Zofia Kurnatowska in her book "Słowiańszczyzna południowa" ("Southern Slavdom") on page 55 writes that great role in Slavic colonization of the Balkans was played by assimilation of non-Slavs (both local inhabitants and prisoners of war) and turning them into Slavic speakers. She also quotes Miracula S. Demetrii, L. II, V, 195 (which says about assimilation of Bulgars, Avars and other tribes into Slavic ethnos, including intermarrying between Slavic people and Bulgars, Avars and other tribes, as the result of which:
"a huge and very numerous nation emerged") to support this claim:
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BTW - there are claims that early Slavic people were "less civilized" than tribes which they conquered, colonized or assimilated.
However, primary sources such as Miracula S. Demetrii, seem to suggest that it was not really the case.
For example L. II, IV, 190, 102 of Miracula S. Demetrii writes that Slavic people had many skilled artisans - carpenters, joiners and smiths.
L. II, IV, 190, 101 - 102 says about a Slavic foreman and his artisans - who were ordered to construct a sophisticated siege tower.
The same source says about specialization among Slavic smiths. For example, some of them specialized in producing arrowheads.
Kurnatowska also writes, that there are proofs for existence of goldsmiths and other metal-workers in early Slavic settlements. She notices that apart from ethnic Slavic artisans there could be artisans recruited from prisoners of war captured by Slavs during their expansion.
While Slavic carpenters, joiners, smiths and producers of arrows were skilled, it seems that there were no professional potters in Slavic societies.
Slavic pottery was produced by each family on its own, rather than by artisans specialized in producing pottery (Kurnatowska, page 69).
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Pangur Ban said:
TW, it is important to stress for those who don't realise how great 'Slavic expansion' was. There were Slavic tribes in the Peloponnese in the 10th century, i.e. near places like Sparta and Argos, and in the same era Slavic had been brought to the vicinity of Hamburg! Were it not for the Franks preserving Roman big-state protection in the era, Slavic may I might suggest have become the language of places like Paris and Cologne not long after.
Map showing Slavic tribes in the Peloponnese from Kurnatowska's book:
Among the southernmost ones were Milingowie (Μιληγγοί; Milengoi) and Jeziercy (Ἐζερῖται; Ezerītai):
http://pl.wikipedia.org/wiki/Milingowie
http://pl.wikipedia.org/wiki/Jeziercy
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Regarding Slavic houses:
According to Kurnatowska early Slavic houses (6th - 7th centuries and earlier) were 0,3 m - over 1 m deep (on average 0,5 m deep) earth-sheltered. Area of Slavic houses varied - usually between 5-6 and 14-15 square meters, with some houses being 25 square meters large. They had hard earthen floors, sometimes hammered out with clay. Walls of Slavic houses were built of wood with use of one of 2 alternative techniques:
1) log cabin technique (more widespread):
Log cabin - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Log_cabin
2) timbering kept up with wooden poles.
Early Slavic houses had wooden pitched roofs, often propped up with wooden poles. Each house had a stove made of stones located in one corner (houses had shape similar to square). Stoves had area of between 0,5 to over 2 square meters. In most regions Slavic stoves were made of stones. In some regions also clay stoves were in use - sometimes located in a hollow in a wall (probably to spare some space).
Here are two examples (plans) of such houses:
Example of reconstruction (with a stone stove):
Other examples:
http://podkarpaccy.pl/wp-content/uploads/2013/08/4.jpg
http://mediadeling.com/addons/albums/images/812a3bb7a7.jpg
Kurnatowska writes that this kind of houses was the most widespread one in all Slavic lands, not only among Southern Slavs.
Only in a few places - mostly in a few parts of Bosnia - smaller and more primitive earth dug huts without stoves inside were found.
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Regarding early Slavic farming techniques, Kurnatowska writes:
"(...) In more recent literature on the subject more and more clearly the old opinion about the widespread use or even exclusive use of slash-and-burn agriculture among first Slavs is being questioned, while a more complex picture of their articulture is presented. Early Slavic agriculture simultaneously knew and used various directions of arable farming, starting from intensive manual cultivation of infield allotments, through fallow system of farming, to peripheral slash-and-burn system. Realizing the many-sidedness of early Slavic agriculture allows to understand its considerable elasticity and its ability to adapt to changing conditions, (...)"
Garden plants - especially leguminous plants - had a very important place in Slavic agriculture, which is confirmed by primary sources.
Kurnatowska writes what is the origin of the opinion, that Slavs knew only slash-and-burn agriculture:
"(...) Slavs, while colonizing the Balkan Peninsula as well as some parts of Eastern Alpine countries, initially settled mainly in abandoned territories, which had not been in agricultural use already for long time and were often overgrown by forests. Due to this fact activity of settlers had to be directed first of all towards recultivation of those areas with use of traditional method of burning forests. In this sense we can talk about intensification of slash-and-burn agriculture in initial period of Slavic colonization of the Balkans. Excerpts from Nomos georgikos (Farmer's law) cited to support this theory, concern the use of fire in order to cleanse pieces of land for agriculture. About this kind of activity of Slavic settlers we are informed also by document of Bavarian duke Tassilon from year 777 written for monastery in Kremsmunster:
"We grant also this land, which had been brought to condition suitable for cultivation by these Slavs without our permission, below the place which is called Las, near Todicha and Sirnik" (...)"
All this confirms what Pangur Ban wrote before - that Slavic agriculture was not backward and that they were considered as specialist farmers.
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Coming back to artisans:
Slavic carpenters and boatbuilders constructed the fleets which attacked Constantinople in 624 AD and Thessaloniki in 614-616 / 620 AD.
Slavic fleet also blockaded the harbour of Thessaloniki in years 674-677.