had to be turned fully up, resulting in large sheets of flame rising towards a ceiling. Also, because the lamp had to be point upwards, the illumination was also directed upwards, i.e. at the ceiling rather than where it was needed. So the usable light for a given amount of gas was minimal.
The invention of the gas mantle changed this. It enabled gas lights to have a small flame and to direct their light downwards.
The reason for the gas jet in an old scullery not having a mantel was because it was the old style of gas light. The scullery was a room for cooking, not socialising, so its light did not have to be ascetically pleasing. Set on a wall in the corner of a room, the naked gas jet offered enough light for cooking and cleaning, but, as it didn't have a mantle, it was also relatively robust if it got knocked.